Hashset as HashMap add方法
public class MapTest {?public static void main(String[] args) {??Map m =new HashMap();??m.put("1", 2);??Object i? = m.get("1");??System.out.println(m.put("1", 3));??System.out.println(m.get("1"));????Set set = new HashSet();??System.out.println(set.add("5"));??System.out.println(set.add("5"));????List li = new ArrayList();??set.add("6");??System.out.println(set.add("6"));??System.out.println("----set size ---? "+set.size());? 结果：
2
3
true
false
false
----set size ---? 2
先看hashset的构造方法
private transient HashMap<E,Object> map; // Dummy value to associate with an Object in the backing Map private static final Object PRESENT = new Object(); /** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * default initial capacity (16) and load factor (0.75). */ public HashSet() {map = new HashMap<E,Object>(); } /** * Constructs a new set containing the elements in the specified * collection. The <tt>HashMap</tt> is created with default load factor * (0.75) and an initial capacity sufficient to contain the elements in * the specified collection. * * @param c the collection whose elements are to be placed into this set * @throws NullPointerException if the specified collection is null */ public HashSet(Collection<? extends E> c) {map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));addAll(c); } /** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * the specified initial capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */ public HashSet(int initialCapacity, float loadFactor) {map = new HashMap<E,Object>(initialCapacity, loadFactor); } /** * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has * the specified initial capacity and default load factor (0.75). * * @param initialCapacity the initial capacity of the hash table * @throws IllegalArgumentException if the initial capacity is less * than zero */ public HashSet(int initialCapacity) {map = new HashMap<E,Object>(initialCapacity); } /** * Constructs a new, empty linked hash set. (This package private * constructor is only used by LinkedHashSet.) The backing * HashMap instance is a LinkedHashMap with the specified initial * capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @param dummy ignored (distinguishes this * constructor from other int, float constructor.) * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */ HashSet(int initialCapacity, float loadFactor, boolean dummy) {map = new LinkedHashMap<E,Object>(initialCapacity, loadFactor); }?? 可以看出hashset其实就是hashMap
?在看看add方法
?
public boolean add(E e) {return map.put(e, PRESENT)==null; }?调用的是hashmap的add方法在看看hshmap的put方法
?? public V put(K key, V value) {??????? if (key == null)??????????? return putForNullKey(value);??????? int hash = hash(key.hashCode());//根据hash算法算出该key在table数组中的位置??????? int i = indexFor(hash, table.length);//推算出hash值在table的大概位置??????? for (Entry<K,V> e = table[i]; e != null; e = e.next) {??????????? Object k;??????????? //根据key的hashcode 判断出 在table数组中i位置中的Entry链表的key是否是一样的??????????? if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {??????????????? V oldValue = e.value;??????????????? e.value = value;//如果是一样的。就替换前面相同key的value，System.out.println(m.get("1"));所以为3??????????????? e.recordAccess(this);??????????????? return oldValue;//返回前面key的value值。所以System.out.println(m.put("1", 3));为2??????????????? /* 因为set里面的value是PRESENT 当set相同的key时，返回前面key的value，也就PRESENT ，????????????????? System.out.println(set.add("6")); PRESENT == null 为false?????????????????? public boolean add(E e) {??????????????????? private static final Object PRESENT = new Object();???????????? return map.put(e, PRESENT)==null;//PRESENT是个常量??????????????????? }??????????????? */??????????? }??????? }??????? modCount++;??????? addEntry(hash, key, value, i);??????? return null;//当第一次的返回的为空，所以当set方法第一次添加一个key时。返回的为map.put(e, PRESENT)==null为true??? }
??