(window.webpackJsonp=window.webpackJsonp||[]).push([[1563],{1972:function(t,s,a){"use strict";a.r(s);var n=a(31),e=Object(n.a)({},(function(){var t=this,s=t.$createElement,a=t._self._c||s;return a("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[a("h1",{attrs:{id:"enums"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#enums"}},[t._v("#")]),t._v(" Enums")]),t._v(" "),a("h2",{attrs:{id:"declaring-and-using-a-basic-enum"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#declaring-and-using-a-basic-enum"}},[t._v("#")]),t._v(" Declaring and using a basic enum")]),t._v(" "),a("p",[a("a",{attrs:{href:"https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("Enum"),a("OutboundLink")],1),t._v(" can be considered to be syntax sugar for a sealed class that is instantiated only a number of times known at compile-time to define a set of constants.")]),t._v(" "),a("p",[t._v("A simple enum to list the different seasons would be declared as follows:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Season")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    WINTER"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    SPRING"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    SUMMER"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    FALL\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("While the enum constants don't necessarily need to be in all-caps, it is Java convention that names of constants are entirely uppercase, with words separated by underscores.")]),t._v(" "),a("p",[t._v("You can declare an Enum in its own file:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n * This enum is declared in the Season.java file.\n*/")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Season")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    WINTER"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    SPRING"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    SUMMER"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    FALL\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("But you can also declare it inside another class:")]),t._v(" "),a("div",{staticClass:"language- extra-class"},[a("pre",{pre:!0,attrs:{class:"language-text"}},[a("code",[t._v("\npublic class Day {\n\n    private Season season;\n\n    public String getSeason() {\n        return season.name();\n    }\n\n    public void setSeason(String season) {\n        this.season = Season.valueOf(season);\n    }\n\n    /**\n     * This enum is declared inside the Day.java file and \n     * cannot be accessed outside because it's declared as private.\n     */\n    private enum Season {\n        WINTER,\n        SPRING,\n        SUMMER,\n        FALL\n    }\n\n}\n\n")])])]),a("p",[t._v("Finally, you cannot declare an Enum inside a method body or constructor:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n     * Constructor\n    */")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Illegal. 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Therefore, that allows to implement "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/130/singletons",target:"_blank",rel:"noopener noreferrer"}},[t._v("Singleton"),a("OutboundLink")],1),t._v(" software design pattern with a single-element enum.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Attendant")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n\n    INSTANCE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Attendant")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// perform some initialization routine")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("sayHello")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Hello!"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Main")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("main")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v(" args"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Attendant")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("INSTANCE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("sayHello")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// instantiated at this point")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v('According to "Effective Java" book by Joshua Bloch, a single-element enum is the best way to implement a singleton. This approach has following advantages:')]),t._v(" "),a("ul",[a("li",[t._v("thread safety")]),t._v(" "),a("li",[t._v("guarantee of single instantiation")]),t._v(" "),a("li",[t._v("out-of-the-box serialization")])]),t._v(" "),a("p",[t._v("And as shown in the section "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/155/enums/1809/implements-interface",target:"_blank",rel:"noopener noreferrer"}},[t._v("implements interface"),a("OutboundLink")],1),t._v(" this singleton might also implement one or more interfaces.")]),t._v(" "),a("h2",{attrs:{id:"using-methods-and-static-blocks"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#using-methods-and-static-blocks"}},[t._v("#")]),t._v(" Using methods and static blocks")]),t._v(" "),a("p",[t._v("An enum can contain a method, just like any class. 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"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("default")]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//This will never happen")]),t._v("\n                "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("This can be improved further through the use of fields and static initializer blocks:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token 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class-name"}},[t._v("Direction")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getOpposite")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" opposite"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    \n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        NORTH"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("opposite "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" SOUTH"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        SOUTH"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("opposite "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" NORTH"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        WEST"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("opposite "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" EAST"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        EAST"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("opposite "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" WEST"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("In this example, the opposite direction is stored in a private instance field "),a("code",[t._v("opposite")]),t._v(", which is statically initialized the first time a "),a("code",[t._v("Direction")]),t._v(" is used.\nIn this particular case (because "),a("code",[t._v("NORTH")]),t._v(" references "),a("code",[t._v("SOUTH")]),t._v(" and conversely), we cannot use "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/155/enums/602/enums-with-constructors#t=201607220740460629821",target:"_blank",rel:"noopener noreferrer"}},[t._v("Enums with constructors"),a("OutboundLink")],1),t._v(" here (Constructors "),a("code",[t._v("NORTH(SOUTH), SOUTH(NORTH), EAST(WEST), WEST(EAST)")]),t._v(" would be more elegant and would allow "),a("code",[t._v("opposite")]),t._v(" to be declared "),a("code",[t._v("final")]),t._v(", but would be self-referential and therefore are not allowed).")]),t._v(" "),a("h2",{attrs:{id:"documenting-enums"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#documenting-enums"}},[t._v("#")]),t._v(" Documenting enums")]),t._v(" "),a("p",[t._v("Not always the "),a("code",[t._v("enum")]),t._v(" name is clear enough to be understood. To document an "),a("code",[t._v("enum")]),t._v(", use standard javadoc:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n * United States coins\n */")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Coins")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    \n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n     * One-cent coin, commonly known as a penny, \n     * is a unit of currency equaling one-hundredth \n     * of a United States dollar\n     */")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("PENNY")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n     * A nickel is a five-cent coin equaling \n     * five-hundredth of a United States dollar\n     */")]),t._v("        \n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("NICKEL")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n     * The dime is a ten-cent coin refers to \n     * one tenth of a United States dollar\n     */")]),t._v("        \n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("DIME")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("10")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n     * The quarter is a US coin worth 25 cents, \n     * one-fourth of a United States dollar\n     */")]),t._v("        \n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("QUARTER")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("25")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Coins")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" \n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("value "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" value"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getValue")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" value"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("h2",{attrs:{id:"enum-as-a-bounded-type-parameter"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#enum-as-a-bounded-type-parameter"}},[t._v("#")]),t._v(" Enum as a bounded type parameter")]),t._v(" "),a("p",[t._v("When writing a class with generics in java, it is possible to ensure that the type parameter is an enum. Since all enums extend the "),a("code",[t._v("Enum")]),t._v(" class, the following syntax may be used.