Collection学习（一）

一、概述

Collection位于java.util包下。此接口继承Iterable接口。

package java.lang;

import java.util.Iterator;

/**
*
* @since 1.5
*/
public interface Iterable<T> {
/**
* Returns an iterator over a set of elements of type T.
*
* @return an Iterator.
*/
Iterator<T> iterator();
}

public interface Collection<E> extends Iterable<E> {
// Query Operations

int size();

boolean isEmpty();

boolean contains(Object o);

Iterator<E> iterator();

Object[] toArray();

<T> T[] toArray(T[] a);

boolean add(E e);

boolean remove(Object o);

boolean containsAll(Collection<?> c);

boolean addAll(Collection<? extends E> c);
boolean removeAll(Collection<?> c);

boolean retainAll(Collection<?> c);

void clear();

boolean equals(Object o);

int hashCode();
}

Collection下面的所有实现：



下面主要说明List和Set
List图解



ArrayList的源码分析：
1.底层使用数组
/**

/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;  

* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
*/
private transient Object[] elementData;

2.构造方法

public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}

public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}

public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

3.存储方式
ArrayList提供了set(int index, E element)、add(E e)、add(int index, E element)、addAll(Collection<? extends E> c)、addAll(int index,Collection<extends E> c)

set方法：用指定的元素替代此列表中指定位置上的元素，并返回以前位于该位置上的元素。
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* Checks if the given index is in range.  If not, throws an appropriate
* runtime exception.  This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

add方法：
//将指定的元素添加到列表的尾部
public boolean add(E e) {
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}
// 将指定的元素插入此列表中的指定位置。
// 如果当前位置有元素，则向右移动当前位于该位置的元素以及所有后续元素（将其索引加1）。
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1);  // Increments modCount!!
// 将 elementData中从Index位置开始、长度为size-index的元素，
// 拷贝到从下标为index+1位置开始的新的elementData数组中。
// 即将当前位于该位置的元素以及所有后续元素右移一个位置。
System.arraycopy(elementData, index, elementData, index + 1, size - index);
elementData[index] = element;
size++;
}

/**
* A version of rangeCheck used by add and addAll.
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

addAll方法：
//按照指定collection的迭代器所返回的元素顺序，将该collection中的所有元素添加到此列表的尾部。
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}

//从指定的位置将指定集合中的所有元素插入此列表。
//变化的元素位置和任何后续元素向右移动(增加他们的指标)。
//新元素的顺序将出现在列表中,指定返回的集合的迭代器。
public boolean addAll(int index, Collection<? extends E> c){
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // Increments modCount

int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);

System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}

remove方法:
//通过角标移除元素
public E remove(int index) {
rangeCheck(index); //判断角标是否越界

modCount++;
E oldValue = elementData(index);

int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work

return oldValue;
}

private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//如果list中存在该元素,就删除该元素。如果列表不包含该元素,list是不变的。
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}

/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}

get方法：获取指定位置的元素
public E get(int index) {
rangeCheck(index);
return elementData(index);
}

/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
* 数组的扩容
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//右移一位，等于oldCapacity/2,由此看出新数组最少是原来的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}