(window.webpackJsonp=window.webpackJsonp||[]).push([[1581],{1989:function(s,t,a){"use strict";a.r(t);var n=a(31),e=Object(n.a)({},(function(){var s=this,t=s.$createElement,a=s._self._c||t;return a("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[a("h1",{attrs:{id:"inheritance"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#inheritance"}},[s._v("#")]),s._v(" Inheritance")]),s._v(" "),a("p",[s._v("Inheritance is a basic object oriented feature in which one class acquires and extends upon the properties of another class, using the keyword "),a("code",[s._v("extends")]),s._v(". For Interfaces and the keyword "),a("code",[s._v("implements")]),s._v(", see "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/102/interfaces#t=201701031900066194592",target:"_blank",rel:"noopener noreferrer"}},[s._v("interfaces"),a("OutboundLink")],1),s._v(".")]),s._v(" "),a("h2",{attrs:{id:"abstract-classes"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#abstract-classes"}},[s._v("#")]),s._v(" Abstract Classes")]),s._v(" "),a("p",[s._v("An abstract class is a class marked with the "),a("code",[s._v("abstract")]),s._v(" keyword. It, contrary to non-abstract class, may contain abstract - implementation-less - methods. It is, however, valid to create an abstract class without abstract methods.")]),s._v(" "),a("p",[s._v("An abstract class cannot be instantiated. It can be sub-classed (extended) as long as the sub-class is either also abstract, or implements all methods marked as abstract by super classes.")]),s._v(" "),a("p",[s._v("An example of an abstract class:")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("abstract")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Component")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("private")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("int")]),s._v(" x"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" y"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n    \n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("setPosition")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("int")]),s._v(" x"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("int")]),s._v(" y"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("x "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" x"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("y "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" y"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("abstract")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("render")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("The class must be marked abstract, when it has at least one abstract method. An abstract method is a method that has no implementation. Other methods can be declared within an abstract class that have implementation in order to provide common code for any sub-classes.")]),s._v(" "),a("p",[s._v("Attempting to instantiate this class will provide a compile error:")]),s._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"}},[s._v("//error: Component is abstract; cannot be instantiated   ")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Component")]),s._v(" myComponent "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Component")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token 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Using the above example this could look like this:")]),s._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"}},[s._v("Component")]),s._v(" myAnonymousComponent "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Component")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[s._v("@Override")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("render")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// render a quick 1-time use component")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("h2",{attrs:{id:"using-final-to-restrict-inheritance-and-overriding"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#using-final-to-restrict-inheritance-and-overriding"}},[s._v("#")]),s._v(" Using 'final' to restrict inheritance and overriding")]),s._v(" "),a("h3",{attrs:{id:"final-classes"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#final-classes"}},[s._v("#")]),s._v(" Final classes")]),s._v(" "),a("p",[s._v("When used in a "),a("code",[s._v("class")]),s._v(" declaration, the "),a("code",[s._v("final")]),s._v(" modifier prevents other classes from being declared that "),a("code",[s._v("extend")]),s._v(" the class.  A "),a("code",[s._v("final")]),s._v(' class is a "leaf" class in the inheritance class hierarchy.')]),s._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"}},[s._v("// This declares a final class")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("final")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyFinalClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("/* some code */")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Compilation error: cannot inherit from final MyFinalClass")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MySubClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("extends")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyFinalClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("/* more code */")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("h3",{attrs:{id:"use-cases-for-final-classes"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#use-cases-for-final-classes"}},[s._v("#")]),s._v(" Use-cases for final classes")]),s._v(" "),a("p",[s._v("Final classes can be combined with a "),a("code",[s._v("private")]),s._v(' constructor to control or prevent the instantiation of a class.  This can be used to create a so-called "utility class" that only defines static members; i.e. constants and static methods.')]