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Holder")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("extends")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Enum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("final")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),t._v(" value"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Holder")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("T")]),t._v(" init"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("value "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" init"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("In this example, the type "),a("code",[t._v("T")]),t._v(" "),a("strong",[t._v("must")]),t._v(" be an enum.")]),t._v(" "),a("h2",{attrs:{id:"enum-constant-specific-body"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#enum-constant-specific-body"}},[t._v("#")]),t._v(" Enum constant specific body")]),t._v(" "),a("p",[t._v("In an "),a("code",[t._v("enum")]),t._v(" it is possible to define a specific behavior for a particular constant of the "),a("code",[t._v("enum")]),t._v(" which overrides the default behavior of the "),a("code",[t._v("enum")]),t._v(", this technique is known as "),a("strong",[t._v("constant specific body")]),t._v(".")]),t._v(" "),a("p",[t._v("Suppose three piano students - John, Ben and Luke - are defined in an "),a("code",[t._v("enum")]),t._v(" named "),a("code",[t._v("PianoClass")]),t._v(", as follows:")]),t._v(" "),a("div",{staticClass:"language- extra-class"},[a("pre",{pre:!0,attrs:{class:"language-text"}},[a("code",[t._v('\n   enum PianoClass {\n    JOHN, BEN, LUKE;\n        public String getSex() {\n            return "Male";\n        }\n        public String getLevel() {\n            return "Beginner";\n        }\n    }\n\n')])])]),a("p",[t._v("And one day two other students arrive - Rita and Tom - with a sex (Female) and level (Intermediate) that do not match the previous ones:")]),t._v(" "),a("div",{staticClass:"language- extra-class"},[a("pre",{pre:!0,attrs:{class:"language-text"}},[a("code",[t._v('\n   enum PianoClass2 {\n    JOHN, BEN, LUKE, RITA, TOM;\n        public String getSex() {\n            return "Male"; // issue, Rita is a female\n        }\n        public String getLevel() {\n            return "Beginner"; // issue, Tom is an intermediate student\n        }\n    }\n\n')])])]),a("p",[t._v("so that simply adding the new students to the constant declaration, as follows, is not correct:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass2")]),t._v(" tom "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass2")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("TOM"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass2")]),t._v(" rita "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass2")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("RITA"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("tom"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getLevel")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// prints Beginner -> wrong Tom's not a beginner")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("rita"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getSex")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// prints Male -> wrong Rita's not a male")]),t._v("\n\n")])])]),a("p",[t._v("It's possible to define a specific behavior for each of the constant, Rita and Tom, which overrides the "),a("code",[t._v("PianoClass2")]),t._v(" default behavior as follows:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass3")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    JOHN"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" BEN"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" LUKE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n    RITA "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@Override")]),t._v("\n  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"),a("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@Override")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getLevel")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n            "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Intermediate"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getSex")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Male"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token 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"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass3")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("TOM"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass3")]),t._v(" rita "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("PianoClass3")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("RITA"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("tom"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getLevel")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// prints Intermediate")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("rita"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getSex")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// prints Female")]),t._v("\n\n")])])]),a("p",[t._v("Another way to define content specific body is by using constructor, for instance:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("MAT")]),a("span",{pre:!0,attrs:{class:"token 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"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" gender"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" gender"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("gender "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" gender"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token 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extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),t._v(" mat "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("MAT"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),t._v(" john "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("JOHN"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),t._v(" jane "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Friend")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("JANE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("mat"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getGender")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("     "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Male")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("john"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("getGender")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Male")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token 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"),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Util")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/* No instances */")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("clamp")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" min"),a("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v("(")]),t._v("i"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" min"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" max"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// other utility methods...")