jdk源码_Collection

技术
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Collection

集合结构中的根接口

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

forEach

继承Iterable接口，故可调用默认方法forEach

AbstractCollection

提供了 Collection 接口的基本实现

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public abstract class AbstractCollection<E> implements Collection<E> {
	
}

iterator()

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/**
 * Returns an iterator over the elements contained in this collection.
 *
 * @return an iterator over the elements contained in this collection
 */
public abstract Iterator<E> iterator();

AbstractList.iterator

public abstract class AbstractList extends AbstractCollection implements List 

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public Iterator<E> iterator() {
    return new Itr();
}

Itr

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private class Itr implements Iterator<E> {
        /**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;

        /**
         * Index of element returned by most recent call to next or
         * previous.  Reset to -1 if this element is deleted by a call
         * to remove.
         */
        int lastRet = -1;

        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size();
        }

        public E next() {
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

size()

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public abstract int size();

public abstract class AbstractList extends AbstractCollection implements List
public class ArrayList extends AbstractList implements List, RandomAccess, Cloneable, java.io.Serializable

ArrayList.size()

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/**
 * The size of the ArrayList (the number of elements it contains).
 *
 * @serial
 */
private int size;
	
/**
 * Returns the number of elements in this list.
 *
 * @return the number of elements in this list
 */
public int size() {
    return size;
}

boolean isEmpty()

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public boolean isEmpty() {
    return size() == 0;
}

boolean contains(Object o)

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public boolean contains(Object o) {
    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;
}

集合可以存入空对象

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public static void main(String[] args) {
    Collection collection = new ArrayList();
    collection.add(null);
    System.out.println(collection.size()==0);//false
    System.out.println(collection.contains(null));//true
}

toArray()

Object[] toArray()

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public Object[] toArray() {
    // 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;
}

先根据Size()方法预建立一个与集合长度相等的数组，然后进行判断，若迭代过程中发现对象没有预期的多，则直接截取当前的数组返回

T[] toArray(T[] a)

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public <T> T[] toArray(T[] a) {
    // 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;
}

若参数数组a的长度大于集合的长度，则直接用a数组赋值，若小于则利用反射建立一个长度等于集合长度的数组

boolean remove(Object o)

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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;
}

Object.equals

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public boolean equals(Object obj) {
    return (this == obj);
}

boolean containsAll(Collection<?> c)

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public boolean containsAll(Collection<?> c) {
    for (Object e : c)
        if (!contains(e))
            return false;
    return true;
}

boolean addAll(Collection<? extends E> c)

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public boolean addAll(Collection<? extends E> c) {
    boolean modified = false;
    for (E e : c)
        if (add(e))
            modified = true;
    return modified;
}

boolean removeAll(Collection<?> c)

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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;
}

Objects.requireNonNull(c)

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public static <T> T requireNonNull(T obj) {
    if (obj == null)
        throw new NullPointerException();
    return obj;
}

default boolean removeIf(Predicate<? super E> filter)

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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;
}

传入一个断言型功能函数，判断，若符合断言要求，remove

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Collection<String> collection = new ArrayList<String>(Arrays.asList("first","second","third","--------"));
collection.removeIf(s->s.equals("first"));

boolean retainAll(Collection<?> c)

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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;
}

取交集 遍历集合，若元素存在于参数集合C，则移除，并将modified置为true返回

boolean equals(Object o)

AbstractList

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public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof List))
        return false;

    ListIterator<E> e1 = listIterator();
    ListIterator<?> e2 = ((List<?>) o).listIterator();
    while (e1.hasNext() && e2.hasNext()) {
        E o1 = e1.next();
        Object o2 = e2.next();
        if (!(o1 == null ? o2 == null : o1.equals(o2)))
            return false;
    }
    return !(e1.hasNext() || e2.hasNext());
}

先比较参数是否和当前List指向同一对象，再比较参数是否为List实例，然后将两个List中的元素一一对比（若List对象元素不为null，则用Object.equals方法对比，若为null则判断参数List对象中元素是否为null），最后判断长度是否相等

hasNext()

ArrayList

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public boolean hasNext() {
    return cursor != size;
}

next()

ArrayList

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public E next() {
    checkForComodification();
    int i = cursor;
    if (i >= size)
        throw new NoSuchElementException();
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
        throw new ConcurrentModificationException();
    cursor = i + 1;
    return (E) elementData[lastRet = i];
}

instanceof

instanceof是Java中的二元运算符，左边是对象，右边是类；当对象是右边类或子类所创建对象时，返回true；否则，返回false。

  • 类的实例包含本身的实例，以及所有直接或间接子类的实例
  • instanceof左边显式声明的类型与右边操作元必须是同种类或存在继承关系，也就是说需要位于同一个继承树，否则会编译错误
  • 左边的对象实例不能是基础数据类型
  • null用instanceof跟任何类型比较时都是false

int hashCode()

重写Object.equals方法时必须同样重写Object.hashCode方法

  • 如果两个对象equals相等，那么这两个对象的HashCode一定也相同
  • 如果两个对象的HashCode相同，不代表两个对象就相同，只能说明这两个对象在散列存储结构中，存放于同一个位置
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/**
 * Returns the hash code value for this collection.  While the
 * <tt>Collection</tt> interface adds no stipulations to the general
 * contract for the <tt>Object.hashCode</tt> method, programmers should
 * take note that any class that overrides the <tt>Object.equals</tt>
 * method must also override the <tt>Object.hashCode</tt> method in order
 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
 * In particular, <tt>c1.equals(c2)</tt> implies that
 * <tt>c1.hashCode()==c2.hashCode()</tt>.
 *
 * @return the hash code value for this collection
 * @see Object#hashCode()
 * @see Object#equals(Object)
 */
int hashCode();

default Stream stream()

返回一个不影响数据源的数据序列

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/**
 * Returns a sequential {@code Stream} with this collection as its source.
 *
 * <p>This method should be overridden when the {@link #spliterator()}
 * method cannot return a spliterator that is {@code IMMUTABLE},
 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 * for details.)
 *
 * @return a sequential {@code Stream} over the elements in this collection
 * @implSpec The default implementation creates a sequential {@code Stream} from the
 * collection's {@code Spliterator}.
 * @since 1.8
 */
default Stream<E> stream() {
    return StreamSupport.stream(spliterator(), false);
}
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