/*
看一下部分的HashSet源碼....
public class HashSet<E>extends AbstractSet<E>implements Set<E>, Cloneable, java.io.Serializable
{static final long serialVersionUID = -5024744406713321676L;private transient HashMap<E,Object> map;private static final Object PRESENT = new Object();public HashSet() {map = new HashMap<>();}public HashSet(Collection<? extends E> 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<E> iterator() {return map.keySet().iterator();}public boolean add(E e) {return map.put(e, PRESENT)==null;//在下面的代碼中我們可以看見map.put()的代碼}
}public class HashMap<K,V>extends AbstractMap<K,V>implements Map<K,V>, 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();//調用了改對象中的hashCode（）方法，由 Object 類定義的 hashCode 方法確實會針對不同的對象返回不同的整數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);//調用了上面的函數int i = indexFor(hash, table.length);for (Entry<K,V> 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;}
}也就是說HashSet內部實現使用HashMap這個類來完成的
TreeSet的內部實現元素之間是否相等？
從上面的比較方法中可以看出，只有兩個對象的hash值相等并且對象的內容也想等，那么兩個對象才相等
并且判斷的方法用的是 equals 方法
注意：當equals()此方法被重寫時，通常有必要重寫 hashCode 方法，以維護 hashCode 方法的常規協定，該協定聲明相等對象必須具有相等的哈希碼。
*/import java.util.*;
public class CompTest{public static void main(String args[]){Set<myClass> st = new HashSet<myClass>();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<myClass> 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){//這里是方法的重寫，參數的類型和個數一定要一樣....return x==((myClass)tmp).x && name.equals( ((myClass)tmp).name);}public String toString(){return x+" "+name;}
}
)