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Just as "),a("code",[t._v("enum")]),t._v(" "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/155/enums/5741/implement-singleton-pattern-with-a-single-element-enum",target:"_blank",rel:"noopener noreferrer"}},[t._v("can be used for singletons"),a("OutboundLink")],1),t._v(" (1 instance classes), it can be used for utility classes (0 instance classes). Just make sure to terminate the (empty) list of enum constants with a "),a("code",[t._v(";")]),t._v(".")]),t._v(" "),a("p",[t._v("See the question "),a("a",{attrs:{href:"http://stackoverflow.com/questions/25137490/zero-instance-enum-vs-private-constructors-for-preventing-instantiation",target:"_blank",rel:"noopener noreferrer"}},[t._v("Zero instance enum vs private constructors for preventing instantiation"),a("OutboundLink")],1),t._v(" for a discussion on pro's and con's compared to private constructors.")]),t._v(" "),a("h2",{attrs:{id:"getting-the-values-of-an-enum"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#getting-the-values-of-an-enum"}},[t._v("#")]),t._v(" Getting the values of an enum")]),t._v(" "),a("p",[t._v("Each enum class contains an implicit static method named "),a("code",[t._v("values()")]),t._v(". This method returns an array containing all values of that enum. You can use this method to iterate over the values. It is important to note however that this method returns a "),a("strong",[t._v("new")]),t._v(" array every time it is called.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    MONDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" TUESDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" WEDNESDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" THURSDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" FRIDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" SATURDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" SUNDAY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    \n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n    * Print out all the values in this enum.\n    */")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printAllDays")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),t._v(" day "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("values")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n            "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("day"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("name")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("If you need a "),a("code",[t._v("Set")]),t._v(" you can use "),a("code",[t._v("EnumSet.allOf(Day.class)")]),t._v(" as well.")]),t._v(" "),a("h2",{attrs:{id:"enum-polymorphism-pattern"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#enum-polymorphism-pattern"}},[t._v("#")]),t._v(" Enum Polymorphism Pattern")]),t._v(" "),a("p",[t._v('When a method need to accept an "extensible" set of '),a("code",[t._v("enum")]),t._v(" values, the programmer can apply polymorphism like on a normal "),a("code",[t._v("class")]),t._v(" by creating an interface which will be used anywere where the "),a("code",[t._v("enum")]),t._v("s shall be used:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtensibleEnum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("name")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("This way, any "),a("code",[t._v("enum")]),t._v(" tagged by (implementing) the interface can be used as a parameter, allowing the programmer to create a variable amount of "),a("code",[t._v("enum")]),t._v("s that will be accepted by the method. This can be useful, for example, in APIs where there is a default (unmodifiable) "),a("code",[t._v("enum")]),t._v(' and the user of these APIs want to "extend" the '),a("code",[t._v("enum")]),t._v(" with more values.")]),t._v(" "),a("p",[t._v("A set of default enum values can be defined as follows:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("DefaultValues")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("implements")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtensibleEnum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    VALUE_ONE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" VALUE_TWO"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Additional values can then be defined like this:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtendedValues")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("implements")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtensibleEnum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    VALUE_THREE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" VALUE_FOUR"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("Sample which shows how to use the enums - note how "),a("code",[t._v("printEnum()")]),t._v(" accepts values from both "),a("code",[t._v("enum")]),t._v(" types:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printEnum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtensibleEnum")]),t._v(" val"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("val"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("name")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("  \n\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printEnum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("DefaultValues")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("VALUE_ONE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// VALUE_ONE")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printEnum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("DefaultValues")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("VALUE_TWO"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// VALUE_TWO")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printEnum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtendedValues")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("VALUE_THREE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// VALUE_THREE")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("printEnum")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ExtendedValues")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("VALUE_FOUR"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("  "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// VALUE_FOUR")]),t._v("\n\n")])])]),a("p",[t._v("Note: This pattern does not prevent you from redefining enum values, which are already defined in one enum, in another enum. These enum values would be different instances then.\nAlso, it is not possible to use switch-on-enum since all we have is the interface, not the real "),a("code",[t._v("enum")]),t._v(".")]),t._v(" "),a("h2",{attrs:{id:"compare-and-contains-for-enum-values"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#compare-and-contains-for-enum-values"}},[t._v("#")]),t._v(" Compare and Contains for Enum values")]),t._v(" "),a("p",[t._v("Enums contains only constants and can be compared directly with "),a("code",[t._v("==")]),t._v(". So, only reference check is needed, no need to use "),a("code",[t._v(".equals")]),t._v(" method. Moreover, if "),a("code",[t._v(".equals")]),t._v(" used incorrectly, may raise the "),a("code",[t._v("NullPointerException")]),t._v(" while that's not the case with "),a("code",[t._v("==")]),t._v(" check.")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    GOOD"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" AVERAGE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" WORST"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token 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day "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("null")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("day"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("GOOD"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//NullPointerException!")]),t._v("\n            "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Good Day!"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("day "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("==")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Day")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("GOOD"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//Always use == to compare enum")]),t._v("\n            "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Good Day!"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n  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String")]),t._v(" "),a("p",[t._v("Sometimes you want to convert your enum to a String, there are two ways to do that.")]),t._v(" "),a("p",[t._v("Assume we have:")]),t._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("enum")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Fruit")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    APPLE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" ORANGE"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" STRAWBERRY"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" BANANA"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" LEMON"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" 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