Java学习笔记之ArrayList基本用法


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ArrayList简介

ArrayList是一个其容量能够动态增长的动态数组。它继承了AbstractList,实现了ListRandomAccess, Cloneable, java.io.Serializable
基本的ArrayList,长于随机访问元素,但是在List中间插入和移除元素时较慢。同时,ArrayList的操作不是线程安全的!一般在单线程中才使用ArrayList,而在多线程中一般使用Vector或者CopyOnWriteArrayList

ArrayList遍历方式

ArrayList有三种遍历方式

迭代器遍历

Iterator it = arrayList.iterator();
while(it.hasNext()){
    System.out.print(it.next() + " ");
}

索引值遍历

for(int i = 0; i < arrayList.size(); i++){
   System.out.print(arrayList.get(i) + " ");
}

for循环遍历

for(Integer number : arrayList){
   System.out.print(number + " ");
}

需要说明的是,遍历ArrayList时,通过索引值遍历效率最高,for循环遍历次之,迭代器遍历最低。

toArray()的使用

有时候,当我们调用ArrayList中的 toArray(),可能遇到过抛出java.lang.ClassCastException异常的情况,这是由于toArray() 返回的是 Object[] 数组,将 Object[] 转换为其它类型(如,将Object[]转换为的Integer[])则会抛出java.lang.ClassCastException异常,因为Java不支持向下转型。
所以一般更常用的是使用另外一种方法进行使用:

 T[] toArray(T[] a)

调用toArray(T[] a)返回T[]可通以下方式进行实现:

 // toArray用法
 // 第一种方式(最常用)
 Integer[] integer = arrayList.toArray(new Integer[0]);

 // 第二种方式(容易理解)
 Integer[] integer1 = new Integer[arrayList.size()];
 arrayList.toArray(integer1);

 // 抛出异常,java不支持向下转型
 //Integer[] integer2 = new Integer[arrayList.size()];
 //integer2 = arrayList.toArray();

ArrayList用法示例

/**
 * @author GongchuangSu
 * @since 2016.05.10
 */
import java.util.ArrayList;
import java.util.Iterator;

public class ArrayListDemo {

    public static void main(String[] srgs){
         ArrayList arrayList = new ArrayList();

         System.out.printf("Before add:arrayList.size() = %d\n",arrayList.size());

         arrayList.add(1);
         arrayList.add(3);
         arrayList.add(5);
         arrayList.add(7);
         arrayList.add(9);
         System.out.printf("After add:arrayList.size() = %d\n",arrayList.size());

         System.out.println("Printing elements of arrayList");
         // 三种遍历方式打印元素
         // 第一种:通过迭代器遍历
         System.out.print("通过迭代器遍历:");
         Iterator it = arrayList.iterator();
         while(it.hasNext()){
             System.out.print(it.next() + " ");
         }
         System.out.println();

         // 第二种:通过索引值遍历
         System.out.print("通过索引值遍历:");
         for(int i = 0; i < arrayList.size(); i++){
             System.out.print(arrayList.get(i) + " ");
         }
         System.out.println();

         // 第三种:for循环遍历
         System.out.print("for循环遍历:");
         for(Integer number : arrayList){
             System.out.print(number + " ");
         }

         // toArray用法
         // 第一种方式(最常用)
         Integer[] integer = arrayList.toArray(new Integer[0]);

         // 第二种方式(容易理解)
         Integer[] integer1 = new Integer[arrayList.size()];
         arrayList.toArray(integer1);

         // 抛出异常,java不支持向下转型
         //Integer[] integer2 = new Integer[arrayList.size()];
         //integer2 = arrayList.toArray();
         System.out.println();

         // 在指定位置添加元素
         arrayList.add(2,2);
         // 删除指定位置上的元素
         arrayList.remove(2);    
         // 删除指定元素
         arrayList.remove((Object)3);
         // 判断arrayList是否包含5
         System.out.println("ArrayList contains 5 is: " + arrayList.contains(5));

