关于comparable接口

参考博客: https://blog.csdn.net/nvd11/article/details/27393445

第一个例子
@Test
    public void fun1(){
        List list = new ArrayList();
        list.add(1);
        list.add(6);
        list.add(3);
        Collections.sort(list);   //这里将list里面的元素进行了排序(为什么可以排序,因为Integer实现了comparable接口且重写了compareTo方法
                                  //list里面的元素进行比较的时候会调用compareTo方法)
        for (Object object : list) {
            System.out.println(object);
        }
    }

结果: 1,3,6

Integer里面的源码:  (同样String类中实现了这个接口)
　　public int compareTo(Integer anotherInteger) {

 　　　　return compare(this.value, anotherInteger.value);
        }

       public static int compare(int x, int y) {

　　　　return (x < y) ? -1 : ((x == y) ? 0 : 1);
       }

因为实现了comparable接口,所以list里面的元素进行比较的时候就会调用compareTo方法

compareTo返回 正数 则x > y  

返回负数 则 x<y

返回0 则 x = y

public class Liyong implements Comparable<Liyong> {
    private int avg;
    private String name;

    public Liyong() {
        super();
    }

    public Liyong(int avg) {
        super();
        this.avg = avg;
    }
    

    public Liyong(String name, int avg) {
        super();
        this.avg = avg;
        this.name = name;
    }
    public Liyong(int avg, String name) {
        super();
        this.avg = avg;
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAvg() {
        return avg;
    }

    public void setAvg(int avg) {
        this.avg = avg;
    }

    @Override
    public int compareTo(Liyong o) {
        return avg - o.getAvg();
    }

    @Override
    public String toString() {
        return "Liyong [avg=" + avg + ", name=" + name + "]";
    }
}

@Test
    public void fun2(){
        List list2 = new ArrayList();
        list2.add(new Liyong("张三", 1));
        list2.add(new Liyong("李四", 6));
        list2.add(new Liyong("王五", 3));
        Collections.sort(list2);   
        for (Object object : list2) {
            System.out.println(object);
        }
    }

结果:

Liyong [avg=1, name=张三]
Liyong [avg=3, name=王五]
Liyong [avg=6, name=李四]

如果Liyong类没有实现comparable接口,则会报错 

java.lang.ClassCastException: com.关于Comparable接口.Liyong cannot be cast to java.lang.Comparable
at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
at java.util.Arrays.sort(Arrays.java:1312)
at java.util.Arrays.sort(Arrays.java:1506)
at java.util.ArrayList.sort(ArrayList.java:1454)
at java.util.Collections.sort(Collections.java:141)
at com.关于Comparable接口.TestMain.fun2(TestMain.java:29)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

 实现并重写compareTo方法,且按照自己定义的规则来比较两个对象,从而进行排序