一.示例代码

package lambda.examples._02;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
public class App { 
    List<Apple> getAppleList() { 
        List<Apple> list = new ArrayList<Apple>();
        list.add(new Apple("green", 100));
        list.add(new Apple("red", 150));
        list.add(new Apple("green", 50));
        return list;
    }
    public void test() { 
        List<Apple> list = getAppleList();
        list.sort(Comparator.comparing(Apple::getWeight));
        list.forEach(System.out::println);
    }
    public static void main(String[] args) { 
        new App().test();
    }
}
class Apple { 
    private String color;
    private int weight;
    public Apple(String color, int weight) { 
        super();
        this.color = color;
        this.weight = weight;
    }
    @Override
    public String toString() { 
        return "Apple [color=" + color + ", weight=" + weight + "]";
    }
    public String getColor() { 
        return color;
    }
    public void setColor(String color) { 
        this.color = color;
    }
    public int getWeight() { 
        return weight;
    }
    public void setWeight(int weight) { 
        this.weight = weight;
    }
}

二.代码分析

输出如下:

Apple [color=green, weight=50]
Apple [color=green, weight=100]
Apple [color=red, weight=150]

在test方法中我们使用了java8中的新特性lambda表达式,
list.sort()需要传入一个Comparator接口的实现类,在java8之前我们通常使用匿名内部类实现,类似这样

list.sort(new Comparator<Apple>() { 
            @Override
            public int compare(Apple a, Apple b) { 
                return a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0);
            }
        });

可以看出来其实有用的就一行代码,还要写那么多,所以对于这种只有一个抽象方法的接口我们称之为函数式接口,我们直接用lambda表达式实现,代码如下:

list.sort((a, b) -> a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0));

分析下什么意思,sort可以接受Comparator类型的接口实现类,Comparator是一个函数式接口,他的抽象方法是

@Override
            public int compare(Apple a, Apple b) { 
                return a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0);
            }

我们简写为

(a, b) -> a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0)

即可
->意为goto,前面的是方法的参数,后面是结果,如果只有一行就不写return,否则需要用{}括起来,并且return返回值,例如:

list.sort((a, b) -> { 
            if (a.getWeight() > b.getWeight())
                return 1;
            if (a.getWeight() < b.getWeight())
                return -1;
            return 0;
        });

这里ab我们并不需要写类型,编译器会根据list的类型做推断

三.lambda如何实现

代码如下:

package lambda.examples._02;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
public class App { 
    List<Apple> getAppleList() { 
        List<Apple> list = new ArrayList<Apple>();
        list.add(new Apple("green", 100));
        list.add(new Apple("red", 150));
        list.add(new Apple("green", 50));
        return list;
    }
    public void test() { 
        List<Apple> list = getAppleList();
        list.sort(new Comparator<Apple>() { 
            @Override
            public int compare(Apple a, Apple b) { 
                return a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0);
            }
        });
        list.sort((a, b) -> a.getWeight() > b.getWeight() ? 1 : (a.getWeight() < b.getWeight() ? -1 : 0));
        list.forEach(System.out::println);
    }
}
class Apple { 
    private String color;
    private int weight;
    public Apple(String color, int weight) { 
        super();
        this.color = color;
        this.weight = weight;
    }
    @Override
    public String toString() { 
        return "Apple [color=" + color + ", weight=" + weight + "]";
    }
    public String getColor() { 
        return color;
    }
    public void setColor(String color) { 
        this.color = color;
    }
    public int getWeight() { 
        return weight;
    }
    public void setWeight(int weight) { 
        this.weight = weight;
    }
}
package lambda.examples._02;
import java.io.IOException;
public class OtherApp { 
    public static void main(String[] args) throws IOException { 
        new App();
        System.in.read();
    }
}

现在用OtherApp的main方法初始化一个App类型的对象(这是为了加载App类),然后阻塞住查看字节码

java -classpath "%JAVA_HOME%/lib/sa-jdi.jar" sun.jvm.hotspot.HSDB

jps查看pid
在这里插入图片描述
使用hsdb attch 4744
打开Tools -> Class Brower
搜索App

查看class,发现其中有一个方法叫

private static [synthetic] int lambda$0(lambda.examples._02.Apple, lambda.examples._02.Apple)@0x000000001da80b80;

synthetic表示这是编译器生成的方法

所以说lambda语句会在所在的类被加载时,被jvm解析为该类中的一个private static 方法,接收lambda语句中的参数并且返回 lambda语句 -> 后面的结果(如果有返回值)

Create .class File

执行

javap -c -p App.class

结果如下:

Compiled from "App.java"
public class lambda.examples._02.App {
  public lambda.examples._02.App();
    Code:
       0: aload_0
       1: invokespecial #8                  // Method java/lang/Object."<init>":()V
       4: return
            .
            .
            .
            略
            .
            .
            .
  private static int lambda$0(lambda.examples._02.Apple, lambda.examples._02.Apple);
    Code:
       0: aload_0
       1: invokevirtual #72                 // Method lambda/examples/_02/Apple.getWeight:()I
       4: aload_1
       5: invokevirtual #72                 // Method lambda/examples/_02/Apple.getWeight:()I
       8: if_icmple     15
      11: iconst_1
      12: goto          31
      15: aload_0
      16: invokevirtual #72                 // Method lambda/examples/_02/Apple.getWeight:()I
      19: aload_1
      20: invokevirtual #72                 // Method lambda/examples/_02/Apple.getWeight:()I
      23: if_icmpge     30
      26: iconst_m1
      27: goto          31
      30: iconst_0
      31: ireturn
}