package java.util;
public class HashSet<E>extends AbstractSet<E>implements Set<E>, Cloneable, java.io.Serializable
{static final long serialVersionUID = -5024744406713321676L;//存储元素的HashMapprivate transient HashMap<E,Object> map;// 一个冗余的空对象，用于在Map中存放key对应的value，Map中所有的键值对的value都是同一个空Object的引用private static final Object PRESENT = new Object();/*** 构造方法，直接生成一个对应的HashMap*/public HashSet() {map = new HashMap<>();}//省略一部分代码.....//返回HashMap的key迭代器，HashSet中的元素实际上就是HashMap中的key元素public Iterator<E> iterator() {return map.keySet().iterator();}//调用HashMap的put方法，将e-PRESENT键值对对象put到map中public boolean add(E e) {return map.put(e, PRESENT)==null;}//省略一部分代码.....
}

package java.util;
public class LinkedHashSet<E>extends HashSet<E>implements Set<E>, Cloneable, java.io.Serializable {private static final long serialVersionUID = -2851667679971038690L;public LinkedHashSet(int initialCapacity, float loadFactor) {super(initialCapacity, loadFactor, true);}public LinkedHashSet(int initialCapacity) {super(initialCapacity, .75f, true);}public LinkedHashSet() {super(16, .75f, true);}public LinkedHashSet(Collection<? extends E> c) {super(Math.max(2*c.size(), 11), .75f, true);addAll(c);}
}

    HashSet(int initialCapacity, float loadFactor, boolean dummy) {map = new LinkedHashMap<>(initialCapacity, loadFactor);}

public class TreeSet<E> extends AbstractSet<E>implements NavigableSet<E>, Cloneable, java.io.Serializable
{/*** TreeMap对象*/private transient NavigableMap<E,Object> m;// 空对象，所有Map键值对中value对象的同一个引用private static final Object PRESENT = new Object();

    TreeSet(NavigableMap<E,Object> m) {this.m = m;}
public TreeSet() {this(new TreeMap<E,Object>());}

 //省略部分代码。。。public NavigableSet<E> descendingSet() {return new TreeSet<>(m.descendingMap());}public int size() {return m.size();}
public boolean isEmpty() {return m.isEmpty();}
//省略部分代码。。。