(window.webpackJsonp=window.webpackJsonp||[]).push([[1617],{2027:function(t,a,s){"use strict";s.r(a);var n=s(31),e=Object(n.a)({},(function(){var t=this,a=t.$createElement,s=t._self._c||a;return s("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[s("h1",{attrs:{id:"lambda-expressions"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#lambda-expressions"}},[t._v("#")]),t._v(" Lambda Expressions")]),t._v(" "),s("p",[t._v("Lambda expressions provide a clear and concise way of implementing a single-method interface using an expression. They allow you to reduce the amount of code you have to create and maintain. While similar to anonymous classes, they have no type information by themselves. Type inference needs to happen.")]),t._v(" "),s("p",[t._v("Method references implement functional interfaces using existing methods rather than expressions. They belong to the lambda family as well.")]),t._v(" "),s("h2",{attrs:{id:"introduction-to-java-lambdas"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#introduction-to-java-lambdas"}},[t._v("#")]),t._v(" Introduction to Java lambdas")]),t._v(" "),s("h3",{attrs:{id:"functional-interfaces"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#functional-interfaces"}},[t._v("#")]),t._v(" Functional Interfaces")]),t._v(" "),s("p",[t._v("Lambdas can only operate on a functional interface, which is an interface with just one abstract method. Functional interfaces can have any number of "),s("a",{attrs:{href:"http://stackoverflow.com/documentation/java/113/default-methods",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("default")]),s("OutboundLink")],1),t._v(" or "),s("code",[t._v("static")]),t._v(" methods. (For this reason, they are sometimes referred to as Single Abstract Method Interfaces, or SAM Interfaces).")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo1")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo2")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("boolean")]),t._v(" baz"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo3")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Object")]),t._v(" baz"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" mink"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo4")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("default")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// default so not counted")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"baz"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("quux")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("p",[t._v("When declaring a functional interface the "),s("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("@FunctionalInterface")]),s("OutboundLink")],1),t._v(" annotation can be added. This has no special effect, but a compiler error will be generated if this annotation is applied to an interface which is not functional, thus acting as a reminder that the interface should not be changed.")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@FunctionalInterface")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo5")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token 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overrides one of Object's method so not counted")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("p",[t._v("Conversely, this is "),s("strong",[t._v("not")]),t._v(" a functional interface, as it has more than "),s("strong",[t._v("one abstract")]),t._v(" method:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("interface")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("BadFoo")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token 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Suppose you have")]),t._v(" "),s("p",[s("code",[t._v("interface Parent { public int parentMethod(); }")])]),t._v(" "),s("p",[t._v("and")]),t._v(" "),s("p",[s("code",[t._v("interface Child extends Parent { public int ChildMethod(); }")])]),t._v(" "),s("p",[t._v("Then "),s("code",[t._v("Child")]),t._v(" "),s("strong",[t._v("cannot")]),t._v(" be a functional interface since it has two\nspecified methods.")]),t._v(" "),s("p",[t._v("Java 8 also provides a number of generic templated functional interfaces in the package "),s("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("java.util.function")]),s("OutboundLink")],1),t._v(". For example, the built-in interface "),s("code",[t._v("Predicate<T>")]),t._v(" wraps a single method which inputs a value of type "),s("code",[t._v("T")]),t._v(" and returns a "),s("code",[t._v("boolean")]),t._v(".")]),t._v(" "),s("h3",{attrs:{id:"lambda-expressions-2"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#lambda-expressions-2"}},[t._v("#")]),t._v(" Lambda Expressions")]),t._v(" "),s("p",[t._v("The basic structure of a Lambda expression is:")]),t._v(" "),s("p",[s("a",{attrs:{href:"http://stackoverflow.com/documentation/java/113/default-methods",target:"_blank",rel:"noopener noreferrer"}},[t._v('<img src="proxy.php?url=https%3A%2F%2Fi.stack.imgur.com%2FRRcfc.png" alt="FunctionalInterface fi = () -> System.out.println("Hello");" />'),s("OutboundLink")],1)]),t._v(" "),s("p",[s("code",[t._v("fi")]),t._v(" will then hold a singleton instance of a class, similar to an anonymous class, which implements "),s("code",[t._v("FunctionalInterface")]),t._v(" and where the one method's definition is "),s("code",[t._v('{ System.out.println("Hello"); }')]),t._v(". In other words, the above is mostly equivalent to:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("FunctionalInterface")]),t._v(" fi "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("FunctionalInterface")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@Override")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("theOneMethod")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Hello"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("p",[t._v('The lambda is only "mostly equivalent" to the anonymous class because in a lambda, the meaning of expressions like '),s("code",[t._v("this")]),t._v(", "),s("code",[t._v("super")]),t._v(" or "),s("code",[t._v("toString()")]),t._v(" reference the class within which the assignment takes place, not the newly created object.")]),t._v(" "),s("p",[t._v("You cannot specify the name of the method when using a lambda—but you shouldn't need to, because a functional interface must have only one abstract method, so Java overrides that one.")]),t._v(" "),s("p",[t._v("In cases where the type of the lambda is not certain, (e.g. overloaded methods) you can add a cast to the lambda to tell the compiler what its type should be, like so:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Object")]),t._v(" fooHolder "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Hello"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("fooHolder "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("instanceof")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// returns true")]),t._v("\n\n")])])]),s("p",[t._v("If the functional interface's single method takes parameters, the local formal names of these should appear between the brackets of the lambda. There is no need to declare the type of the parameter or return as these are taken from the interface (although it is not an error to declare the parameter types if you want to). Thus, these two examples are equivalent:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo2")]),t._v(" longFoo "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo2")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@Override")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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"),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// not