java基础类库学习（二.2）Set子接口的实现类

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本文深入解析了Set集合的基本概念，探讨了其与Collection接口的区别，详细分析了HashSet的内部实现机制，包括线程安全性、元素顺序及重复性处理，并介绍了HashSet的源码细节。

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Set子接口的实现类：HashSet/LinkedHashSet/TreeSet/EnumSet/SortedSet

Set子接口和Collection父接口源码对比？

1Collection父接口源码

public interface Collection<E> extends Iterable<E> {

int size();
boolean isEmpty();
boolean contains(Object o);
Iterator<E> iterator();
Object[] toArray();
boolean add(E e);
boolean remove(Object o);
boolean containsAll(Collection<?> c);
boolean addAll(Collection<? extends E> c);
boolean removeAll(Collection<?> c);
@since 1.8从jdk8开始接口支持有方法体，必须为static/default修饰
default boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
boolean removed = false;
final Iterator<E> each = iterator();
while (each.hasNext()) {
if (filter.test(each.next())) {
each.remove();
removed = true;
}
}
return removed;
}
boolean retainAll(Collection<?> c);
void clear();
boolean equals(Object o);
int hashCode();
}

2Set子接口源码

public interface Set<E> extends Collection<E> {
int size();
boolean isEmpty();
Iterator<E> iterator();
Object[] toArray();
boolean add(E e);
boolean remove(Object o);
boolean containsAll(Collection<?> c);
boolean addAll(Collection<? extends E> c);
boolean retainAll(Collection<?> c);

boolean removeAll(Collection<?> c);
void clear();
boolean equals(Object o);
int hashCode();

}

3结论

可以发现Set集合基本与Collection集合基本相同，只是Set集合不允许包含重复元素

Set子接口的实现类？HashSet

HashSet的说明

1HashSet实现类没有实现线程同步，不支持多线程安全，需手动实现线程安全

2不能保证元素的顺序，元素不重复

3hashSet源码

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;

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 int size() {
    return map.size();
}

public boolean isEmpty() {
    return map.isEmpty();
}

public boolean contains(Object o) {
    return map.containsKey(o);
}

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

public boolean remove(Object o) {
    return map.remove(o)==PRESENT;
}

public void clear() {
    map.clear();
}

public Object clone() {
    try {
        HashSet<E> newSet = (HashSet<E>) super.clone();
        newSet.map = (HashMap<E, Object>) map.clone();
        return newSet;
    } catch (CloneNotSupportedException e) {
        throw new InternalError(e);
    }
}

private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException {
    // Write out any hidden serialization magic
    s.defaultWriteObject();

    // Write out HashMap capacity and load factor
    s.writeInt(map.capacity());
    s.writeFloat(map.loadFactor());

    // Write out size
    s.writeInt(map.size());

    // Write out all elements in the proper order.
    for (E e : map.keySet())
        s.writeObject(e);
}

}

注意到HashSet中并没有重写Object祖先类的hashCode（）和equals（）方法，但其父类 AbstractSet<E>实现了，因此用的就是 AbstractSet<E>的这两个方法，

public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {

public boolean equals(Object o) {
    if (o == this)
        return true;

    if (!(o instanceof Set))
        return false;
    Collection<?> c = (Collection<?>) o;
    if (c.size() != size())
        return false;
    try {
        return containsAll(c);
    } catch (ClassCastException unused)   {
        return false;
    } catch (NullPointerException unused) {
        return false;
    }
}

public int hashCode() {
    int h = 0;
    Iterator<E> i = iterator();
    while (i.hasNext()) {
        E obj = i.next();
        if (obj != null)
            h += obj.hashCode();
    }
    return h;
}

}

HashSet集合在执行add（）操作时，判断两个元素是否相同的依据是euqals（）和hashCode()方法，如果euqals（）为true并且两个对象的hashCode（）也相等，那么这两个元素就是相等的，不会纳入HashSet集合中