TreeSet——實現Comparable接口並重寫CompareTo()方法

TreeSet是以自然順序存的數據，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());
        }

輸出結果為111  222  333

而且這時候的Student必須繼承Comparable接口，重寫抽象方法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;
    }

}

出現這樣的效果是因為存儲的時候的代碼是這樣的

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;
    }

看紅色的代碼，存儲的時候執行compareTo方法，這個時候就會判斷你存的值得大小順序，然后判斷你該存儲的順序，就是自然順序了。。