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("final")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("UtilityClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Private constructor to replace the default visible constructor")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("private")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("UtilityClass")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Static members can still be used as usual")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("int")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("doSomethingCool")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[s._v("123")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("Immutable classes should also be declared as "),a("code",[s._v("final")]),s._v(".  (An immutable class is one whose instances cannot be changed after they have been created; see the I"),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/2807/immutable-objects",target:"_blank",rel:"noopener noreferrer"}},[s._v("mmutable Objects"),a("OutboundLink")],1),s._v(" topic. ) By doing this, you make it impossible to create a mutable subclass of an immutable class. That would violate the "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/87/inheritance/3106/the-liskov-substitution-principle",target:"_blank",rel:"noopener noreferrer"}},[s._v("Liskov Substitution Principle"),a("OutboundLink")],1),s._v(' which requires that a subtype should obey the "behavioral contract" of its supertypes.')]),s._v(" "),a("p",[s._v("From a practical perspective, declaring an immutable class to be "),a("code",[s._v("final")]),s._v(" makes it easier to reason about program behavior.  It also addresses security concerns in the scenario where untrusted code is executed in a security sandbox.  (For instance, since "),a("code",[s._v("String")]),s._v(" is declared as "),a("code",[s._v("final")]),s._v(", a trusted class does not need to worry that it might be tricked into accepting mutable subclass, which the untrusted caller could then surreptitiously change.)")]),s._v(" "),a("p",[s._v("One disadvantage of "),a("code",[s._v("final")]),s._v(" classes is that they do not work with some mocking frameworks such as Mockito. Update: Mockito version 2 now support mocking of final classes.")]),s._v(" "),a("h3",{attrs:{id:"final-methods"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#final-methods"}},[s._v("#")]),s._v(" Final methods")]),s._v(" "),a("p",[s._v("The "),a("code",[s._v("final")]),s._v(" modifier can also be applied to methods to prevent them being overridden in sub-classes:")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyClassWithFinalMethod")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("final")]),s._v(" 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entirely.")]),s._v(" "),a("p",[s._v("The "),a("code",[s._v("final")]),s._v(" modifier can also be applied to variables, but the meaning of "),a("code",[s._v("final")]),s._v(" for variables is unrelated to inheritance.")]),s._v(" "),a("h2",{attrs:{id:"inheritance-2"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#inheritance-2"}},[s._v("#")]),s._v(" Inheritance")]),s._v(" "),a("p",[s._v("With the use of the "),a("code",[s._v("extends")]),s._v(" keyword among classes, all the properties of the superclass (also known as the "),a("strong",[s._v("Parent Class")]),s._v(" or "),a("strong",[s._v("Base Class")]),s._v(") are present in the subclass (also known as the "),a("strong",[s._v("Child Class")]),s._v(" or "),a("strong",[s._v("Derived Class")]),s._v(")")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token 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Doing so allows for additional functionality in the subclass without any change to the base class or any other subclasses from that same base class:")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Subclass2")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("extends")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("BaseClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token 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"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("printX")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("x"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//Illegal, can't access private field x")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("getX")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//Legal, prints 5")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token 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multiple inheritance, and while it is legal in some languages, Java does not permit it with classes.")]),s._v(" "),a("p",[s._v("As a result of this, every class has an unbranching ancestral chain of classes leading to "),a("code",[s._v("Object")]),s._v(", from which all classes descend.")]),s._v(" "),a("h2",{attrs:{id:"the-liskov-substitution-principle"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#the-liskov-substitution-principle"}},[s._v("#")]),s._v(" The Liskov Substitution Principle")]),s._v(" "),a("p",[s._v("Substitutability is a principle in object-oriented programming introduced by Barbara Liskov in a 1987 conference keynote stating that, if class "),a("code",[s._v("B")]),s._v(" is a subclass of class "),a("code",[s._v("A")]),s._v(", then wherever "),a("code",[s._v("A")]),s._v(" is expected, "),a("code",[s._v("B")]),s._v(" can be used instead:")]),s._v(" "),a("div",{staticClass:"language-java 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keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("StaticOverride")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("extends")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("BaseClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//Hides the num field from BaseClass")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//You can even change the type, since this doesn't affect the