okay")]),t._v("\n\n")])])]),s("h3",{attrs:{id:"implicit-returns"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#implicit-returns"}},[t._v("#")]),t._v(" Implicit Returns")]),t._v(" "),s("p",[t._v("If the code placed inside a lambda is a Java "),s("strong",[t._v("expression")]),t._v(" rather than a "),s("strong",[t._v("statement")]),t._v(", it is treated as a method which returns the value of the expression. Thus, the following two are equivalent:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" addOneShort "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" addOneLong "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("h3",{attrs:{id:"accessing-local-variables-value-closures"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#accessing-local-variables-value-closures"}},[t._v("#")]),t._v(" Accessing Local Variables (value closures)")]),t._v(" "),s("p",[t._v("Since lambdas are syntactic shorthand for anonymous classes, they follow the same rules for accessing local variables in the enclosing scope; the variables must be treated as "),s("code",[t._v("final")]),t._v(" and not modified inside the lambda.")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("makeAdder")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" amount"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" amount"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Legal even though amount will go out of scope")]),t._v("\n                                "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// because amount is not modified")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("makeAccumulator")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Will not compile")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("p",[t._v("If it is necessary to wrap a changing variable in this way, a regular object that keeps a copy of the variable should be used. Read more in "),s("a",{attrs:{href:"http://stackoverflow.com/documentation/java/91/lambda-expressions/14441/java-closures-with-lambda-expressions",target:"_blank",rel:"noopener noreferrer"}},[t._v("Java Closures with lambda expressions."),s("OutboundLink")],1)]),t._v(" "),s("h3",{attrs:{id:"accepting-lambdas"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#accepting-lambdas"}},[t._v("#")]),t._v(" Accepting Lambdas")]),t._v(" "),s("p",[t._v("Because a lambda is an implementation of an interface, nothing special needs to be done to make a method accept a lambda: any function which takes a functional interface can also accept a lambda.")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("passMeALambda")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo1")]),t._v(" f"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    f"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("bar")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("passMeALambda")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Lambda called"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("h3",{attrs:{id:"the-type-of-a-lambda-expression"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#the-type-of-a-lambda-expression"}},[t._v("#")]),t._v(" The Type of a Lambda Expression")]),t._v(" "),s("p",[t._v("A lambda expression, by itself, does not have a specific type. While it is true that the types and number of parameters, along with the type of a return value can convey some type information, such information will only constrain what types it can be assigned to. The lambda receives a type when it is assigned to a functional interface type in one of the following ways:")]),t._v(" "),s("ul",[s("li",[t._v("Direct assignment to a functional type, e.g. "),s("code",[t._v("myPredicate = s -> s.isEmpty()")])]),t._v(" "),s("li",[t._v("Passing it as a parameter that has a functional type, e.g. "),s("code",[t._v("stream.filter(s -> s.isEmpty())")])]),t._v(" "),s("li",[t._v("Returning it from a function that returns a functional type, e.g. "),s("code",[t._v("return s -> s.isEmpty()")])]),t._v(" "),s("li",[t._v("Casting it to a functional type, e.g. "),s("code",[t._v("(Predicate<String>) s -> s.isEmpty()")])])]),t._v(" "),s("p",[t._v("Until any such assignment to a functional type is made, the lambda does not have a definite type.  To illustrate, consider the lambda expression "),s("code",[t._v("o -> o.isEmpty()")]),t._v(". The same lambda expression can be assigned to many different functional types:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Predicate")]),s("span",{pre:!0,attrs:{class:"token generics"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(">")])]),t._v(" javaStringPred "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" o "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" o"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("isEmpty")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token 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This is mostly useful with marker interfaces (such as "),s("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/io/Serializable.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("java.io.Serializable"),s("OutboundLink")],1),t._v(") since they don't add abstract methods.")]),t._v(" "),s("p",[t._v("For example, you want to create a "),s("a",{attrs:{href:"https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("TreeSet")]),s("OutboundLink")],1),t._v(" with a custom "),s("code",[t._v("Comparator")]),t._v(" and then serialize it and send it over the network. 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This example shows how to implement an anonymous listener within the scope of "),s("code",[t._v("btn.addActionListener")]),t._v(".")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("JButton")]),t._v(" btn "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("JButton")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"My Button"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nbtn"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("addActionListener")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ActionListener")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token annotation punctuation"}},[t._v("@Override")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("actionPerformed")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ActionEvent")]),t._v(" e"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Button was pressed"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("p",[s("strong",[t._v("Lambda listener")])]),t._v(" "),s("p",[t._v("Because the "),s("code",[t._v("ActionListener")]),t._v(" interface defines only one method "),s("code",[t._v("actionPerformed()")]),t._v(", it is a functional interface which means there’s a place to use Lambda expressions to replace the boilerplate code. The above example can be re-written using Lambda expressions as follows:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("JButton")]),t._v(" btn "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("JButton")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"My Button"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nbtn"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("addActionListener")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("e "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("System")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("out"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("println")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v('"Button was pressed"')]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("h2",{attrs:{id:"java-closures-with-lambda-expressions"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#java-closures-with-lambda-expressions"}},[t._v("#")]),t._v(" Java Closures with lambda expressions.")]),t._v(" "),s("p",[t._v("A lambda closure is created when a lambda expression references the variables of an enclosing scope (global or local). The rules for doing this are the same as those for inline methods and anonymous classes.")]),t._v(" "),s("p",[s("strong",[t._v("Local variables")]),t._v(" from an enclosing scope that are used within a lambda have to be "),s("code",[t._v("final")]),t._v(". With Java 8 (the earliest version that supports lambdas), they don't need to be "),s("strong",[t._v("declared")]),t._v(" "),s("code",[t._v("final")]),t._v(" in the outside context, but must be treated that way.  