Effective Java 【考虑实现Comparable接口】

Effective Java --Comparable接口

compareTo方法是Comparable接口的唯一方法。类实现了Comparable接口，表明它的实例具有内在的排序关系。

自己实现compareTo方法应该注意的点：

自反性： x.compareTo(x) == 0

可传递性： x.compareTo(y)>0 && y.compareTo(z)>0 ==> x.compareTo(z) > 0

可逆性： x.compareTo(y) == -y.compareTo(x)

建议： (x.compareTo(y) == 0) == x.equals(y) 

例子：

public class ComparableImpl  implements Comparable<ComparableImpl>{

    public static void main(String[] args) {

        ComparableImpl A = new ComparableImpl("A",9);
        ComparableImpl B = new ComparableImpl("B",5);
        ComparableImpl C = new ComparableImpl("B",3);

        //TreeSet 去重 排序
        TreeSet set = new TreeSet();
        set.add(A);
        set.add(B);
        set.add(C);

        while(!set .isEmpty()){
            System.out.println(set.pollFirst());
        }
    }

    private String name;
    private Integer age;

    public int getAge() {
        return age;
    }
    public String getName() {
        return name;
    }

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

    @Override
    public int compareTo(ComparableImpl o) {
        return this.name.compareTo(o.getName()) == 0 ? this.age.compareTo(o.getAge()) : this.name.compareTo(o.getName());
    }

    @Override
    public String toString() {
        return "ComparableImpl{" +"name='" + name + '\'' +", age=" + age +'}';
    }
}

运行结果：

ComparableImpl{name='A', age=9}
ComparableImpl{name='B', age=3}
ComparableImpl{name='B', age=5}