7、集合--ArrayList的测试以及相关方法的源码解析
ArrayList的测试
public static void main(String[] args) { List list = new ArrayList(); list.add("1"); list.add("2"); list.add("3"); list.add("a"); list.add("a"); list.add("b"); System.out.println(list); System.out.println("长度：" + list.size()); //遍历 for (int i = 0; i < list.size();i++){ Object obj = list.get(i); System.out.println("for循环遍历list：" + obj); } //迭代器遍历 Iterator it = list.iterator(); while (it.hasNext()){ System.out.println("迭代器遍历list：" + it.next()); } //在指定位置添加数据 list.add(0,"news"); System.out.println("指定0位置的数据：" + list.get(0)); //替代指定位置上的元素 list.set(0,"old"); System.out.println("替代0位置的元素:" + list.get(0)); //获取第一次出现元素a的位置 System.out.println("元素a第一次出现的位置：" + list.indexOf("a") ); //获取最后一次出现元素a的位置 System.out.println("元素a第一次出现的位置：" + list.lastIndexOf("a")); //移除指定位置上的元素 list.remove(0); System.out.println("第一个元素：" + list.get(0)); //清空集合 list.clear(); }
相关方法的解析：
在new ArrayList之后：
底层的实现是数组
size用于确定此时操作的位数
transient Object[] elementData;
private int size;
protected transient int modCount = 0;//用来迭代操作的数据
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
add(E e）方法
在数组中进行添加是size会自增，将数据存放在数组中
此时会返回true
同时会执行一下相关的方法
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
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);
}
add(int index,E e)方法
首先检查index是否有误
然后执行System.arraycopy(elementData, index, elementData, index + 1,size - index);进行复制数组
将index位置空出，在进行在elementDate【index】位置上设置数据
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
......
size()方法
此时返回的是size数值
public int size() { return size; }
get()方法
首先会对传入的索引值进行判断
如果索引值小于index则会抛出异常
否则将会返回数组指定索引的值
public E get(int index) { rangeCheck(index); return elementData(index); }
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
iterator()方法
会返回一个Itr对象
Itr对象中会有hasNext()、next()方法
此时的操作需要注意的是数据：modCount
在之前的操作中modCount的数据处于自增状态
public Iterator<E> iterator() { return new Itr(); } private class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") 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]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }
set(int index,E element)方法
用于修改指定位置的元素值
首先调用rangeCheck(index)来判断索引值是否越界
将之前的值进行保存
将需要改的值设置在指定索引的位置
最后返回旧值（旧值可能需要）
public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
IndexOf（Object o）方法
首先判断其值是否为null
如果为null在依次进行循环判断，返值为i，此时的i则是第一次出现的位置
如果不为空在进行判断
此时使用equals()方法和数组中的每一个值进行判断
返回第一次相同的位置，返回此时的索引值为i
如果都没有则返回-1
public int indexOf(Object o) { if (o == null) { for (int i = 0; i < size; i++) if (elementData[i]==null) return i; } else { for (int i = 0; i < size; i++) if (o.equals(elementData[i])) return i; } return -1; }
lastIndexOf(Object o)方法
此时的判断方法和IndexOf思想一致
只是从索引的最大值向下递减来判断
public int lastIndexOf(Object o) { if (o == null) { for (int i = size-1; i >= 0; i--) if (elementData[i]==null) return i; } else { for (int i = size-1; i >= 0; i--) if (o.equals(elementData[i])) return i; } return -1; }
clear()方法
遍历数组，将索引值都置为null
并且将size的值置为0
public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }
remove(int index)删除指定索引位置的元素
首先调用rangeCheck（）方法来检测index是否有越界错误
在调用System.array()方法进行复制数组
在让size自减并且将最后的一个值设置为null
public E remove(int index) { rangeCheck(index); 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));
}
remove(Object o)删除指定的对象
首先将传入的对象和null进行对比
如果未null则进行遍历在进行调用fastRemove（int index)方法
如果传入的对象不为null
则使用equals()方法进行判断两个对象是相同，在调用fastRemove（）fangfa
如果以上都不满足，返回false
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 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
}
removeAll(Collection<?> c)从集合中删除集合c中包含的元素
此时依次调用requireNonNull()方法
在此调用batchRemove()方法
public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); }
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) {
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
contains(Object o)集合中是都包含元o
会返回indexOf（Object o)方法进行返回值的设置
public boolean contains(Object o) { return indexOf(o) >= 0; } public int indexOf(Object o) { if (o == null) { for (int i = 0; i < size; i++) if (elementData[i]==null) return i; } else { for (int i = 0; i < size; i++) if (o.equals(elementData[i])) return i; } return -1; }
isEmpty()方法测试数组是否为空
public boolean isEmpty() { return size == 0; }
对于ArrayList的基本方法分析到此结束
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