集合框架-TreeSet的add()方法的源码解析

interface Collection {...}interface Set extends Collection {...}interface NavigableMap {}class TreeMap implements NavigableMap {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();            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;    }}class TreeSet implements Set {private transient NavigableMap<E,Object> m;public TreeSet() { this(new TreeMap<E,Object>());}public boolean add(E e) {        return m.put(e, PRESENT)==null;    }}/*真正的比较是依赖于元素的compareTo()方法，而这个方法是定义在 Comparable里面的。所以，你要想重写该方法，就必须实现Comparable接口。这个接口表示的就是自然排序。*/