signature")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),s._v(" num "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token string"}},[s._v('"test"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n        \n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//Cannot use @Override annotation, since this is static")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//This overrides the sayHello method from BaseClass")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("sayHello")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        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You can always call the base "),a("code",[s._v("sayHello")]),s._v(" method by using "),a("code",[s._v("BaseClass.sayHello()")]),s._v(". But classes do inherit static methods "),a("strong",[s._v("if")]),s._v(" no methods with the same signature are found in the subclass. If two method's signatures vary, both methods can be run from the subclass, even if the name is the same.")]),s._v(" "),a("p",[s._v("Static fields hide each other in a similar way.")]),s._v(" "),a("h2",{attrs:{id:"abstract-class-and-interface-usage-is-a-relation-vs-has-a-capability"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#abstract-class-and-interface-usage-is-a-relation-vs-has-a-capability"}},[s._v("#")]),s._v(' Abstract class and Interface usage: "Is-a" relation vs "Has-a" capability')]),s._v(" "),a("p",[s._v("When to use abstract classes:  To implement the same or different behaviour among multiple related objects")]),s._v(" "),a("p",[s._v("When to use interfaces: to implement a contract by multiple unrelated objects")]),s._v(" "),a("p",[a("strong",[s._v('Abstract classes create "is a" relations while interfaces provide "has a" capability.')])]),s._v(" "),a("p",[s._v("This can be seen in the code below:")]),s._v(" "),a("div",{staticClass:"language-java 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This is done with Interfaces.\n")]),s._v(" "),a("li",[s._v("\nEven though `Animal` is an abstract class, it has a constructor, unlike an interface.\n")]),s._v(" "),a("p",[s._v("TL;DR:")]),s._v(" "),a("p",[a("strong",[s._v("Unrelated classes can have capabilities through interfaces, but related classes change the behaviour through extension of base classes.")])]),s._v(" "),a("p",[s._v("Refer to the Java documentation "),a("a",{attrs:{href:"https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html",target:"_blank",rel:"noopener noreferrer"}},[s._v("page"),a("OutboundLink")],1),s._v(" to understand which one to use in a specific use case.")]),s._v(" "),a("p",[a("strong",[s._v("Consider using abstract classes")]),s._v(" if...")]),s._v(" "),a("ol",[a("li",[s._v("You want to share code among several closely related classes.")]),s._v(" "),a("li",[s._v("You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).")]),s._v(" "),a("li",[s._v("You want to declare non-static or non-final fields.")])]),s._v(" "),a("p",[a("strong",[s._v("Consider using interfaces")]),s._v(" if...")]),s._v(" "),a("ol",[a("li",[s._v("You expect that unrelated classes would implement your interface. For example, many unrelated objects can implement the "),a("code",[s._v("Serializable")]),s._v(" interface.")]),s._v(" "),a("li",[s._v("You want to specify the behaviour of a particular data type but are not concerned about who implements its behaviour.")]),s._v(" "),a("li",[s._v("You want to take advantage of multiple inheritance of type.")])]),s._v(" "),a("h2",{attrs:{id:"inheritance-and-static-methods"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#inheritance-and-static-methods"}},[s._v("#")]),s._v(" Inheritance and Static Methods")]),s._v(" "),a("p",[s._v("In Java, parent and child class both can have static methods with the same name. But in such cases implementation of static method in child is "),a("a",{attrs:{href:"https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.4.8.2",target:"_blank",rel:"noopener noreferrer"}},[s._v("hiding"),a("OutboundLink")],1),s._v(" parent class' implementation, it's not method overriding. For example:")]),s._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"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("StaticMethodTest")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n  "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// static method and inheritance")]),s._v("\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("main")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token 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Since in the first call to "),a("code",[s._v("staticMethod()")]),s._v(", parent class reference "),a("code",[s._v("p")]),s._v(" was used, "),a("code",[s._v("Parent")]),s._v("'s version of "),a("code",[s._v("staticMethod()")]),s._v(" is invoked. In second case, we did cast "),a("code",[s._v("p")]),s._v(" into "),a("code",[s._v("Child")]),s._v(" class, "),a("code",[s._v("Child")]),s._v("'s "),a("code",[s._v("staticMethod()")]),s._v(" executed.")]),s._v(" "),a("h2",{attrs:{id:"variable-shadowing"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#variable-shadowing"}},[s._v("#")]),s._v(" Variable shadowing")]),s._v(" "),a("p",[s._v("Variables are SHADOWED and methods are OVERRIDDEN.\nWhich variable will be used depends on the class that the variable is declared of.