Java's Comparable interface

　　Comparable interface functions provide a comparison object size, the object implements compareTo method to compare the size of this interface class provides the interface through.

　　This method returns an int type, three cases.

• Returns a positive number, the current object is greater than the target object
• Returns negative, the current object is less than the target object
• Returns 0, the current object is equal to the target object

　　TreeSet object provides a sorting function (by default ascending order), if the storage container is a custom class that will implement these features for Comparable TreeSet object-size comparison, otherwise an error.

　　

public class TestCompareTo implements Comparable<TestCompareTo> {
    private Integer a = null;

    public TestCompareTo(Integer a) {
        this.a = a;
    }

    public Integer getA() {
        return a;
    }

    @Override
    public int compareTo(TestCompareTo o) {
        return this.a > o.a ? -1 : (this.a == o.a ? 0 : 1);
    }

}

public class Test{
    public static void main(String[] args) {
        //test TreeSet
        Set<TestCompareTo> set = new TreeSet<>();
        set.add(new TestCompareTo(-10));
        set.add(new TestCompareTo(-20));
        set.add(new TestCompareTo(-30));
        set.add(new TestCompareTo(0));
        set.add(new TestCompareTo(10));
        set.add(new TestCompareTo(-100));
        Iterator iterator = set.iterator();
        while(iterator.hasNext()){
            TestCompareTo tmp = (TestCompareTo) iterator.next();
            System.out.println(tmp.getA());
        }
    }
}