Java 双冒号(::)运算符

双冒号（::）操作符，在Java中也被称为方法引用操作符，用于通过直接引用一个方法的类来调用它。它们的行为与lambda表达式完全一样。它与lambda表达式的唯一区别是，它使用直接引用方法的名称，而不是提供方法的委托。

语法。

<Class name>::<method name>

例子。要打印流中的所有元素。

stream.forEach( s-> System.out.println(s));

程序:

// Java code to print the elements of Stream// without using double colon operator  import java.util.stream.*;  class GFG {    public static void main(String[] args)    {          // Get the stream        Stream<String> stream            = Stream.of("Geeks", "For",                        "Geeks", "A",                        "Computer",                        "Portal");          // Print the stream        stream.forEach(s -> System.out.println(s));    }}

输出:

GeeksForGeeksAComputerPortal

stream.forEach( System.out::println);

程序: 为了演示双冒号运算符的使用

// Java code to print the elements of Stream// using double colon operator  import java.util.stream.*;  class GFG {    public static void main(String[] args)    {          // Get the stream        Stream<String> stream            = Stream.of("Geeks", "For",                        "Geeks", "A",                        "Computer",                        "Portal");          // Print the stream        // using double colon operator        stream.forEach(System.out::println);    }}

输出:

GeeksForGeeksAComputerPortal

何时以及如何使用双冒号操作符？

方法引用或双冒号操作符可以用来引用。

如何在Java中使用方法引用。

Static 方法

语法:

(ClassName::methodName)

示例:

SomeClass::someStaticMethod

程序:

// Java code to show use of double colon operator// for static methods  import java.util.*;  class GFG {      // static function to be called    static void someFunction(String s)    {        System.out.println(s);    }      public static void main(String[] args)    {          List<String> list = new ArrayList<String>();        list.add("Geeks");        list.add("For");        list.add("GEEKS");          // call the static method        // using double colon operator        list.forEach(GFG::someFunction);    }}

输出:

GeeksForGEEKS

Instance 方法

语法:

(objectOfClass::methodName)

示例:

System.out::println

程序:

// Java code to show use of double colon operator// for instance methods  import java.util.*;  class GFG {      // instance function to be called    void someFunction(String s)    {        System.out.println(s);    }      public static void main(String[] args)    {          List<String> list = new ArrayList<String>();        list.add("Geeks");        list.add("For");        list.add("GEEKS");          // call the instance method        // using double colon operator        list.forEach((new GFG())::someFunction);    }}

输出:

GeeksForGEEKS

Super 方法

语法:

(super::methodName)

示例:

super::someSuperClassMethod

程序:

// Java code to show use of double colon operator// for super methods  import java.util.*;import java.util.function.*;  class Test {      // super function to be called    String print(String str)    {        return ("Hello " + str + "\n");    }}  class GFG extends Test {      // instance method to override super method    @Override    String print(String s)    {          // call the super method        // using double colon operator        Function<String, String>            func = super::print;          String newValue = func.apply(s);        newValue += "Bye " + s + "\n";        System.out.println(newValue);          return newValue;    }      // Driver code    public static void main(String[] args)    {          List<String> list = new ArrayList<String>();        list.add("Geeks");        list.add("For");        list.add("GEEKS");          // call the instance method        // using double colon operator        list.forEach(new GFG()::print);    }}

输出:

Hello GeeksBye GeeksHello ForBye ForHello GEEKSBye GEEKS

一个特定类型的任意对象的实例方法

语法:

(ClassName::methodName)

示例:

SomeClass::someInstanceMethod

程序:

// Java code to show use of double colon operator // for instance method of arbitrary type   import java.util.*;   class Test {     String str=null;      Test(String s)    {        this.str=s;    }    // instance function to be called     void someFunction()     {         System.out.println(this.str);     } }   class GFG {       public static void main(String[] args)     {           List<Test> list = new ArrayList<Test>();         list.add(new Test("Geeks"));         list.add(new Test("For"));         list.add(new Test("GEEKS"));           // call the instance method         // using double colon operator         list.forEach(Test::someFunction);     } }

输出:

GeeksForGEEKS

Class 构造函数

语法:

(ClassName::new)

示例:

ArrayList::new

程序:

// Java code to show use of double colon operator// for class constructor  import java.util.*;  class GFG {      // Class constructor    public GFG(String s)    {        System.out.println("Hello " + s);    }      // Driver code    public static void main(String[] args)    {          List<String> list = new ArrayList<String>();        list.add("Geeks");        list.add("For");        list.add("GEEKS");          // call the class constructor        // using double colon operator        list.forEach(GFG::new);    }}

输出:

Hello GeeksHello ForHello GEEKS