Comparable 和 Comparator

Comparable和 Comparator是两个接口，从字面上看这两个接口都是用来做比较用的，但是jdk里面不可能定义两个功能相同的接口，所以他们肯定有不同的用处。

一、Comparable

1.1说明

Comparable可以认为是一个内比较器，实现了Comparable接口的类有一个特点，就是这些 类是可以和自己比较的，至于具体和另一个实现了Comparable接口的类如何比较，则依赖compareTo方法的实现。

public class Boy implements Comparable<Boy>{
    private String name;
    private int age;

    public Boy(int age, String name){
        this.age = age;
        this.name = name;
    }

    /**
      Compares this object with the specified object for order.  Returns a
     * negative integer, zero, or a positive integer as this object is less
     * than, equal to, or greater than the specified object.
    */
    @Override
    public int compareTo(Boy o) {
        return this.age - o.age;
    }

    @NonNull
    @Override
    public String toString() {
        return "Age: " + age + ", Name: " + name;
    }
}

测试：

    Boy boy = new Boy(12, "Jone");
    Boy boy1 = new Boy(22, "Jonson");
    Boy boy2 = new Boy(18, "Jack");

    List boys = new ArrayList();
    boys.add(boy);
    boys.add(boy1);
    boys.add(boy2);

    for (int i = 0; i < boys.size(); i++) {
        System.out.println(" " + boys.get(i));
    }

    System.out.println("=========================");

    //排序
    Collections.sort(boys);
    //bubbleSort(boys);

    for (int i = 0; i < boys.size(); i++) {
        System.out.println(" " + boys.get(i));
    }

结果：

 Age: 12, Name: Jone
 Age: 22, Name: Jonson
 Age: 18, Name: Jack
=========================
 Age: 12, Name: Jone
 Age: 18, Name: Jack
 Age: 22, Name: Jonson

也可以自己手写排序算法，比如冒泡：

private <T extends Comparable<? super  T>> void bubbleSort(List<T> list){
        for (int i = list.size() - 1; i > 0; i--) {

            boolean flag = true;
            for (int j = 0; j < i; j++) {

                T obj = list.get(j);
                T obj2 = list.get(j + 1);

                if (obj.compareTo(obj2) > 0){
                    T tmp = obj;
                    list.set(j, obj2);
                    list.set(j + 1, tmp);
                    flag = false;
                }
            }

            if (flag){
                break;
            }
        }
    }

二、Comparator

2.1说明

Comparator可以认为是是一个外比较器，有两种情况可以使用实现Comparator接口的方式：
1、 一个对象不支持自己和自己比较（没有实现Comparable接口），但是又想对两个对象进行比较。
2、 一个对象实现了Comparable接口，但是开发者认为compareTo方法中的比较方式并不是自己想要的那种比较方式。
Comparator接口里面有一个compare方法，方法有两个参数T o1和T o2，是泛型的表示方式，分别表示待比较的两个对象

public class Girl {
    private String name;
    private int age;

    public Girl(int age, String name){
        this.age = age;
        this.name = name;
    }

    @NonNull
    @Override
    public String toString() {
        return "Age: " + age + ", Name: " + name;
    }

    public int getAge(){
        return age;
    }
}

//创建比较器
public class GrilComparator implements Comparator<Girl> {
    @Override
    public int compare(Girl o1, Girl o2) {
        return o1.getAge() - o2.getAge();
    }
}

测试：

    Girl girl = new Girl(2, "Lucy");
    Girl girl1 = new Girl(22, "LiLi");
    Girl girl2 = new Girl(8, "Anne");

    List girls = new ArrayList();
    girls.add(girl);
    girls.add(girl1);
    girls.add(girl2);

    for (int i = 0; i < girls.size(); i++) {
        System.out.println(" " + girls.get(i));
    }

    System.out.println("=========================");

    GrilComparator comparator = new GrilComparator();
    Collections.sort(girls, comparator);
    //insertSort(girls, comparator);
    
    for (int i = 0; i < girls.size(); i++) {
        System.out.println(" " + girls.get(i));
    }

结果：

 Age: 2, Name: Lucy
 Age: 22, Name: LiLi
 Age: 8, Name: Anne
=========================
 Age: 2, Name: Lucy
 Age: 8, Name: Anne
 Age: 22, Name: LiLi

也可以自己手写排序算法，比如选择排序：

private <T> void insertSort(List<T> list, Comparator<? super T> comparator) {
    for (int i = 1; i < list.size(); i++) {

        T target = list.get(i);

        for (int j = i; j > 0; j--) {
            T obj = list.get(j);
            T obj2 = list.get(j - 1);
            int res = comparator.compare(obj, obj2);
            if(res < 0){
                list.set(j, obj2);
            }else{
                list.set(j, target);
                break;
            }
        }
    }
}

参考：https://blog.csdn.net/wlh2015/java/article/details/83959462