ArrayList的特点有:存取有序,元素可以重复,可以存null值,因为底层是数组,所以查询快,增删相对慢,非线程安全。
先看成员属性:
//默认初始容量
private static final int DEFAULT_CAPACITY = 10;
//被用于空实例的共享空数组实例
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//存放元素的数组,数组的长度为List的size
//当元素被添加的时候,List的容量会被扩充到 DEFAULT_CAPACITY(10)
// 非private是为了方便嵌套类的访问
//transient 修饰,该属性不会序列化,ArrayList有自己的序列化方法
transient Object[] elementData;
//size,List的大小
private int size;
构造函数:
//无参构造,对容器进行初始化赋值{}。
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
//自定义容量的构造函数,必须大于等于0。
public ArrayList(int initialCapacity) {
//如果大于0
if (initialCapacity > 0) {
//对存元素的数组进行初始化
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
//和无参构造一样,将存放元素的数组进行默认初始化{}
this.elementData = EMPTY_ELEMENTDATA;
} else {
//小于0,抛异常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
//集合构造函数
public ArrayList(Collection<? extends E> c) {
//将集合转换为数组,并赋值给元素容器
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// 如果不是空集合,检查是否成功转换为数组
if (elementData.getClass() != Object[].class)
//如果转换不成功,直接copy
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 空集合,
this.elementData = EMPTY_ELEMENTDATA;
}
}
添加指定元素:
//将元素添加到该List的末尾.
public boolean add(E e) {
//size为当前List大小
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
//如果是第一次添加元素
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
//在DEFAULT_CAPACITY(10)和当前的size+1取一个最大值
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
//记录结构性修改次数
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//增加数组的容量,确保它至少能容纳指定的最小容量的元素量。
//minCapacity 期望的最小容量
private void grow(int minCapacity) {
// overflow-conscious code
//记录当前容量大小
int oldCapacity = elementData.length;
//新容量为旧容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//保证新容量最小为当前容量
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//保证新容量不大于Integer.MAX_VALUE - 8
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);
}
指定位置添加:
//将元素插入到List的指定位置
public void add(int index, E element) {
//检查index,保证size>index>0。
rangeCheckForAdd(index);
//如果容量不足,将进行扩容
ensureCapacityInternal(size + 1); // Increments modCount!!
//将数组的元素从index开始后面的元素(size-index),统一后移一位。
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
添加指定集合:
//将新Collection(按照迭代器所返回的元素顺序)的所有元素添加到该List的尾部.
//Collection为空将NullPointerException.
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
//将a[0--(numNew-1)] copy 到 elementData[size--(size+numNew-1)]
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
//添加指定集合到指定位置。
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;
}
删除指定元素:
// 删除List指定位置上的元素
public E remove(int index) {
//确保size>=index
rangeCheck(index);
//
modCount++;
//记录指定索引上的元素,作为返回值
E oldValue = elementData(index);
//计算出index后面元素前移一位的位置,准备copy
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//将数组最后一个元素设置为null,由GC清理
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
删除首次出现的指定元素:
//删除List中首次出现的指定元素(如果存在),
//ArrayList可以存放重复元素。
public boolean remove(Object o) {
//如果元素为null
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
//不为null
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
//删除指定位置元素,跳过index参数的边界检查以及不返回任何值
private void fastRemove(int index) {
modCount++;
//计算出index后面元素前移一位的位置,准备copy
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//将数组最后一个元素设置为null,由GC清理
elementData[--size] = null; // clear to let GC do its work
}
删除List中指定的集合:
//删除List中指定Collection(非空)里面的元素
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
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++)
//判断容器中是否包含elementData[r]元素
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;
}
删除指定边界的元素:
//删除指定边界的元素,包小不包大。
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
//计算出index后面元素前移一位的位置,准备copy
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// GC处理
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
1.8新增的remove方法:
//刪除所有滿足指定条件的元素
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
// figure out which elements are to be removed
// any exception thrown from the filter predicate at this stage
// will leave the collection unmodified
int removeCount = 0;
final BitSet removeSet = new BitSet(size);
final int expectedModCount = modCount;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
@SuppressWarnings("unchecked")
final E element = (E) elementData[i];
if (filter.test(element)) {
removeSet.set(i);
removeCount++;
}
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
// shift surviving elements left over the spaces left by removed elements
final boolean anyToRemove = removeCount > 0;
if (anyToRemove) {
final int newSize = size - removeCount;
for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
for (int k=newSize; k < size; k++) {
elementData[k] = null; // Let gc do its work
}
this.size = newSize;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
return anyToRemove;
}
ArrayList的get方法:
//获得指定位置的元素index<=size
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
ArrayList的set方法:
//将指定位置的值替换为新值
public E set(int index, E element) {
//
rangeCheck(index);
//记录指定位置上的元素,作为返回值
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
查找元素方法:
//顺序查找,返回元素第一次出现的位置,如果元素不存在,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;
}
//逆序查找,返回元素最后一次出现的位置,如果元素不存在,则返回-1
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;
}
如果有不对的地方欢迎指出。
知识的学习,要真诚与谦虚才不会有眼无珠,人生苦短,不能浪费时间做无用功。 人生学习最悲哀的不过是,因为无知傲慢错过真正的好东西,又因为无知贪婪在假东西上耗费生命。