TreeMap——实现comparable接口并重写CompareTo方法

public class TreeMapTest {

    public static void main(String[] args) {
        Map<Student,Integer> students = new TreeMap<>();
        students.put(new Student("11"),1);
        students.put(new Student("11"),1);
        System.out.println(students.size());
    }
}

输出结果为2

因为

 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;
    }

上面红色字体：调用compareTo方法，看结果来看新存入的值放在左侧（cmp<0），还是右侧(cmp>0)，还是现在的value值把原来的value值覆盖(cmp=0)

需要在Student类中重写compareTo方法

    @Override
    public int compareTo(Student o) {
        return 0;
    }

按照你自己的要求重写compareTo方法就行了~