TreeSet——实现Comparable接口并重写CompareTo()方法

Posted 2020-12-29 lyxcode

TreeSet是以自然顺序存的数据，例如

Set<Student> students=new TreeSet();
        students.add(new Student("111"));
        students.add(new Student("333"));
        students.add(new Student("222"));
        
        for (Student student : students) {
            System.out.println(student.getId());
        }

输出结果为111  222  333

而且这时候的Student必须继承Comparable接口，重写抽象方法CompareTo方法

public class Student implements Comparable<Student> {

    private String id;

    
    public Student(String id) {
        this.id = id;
    }




    @Override
    public int compareTo(Student o) {
        return 1;
    }

}

出现这样的效果是因为存储的时候的代码是这样的

public V put(K key, V value) {
        Entry<K,V> t = root;
        if (t == null) {
            compare(key, key); // type (and possibly null) check

            root = new Entry<>(key, value, null);
            size = 1;
            modCount++;
            return null;
        }
        int cmp;
        Entry<K,V> parent;
        // split comparator and comparable paths
        Comparator<? super K> cpr = comparator;
        if (cpr != null) {
            do {
                parent = t;
                cmp = cpr.compare(key, t.key);
                if (cmp < 0)
                    t = t.left;
                else if (cmp > 0)
                    t = t.right;
                else
                    return t.setValue(value);
            } while (t != null);
        }
        else {
            if (key == null)
                throw new NullPointerException();
            @SuppressWarnings("unchecked")
                Comparable<? super K> k = (Comparable<? super K>) key;
            do {
                parent = t;
                cmp = k.compareTo(t.key);
                if (cmp < 0)
                    t = t.left;
                else if (cmp > 0)
                    t = t.right;
                else
                    return t.setValue(value);
            } while (t != null);
        }
        Entry<K,V> e = new Entry<>(key, value, parent);
        if (cmp < 0)
            parent.left = e;
        else
            parent.right = e;
        fixAfterInsertion(e);
        size++;
        modCount++;
        return null;
    }

看红色的代码，存储的时候执行compareTo方法，这个时候就会判断你存的值得大小顺序，然后判断你该存储的顺序，就是自然顺序了。。