         // 清空ArrayList
         arrayList.clear();
         // 判断ArrayList是否为空
         System.out.println("ArrayList is empty: " + arrayList.isEmpty());
    }
}
/**
Before add:arrayList.size() = 0
After add:arrayList.size() = 5
Printing elements of arrayList
通过迭代器遍历:1 3 5 7 9 
通过索引值遍历:1 3 5 7 9 
for循环遍历:1 3 5 7 9 
ArrayList contains 5 is: true
ArrayList is empty: true
*/

ArrayList源码解析

package java.util;

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    // 序列版本号
    private static final long serialVersionUID = 8683452581122892189L;

    // 默认容量大小
    private static final int DEFAULT_CAPACITY = 10;

    // 空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    // 用于保存ArrayList中数据的数组
    private transient Object[] elementData;

    // ArrayList中所包含元素的个数
    private int size;

    // 带初始容量参数的构造函数
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    // 默认构造函数,其默认初始容量为10
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    // 带Collection参数的构造函数
    public ArrayList(Collection 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);
    }

    // 将此 ArrayList 实例的容量调整为列表的当前大小(实际元素个数)
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }

    // 如有必要,增加此 ArrayList 实例的容量,以确保它至少能够容纳最小容量参数所
    // 指定的元素数
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != EMPTY_ELEMENTDATA)
            // any size if real element table
            ? 0
            // larger than default for empty table. It's already supposed to be
            // at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == 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 static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;


    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);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    // 返回ArrayList中的元素个数
    public int size() {
        return size;
    }

    // 判断ArrayList是否为空
    public boolean isEmpty() {
        return size == 0;
    }

    // 判断ArrayList是否包含Object(o)
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    // 返回ArrayList中首次出现的指定元素的索引,或如果此列表不包含元素,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;
    }

    // 返回ArrayList中最后一次出现的指定元素的索引,或如果此列表不包含索引,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;
    }

    // 返回此 ArrayList 实例的浅表副本
    public Object clone() {
        try {
            @SuppressWarnings("unchecked")
            ArrayList v = (ArrayList) super.clone();
            // 将当前ArrayList的全部元素拷贝到v中
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
    }

    // 按适当顺序(从第一个到最后一个元素)返回包含此列表中所有元素的数组
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    // 返回ArrayList的模板数组。所谓模板数组,即可以将T设为任意的数据类型
    @SuppressWarnings("unchecked")
    public  T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    // 位置访问操作   
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    // 返回ArrayList中指定位置上的元素
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    // 用指定的元素替代ArrayList中指定位置上的元素,并返回替代前的元素
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    // 将指定的元素添加到ArrayList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ArrayList中的指定位置
    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++;
    }

    // 移除ArrayList中指定位置上的元素,并返回该位置上的元素
    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;
    }

    // 移除ArrayList中首次出现的指定元素(如果存在则移除并返回true,否则返回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;
    }

    // 私有方法,用于快速移除
    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
    }

    // 移除ArrayList中的所有元素
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    // 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ArrayList的尾部
    public boolean addAll(Collection 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;
    }

    // 从指定的位置开始,将指定 collection 中的所有元素插入到ArrayList中
    public boolean addAll(int index, Collection 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;
    }

    // 移除列表中索引在 fromIndex(包括)和 toIndex(不包括)之间的所有元素
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    // 私有方法,用于范围检测
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    // 私有方法,用于add和addAll
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }


    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    // 移除ArrayList中Collection所包含的所有元素
    public boolean removeAll(Collection c) {
        return batchRemove(c, false);
    }

    // 保留所有ArrayList和Collection共有的元素
    public boolean retainAll(Collection c) {
        return batchRemove(c, true);
    }

    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;
    }

    // java.io.Serializable的写入函数
    // 将ArrayList的“容量,所有的元素值”都写入到输出流中
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; iif (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    // java.io.Serializable的读取函数:根据写入方式读出
    // 先将ArrayList的“容量”读出,然后将“所有的元素值”读出
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i// 返回一个从指定位置开始遍历的ListIterator迭代器
    public ListIterator listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    // 返回一个ListIterator迭代器
    public ListIterator listIterator() {
        return new ListItr(0);
    }

    // 返回一个Iterator迭代器
    public Iterator iterator() {
        return new Itr();
    }

    // 返回一个指定范围的子List列表
    public List subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }
}

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