Java自定义排序

参考：https://www.cnblogs.com/huangjinyong/p/9037588.html

java集合的工具类Collections中提供了两种排序的方法,分别是:

1. Collections.sort(List list)
2. Collections.sort(List list,Comparator c)

第一种称为自然排序,参与排序的对象需实现comparable接口,重写其compareTo()方法,方法体中实现对象的比较大小规则,示例如下: 
实体类:(基本属性,getter/setter方法,有参无参构造方法,toString方法)

package test;

public class Emp implements Comparable {

    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Emp() {
        super();
    }
    public Emp(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Emp [name=" + name + ", age=" + age + "]";
    }
    @Override
    public int compareTo(Object o) {
        if(o instanceof Emp){//判断是否可以转换，不能则抛出异常
            Emp emp = (Emp) o;
//          return this.age-emp.getAge();//按照年龄升序排序
            return this.name.compareTo(emp.getName());//换姓名升序排序
        }
        throw new ClassCastException("不能转换为Emp类型的对象...");
    }

}

显然这种方法限制较多，一般用第二种匿名内部类

 Collections.sort(ans,new Comparator(){
            @Override
            public int compare(Object a1, Object b1){
                if(a1 instanceof ArrayList && b1 instanceof ArrayList) {
                    ArrayList<TreeNode> a = (ArrayList<TreeNode>) a1;
                    ArrayList<TreeNode> b = (ArrayList<TreeNode>) b1;

                    if (a.size() > b.size()) return -1;
                    else if (a.size() == b.size()) return 0;
                    else return 1;
                }
                throw new ClassCastException("不能转换为ArrayList类型");
            }
        });