\nWhich method will be used depends on the actual class of the object that is referenced by the variable.")]),s._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"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Car")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    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"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Car")]),s._v(" car "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("SportsCar")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("car"),a("span",{pre:!0,attrs:{class:"token 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punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("h2",{attrs:{id:"narrowing-and-widening-of-object-references"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#narrowing-and-widening-of-object-references"}},[s._v("#")]),s._v(" Narrowing and Widening of object references")]),s._v(" "),a("p",[s._v("Casting an instance of a base class to a subclass as in : "),a("code",[s._v("b = (B) a;")]),s._v(" is called "),a("strong",[s._v("narrowing")]),s._v(" (as you are trying to narrow the base class object to a more specific class object) and needs an explicit type-cast.")]),s._v(" "),a("p",[s._v("Casting an instance of a subclass to a base class as in: "),a("code",[s._v("A a = b;")]),s._v(" is called "),a("strong",[s._v("widening")]),s._v(" and does not need a type-cast.")]),s._v(" "),a("p",[s._v("To illustrate, consider the following class declarations, and test code:")]),s._v(" "),a("div",{staticClass:"language-java 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The assignment cannot be allowed, since that might lead to")]),s._v("car"),a("code",[s._v("referring to a")]),s._v("Truck` instance.")]),s._v(" "),a("p",[s._v("To prevent this situation, we need to add an explicit type-cast:")]),s._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"}},[s._v("Car")]),s._v(" c "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Car")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" vehicle"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n")])])]),a("p",[s._v("The type-cast tells the compiler that we "),a("strong",[s._v("expect")]),s._v(" the value of "),a("code",[s._v("vehicle")]),s._v(" to be a "),a("code",[s._v("Car")]),s._v(" or a subclass of "),a("code",[s._v("Car")]),s._v(".  If necessary, compiler will insert code to perform a run-time type check.  If the check fails, then a "),a("code",[s._v("ClassCastException")]),s._v(" will be thrown when the code is executed.")]),s._v(" "),a("p",[s._v("Note that not all type-casts are valid.  For example:")]),s._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"}},[s._v("String")]),s._v(" s "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" vehicle"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("  "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// not valid")]),s._v("\n\n")])])]),a("p",[s._v("The Java compiler knows that an instance that is type compatible with "),a("code",[s._v("Vehicle")]),s._v(" "),a("strong",[s._v("cannot ever be")]),s._v(" type compatible with "),a("code",[s._v("String")]),s._v(".  The type-cast could never succeed, and the JLS mandates that this gives in a compilation error.")]),s._v(" "),a("h2",{attrs:{id:"programming-to-an-interface"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#programming-to-an-interface"}},[s._v("#")]),s._v(" Programming to an interface")]),s._v(" "),a("p",[s._v("The idea behind programming to an interface is to base the code primarily on interfaces and only use concrete classes at the time of instantiation. In this context, good code dealing with e.g. Java collections will look something like this (not that the method itself is of any use at all, just illustration):")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("toSet")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Collection")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" collection"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Sets")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("newHashSet")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("collection"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("while bad code might look like this:")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("HashSet")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("toSet")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("ArrayList")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" collection"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Sets")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("newHashSet")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("collection"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("Not only the former can be applied to a wider choice of arguments, its results will be more compatible with code provided by other developers that generally adhere to the concept of programming to an interface. However, the most important reasons to use the former are:")]),s._v(" "),a("ul",[a("li",[s._v("most of the time the context, in which the result is used, does not and should not need that many details as the concrete implementation provides;")]),s._v(" "),a("li",[s._v("adhering to an interface forces cleaner code and less hacks such as yet another public method gets added to a class serving some specific scenario;")]),s._v(" "),a("li",[s._v("the code is more testable as interfaces are easily mockable;")]),s._v(" "),a("li",[s._v("finally, the concept helps even if only one implementation is expected (at least for testability).")])]),s._v(" "),a("p",[s._v("So how can one easily apply the concept of programming to an interface when writing new code having in mind one particular implementation? One option that we commonly use is a combination of the following patterns:")]),s._v(" "),a("ul",[a("li",[s._v("programming to an interface")]),s._v(" "),a("li",[s._v("factory")]),s._v(" "),a("li",[s._v("builder")])]),s._v(" "),a("p",[s._v("The following example based on these principles is a simplified and truncated version of an RPC implementation written for a number of different protocols:")]),s._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"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("interface")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("RemoteInvoker")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token 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