/*看一下部分的HashSet源码....public class HashSet
     
          extends AbstractSet
      
           implements Set
       
        , Cloneable, java.io.Serializable{    static final long serialVersionUID = -5024744406713321676L;    private transient HashMap
        
          map;    private static final Object PRESENT = new Object();    public HashSet() {        map = new HashMap<>();    }    public HashSet(Collection
          c) {        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));        addAll(c);    }    public HashSet(int initialCapacity, float loadFactor) {        map = new HashMap<>(initialCapacity, loadFactor);    }    public HashSet(int initialCapacity) {        map = new HashMap<>(initialCapacity);    }    HashSet(int initialCapacity, float loadFactor, boolean dummy) {        map = new LinkedHashMap<>(initialCapacity, loadFactor);    }    public Iterator
         
           iterator() {        return map.keySet().iterator();    }    public boolean add(E e) {        return map.put(e, PRESENT)==null;//在下面的代码中我们可以看见map.put()的代码    }}public class HashMap
          
            extends AbstractMap
           
             implements Map
            
             , Cloneable, Serializable{ ..... final int hash(Object k) { int h = 0; if (useAltHashing) { if (k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h = hashSeed; } h ^= k.hashCode();//调用了改对象中的hashCode（）方法，由 Object 类定义的 hashCode 方法确实会针对不同的对象返回不同的整数 h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); } public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key);//调用了上面的函数 int i = indexFor(hash, table.length); for (Entry
             
               e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); return null; }}也就是说HashSet内部实现使用HashMap这个类来完成的TreeSet的内部实现元素之间是否相等？从上面的比较方法中可以看出，只有两个对象的hash值相等并且对象的内容也想等，那么两个对象才相等并且判断的方法用的是 equals 方法注意：当equals()此方法被重写时，通常有必要重写 hashCode 方法，以维护 hashCode 方法的常规协定，该协定声明相等对象必须具有相等的哈希码。*/import java.util.*;public class CompTest{ public static void main(String args[]){ Set
              
                st = new HashSet
               
                (); st.add(new myClass(1, "fd")); st.add(new myClass(2, "fff")); st.add(new myClass(2, "tttt")); st.add(new myClass(1, "fd")); for(Iterator
                
                  it = st.iterator(); it.hasNext();) System.out.println(it.next()); }}class myClass{ public int x; public String name; public myClass(int x, String name){ this.x=x; this.name=name; } public int hashCode(){ return x; } public boolean equals(Object tmp){ //这里是方法的重写，参数的类型和个数一定要一样.... return x==((myClass)tmp).x && name.equals( ((myClass)tmp).name); } public String toString(){ return x+" "+name; }}