For example:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// With Java 8 there is no need to explicit final")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Runnable")]),t._v(" r "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Using lambda")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// do something")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("p",[t._v("This is legal as long as the value of the "),s("code",[t._v("n")]),t._v(" variable is not changed.  If you try to change the variable, inside or outside the lambda, you will get the following compilation error:")]),t._v(" "),s("blockquote"),t._v(" "),s("p",[t._v('"local variables referenced from a lambda expression must be '),s("strong",[t._v("final")]),t._v(" or "),s("strong",[t._v("effectively final")]),t._v('".')]),t._v(" "),s("p",[t._v("For example:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Runnable")]),t._v(" r "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Using lambda")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// do something")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nn"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("++")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Will generate an error.")]),t._v("\n\n")])])]),s("p",[t._v("If it is necessary to use a changing variable within a lambda, the normal approach is to declare a "),s("code",[t._v("final")]),t._v(" copy of the variable and use the copy.  For example")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" n "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("final")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" k "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// With Java 8 there is no need to explicit final")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Runnable")]),t._v(" r "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Using lambda")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" k"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// do something")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nn"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("++")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("      "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Now will not generate an error")]),t._v("\nr"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("run")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("  "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Will run with i = 0 because k was 0 when the lambda was created")]),t._v("\n\n")])])]),s("p",[t._v("Naturally, the body of the lambda does not see the changes to the original variable.")]),t._v(" "),s("p",[t._v("Note that Java does not support true closures. A Java lambda cannot be created in a way that allows it to see changes in the environment in which it was instantiated. If you want to implement a closure that observes or makes changes to its environment, you should simulate it using a regular class. For example:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Does not compile ...")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("createAccumulator")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" accumulate "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" accumulate"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("p",[t._v("The above example will not compile for reasons discussed previously. We can work around the compilation error as follows:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Compiles, but is incorrect ...")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("AccumulatorGenerator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("createAccumulator")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("IntUnaryOperator")]),t._v(" accumulate "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("->")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" accumulate"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("p",[t._v("The problem is that this breaks the design contract for the "),s("code",[t._v("IntUnaryOperator")]),t._v(" interface which states that instances should be functional and stateless.  If such a closure is passed to built-in functions that accept functional objects, it is liable to cause crashes or erroneous behavior.  Closures that encapsulate mutable state should be implemented as regular classes.  For example.")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// Correct ...")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Accumulator")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n   "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("private")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n   "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("accumulate")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("int")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n      value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" x"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n      "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" value"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n   "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),s("h2",{attrs:{id:"using-lambda-expression-with-your-own-functional-interface"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#using-lambda-expression-with-your-own-functional-interface"}},[t._v("#")]),t._v(" Using lambda expression with your own functional interface")]),t._v(" "),s("p",[t._v("Lambdas are meant to provide inline implementation code for single method interfaces and the ability to pass them around as we have been doing with normal variables. We call them Functional Interface.")]),t._v(" "),s("p",[t._v("For example, writing a Runnable in anonymous class and starting a Thread looks like:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//Old way")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Thread")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n        "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Runnable")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n            "),s("span",{pre:!0,attrs:{class:"token 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Same as Runnable example shown above. See the driver program below:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("CustomLambda")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("static")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("void")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("main")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("String")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token 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It can apply to any scenario where similar setup/tear down tasks are applicable with minor variations. 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But this is "),s("strong",[t._v("not")]),t._v(" more consistent. 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