java8中的lambda表达式简介

(parameters) -> expression
或
(parameters) -> { statements; }

// 1. 不需要参数,返回值为5
() -> 5

// 2. 接收一个参数(数字类型),返回其2倍的值
x -> 2 * x

// 3. 接受2个参数(数字),并返回他们的差值
(x, y) -> x – y

// 4. 接收2个int型整数,返回他们的和
(int x, int y) -> x + y

// 5. 接受一个 string 对象,并在控制台打印,不返回任何值(看起来像是返回void)
(String s) -> System.out.print(s)

Function<String, Integer> function1 = Integer::parseInt; // 等价于下面
Function<String, Integer> function2 = (String i) -> Integer.parseInt(i);

Function<String, String> function3 = String::toLowerCase; // 等价于下面
Function<String, String> function4 = (String i) -> i.toLowerCase();

BiFunction<String, Integer, String> biFunction = (String s, Integer i) -> s.substring(i);
BiFunction<String, Integer, String> biFunction2 = String::substring;

String str = "hello";
Supplier<Integer> supplier = () -> str.length();
Supplier<Integer> supplier2 = str::length;

Function<Integer, String> function5 = (Integer i) -> str.substring(i);
Function<Integer, String> function6 = str::substring;

package com.morris.java8.lamdba;

import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Supplier;

public class MethodReferenceExample {

    public static void main(String[] args) {

        // 构造函数引用
        Supplier<String> stringSupplier = () -> new String();
        Supplier<String> stringSupplier2 = String::new;

        Function<String, String> stringFunction = (String s)->new String(s);
        Function<String, String> stringFunction2 = String::new;

        BiFunction<Runnable, String, Thread> stringBiFunction = (Runnable r, String b)-> new Thread(r, b);
        BiFunction<Runnable, String, Thread> stringBiFunction2 = Thread::new;

        ThreeFunction<ThreadGroup, Runnable, String, Thread> threeFunction = (ThreadGroup g, Runnable r, String b)-> new Thread(g, r, b);
        ThreeFunction<ThreadGroup, Runnable, String, Thread> threeFunction2 = Thread::new;

    }

    interface ThreeFunction<A, B, C, D> {
        D triple(A a, B b, C c);
    }

}

public class AnonymousClassDemo {

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("this is an Anonymous class demo");
            }
        });
    }
}

AnonymousClassDemo.class
AnonymousClassDemo$1.class

$ javap -v -p AnonymousClassDemo.class
...
  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=1, args_size=1
         0: new           #2                  // class java/lang/Thread
         3: dup
         4: new           #3                  // class AnonymousClassDemo$1 创建匿名内部类
         7: dup
         8: invokespecial #4                  // Method AnonymousClassDemo$1."<init>":()V
        11: invokespecial #5                  // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
        14: pop
        15: return
      LineNumberTable:
        line 5: 0
        line 11: 15
}
SourceFile: "AnonymousClassDemo.java"
InnerClasses:
     static #3; //class AnonymousClassDemo$1

public class LambdaDemo {
    public static void main(String[] args) {
        new Thread(()-> System.out.println("this is a lambda demo"));
    }
}

LambdaDemo.class

$ javap -v -p LambdaDemo.class
...
  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=1, args_size=1
         0: new           #2                  // class java/lang/Thread
         3: dup
         4: invokedynamic #3,  0              // InvokeDynamic #0:run:()Ljava/lang/Runnable; 使用invokedynamic指令调用
         9: invokespecial #4                  // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
        12: pop
        13: return
      LineNumberTable:
        line 4: 0
        line 5: 13

  private static void lambda$main$0(); // Lambda表达式被封装成主类的私有方法
    descriptor: ()V
    flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
    Code:
      stack=2, locals=0, args_size=0
         0: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #6                  // String this is a lambda demo
         5: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 4: 0
}
SourceFile: "LambdaDemo.java"
InnerClasses:
     public static final #51= #50 of #54; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
  0: #22 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #23 ()V
      #24 invokestatic LambdaDemo.lambda$main$0:()V
      #23 ()V