Java集合之Set

Set也是继承自Collection，Set也是集合的一种，同时Set不允许重复的元素存在。Set的实现类都是基于Map来实现的，其中HashSet是通过HashMap来实现的，TreeSet是通过TreeMap实现的。

Set架构：

（1）Set是继承于Collection的接口，它是一个不允许重复元素的集合。

（2）AbstractSet是一个抽象类，继承了AbstractCollection，AbstractCollection实现了Set中的绝大部分函数，为Set的实现类提供了便利。

（3）HashSet和TreeSet是Set的两个实现类。HashSet依赖于HashMap，实际上是通过HashMap来实现的，HashSet中的元素是无序的。TreeSet依赖于TreeMap，实际上是通过TreeMap实现的，TreeSet中的元素是有序的。

基于Java8的AbstractCollection源码：

public abstract class AbstractCollection<E> implements Collection<E> {
    protected AbstractCollection() {//构造函数
    }
    public abstract Iterator<E> iterator();//迭代器

    public abstract int size();//集合大小

    public boolean isEmpty() {//集合是否为空
        return size() == 0;
    }
    public boolean contains(Object o) {//判断是否包含某个元素，通过迭代遍历的方式
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }
    public Object[] toArray() {//生成数组
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {//泛型方式生成数组
        // Estimate size of array; be prepared to see more or fewer elements
        int size = size();
        T[] r = a.length >= size ? a :
                (T[])java.lang.reflect.Array
                        .newInstance(a.getClass().getComponentType(), size);
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) { // fewer elements than expected
                if (a == r) {
                    r[i] = null; // null-terminate
                } else if (a.length < i) {
                    return Arrays.copyOf(r, i);
                } else {
                    System.arraycopy(r, 0, a, 0, i);
                    if (a.length > i) {
                        a[i] = null;
                    }
                }
                return a;
            }
            r[i] = (T)it.next();
        }
        // more elements than expected
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    @SuppressWarnings("unchecked")
    private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
        int i = r.length;
        while (it.hasNext()) {
            int cap = r.length;
            if (i == cap) {
                int newCap = cap + (cap >> 1) + 1;
                // overflow-conscious code
                if (newCap - MAX_ARRAY_SIZE > 0)
                    newCap = hugeCapacity(cap + 1);
                r = Arrays.copyOf(r, newCap);
            }
            r[i++] = (T)it.next();
        }
        // trim if overallocated
        return (i == r.length) ? r : Arrays.copyOf(r, i);
    }
    //对输入的minCapacity判断最大容量
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError
                    ("Required array size too large");
        return (minCapacity > MAX_ARRAY_SIZE) ?
                Integer.MAX_VALUE :
                MAX_ARRAY_SIZE;
    }
    //添加对象
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }
    //通过迭代查找，删除对象
    public boolean remove(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext()) {
                if (it.next()==null) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (o.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }
    //判断集合C中的元素是否都存在
    public boolean containsAll(Collection<?> c) {
        for (Object e : c)
            if (!contains(e))
                return false;
        return true;
    }
    //将集合c中的元素添加
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }
    //删除掉集合c中在此集合中的元素
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    //删除掉此集合中在c中不存在的对象
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            if (!c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    //清空集合
    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }
    //通过StringBuilder生成string
    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";

        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }

}

基于Java8的AbstractSet源代码：

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

    protected AbstractSet() {
    }
    public boolean equals(Object o) {//判断两个集合是否相同
        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;
        }
    }
    //计算hashCode
    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;
    }
    //删除此集合中与c中相同的对象
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;

        if (size() > c.size()) {
            for (Iterator<?> i = c.iterator(); i.hasNext(); )
                modified |= remove(i.next());
        } else {
            for (Iterator<?> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    i.remove();
                    modified = true;
                }
            }
        }
        return modified;
    }

}

基于Java8的SortSet源码：

public interface SortedSet<E> extends Set<E> {
    Comparator<? super E> comparator();
    SortedSet<E> subSet(E fromElement, E toElement);
    SortedSet<E> headSet(E toElement);
    SortedSet<E> tailSet(E fromElement);
    E first();
    E last();
    @Override
    default Spliterator<E> spliterator() {
        return new Spliterators.IteratorSpliterator<E>(
                this, Spliterator.DISTINCT | Spliterator.SORTED | Spliterator.ORDERED) {
            @Override
            public Comparator<? super E> getComparator() {
                return SortedSet.this.comparator();
            }
        };
    }
}

基于Java8的NavigableSet源码：

public interface NavigableSet<E> extends SortedSet<E> {
    E lower(E e);
    E floor(E e);
    E ceiling(E e);
    E higher(E e);
    E pollFirst();
    E pollLast();
    Iterator<E> iterator();
    NavigableSet<E> descendingSet();
    Iterator<E> descendingIterator();
    NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
                           E toElement,   boolean toInclusive);
    NavigableSet<E> headSet(E toElement, boolean inclusive);
    NavigableSet<E> tailSet(E fromElement, boolean inclusive);
    SortedSet<E> subSet(E fromElement, E toElement);
    SortedSet<E> headSet(E toElement);
    SortedSet<E> tailSet(E fromElement);
}

基于Java8的Set源码：

public interface Set<E> extends Collection<E> {
int size(); //大小
boolean isEmpty();//是否为空
boolean contains(Object o); //是否包含某个对象
Iterator<E> iterator(); //生成迭代器
Object[] toArray(); //返回Object数组
<T> T[] toArray(T[] a); //返回泛型数组
boolean add(E e); //向set中添加元素
boolean remove(Object o); //从set中删除某个元素
boolean containsAll(Collection<?> c); //某个Collection是否都包含在此lset中
boolean addAll(Collection<? extends E> c); //将某个Collection追加到此set中
boolean retainAll(Collection<?> c); //删除不存在于Collection中的set中的元素
boolean removeAll(Collection<?> c); //删除包含在此Collection中的元素
void clear(); //清空set
boolean equals(Object o);//判断两个set是否相同
int hashCode(); //计算set的hashCode
@Override
default Spliterator<E> spliterator() {
return Spliterators.spliterator(this, Spliterator.DISTINCT);
}
}