源码解析:Oops ! JDK源码---集合类(一)之ArrayList源码

本文基于JDK1.8

0x 00 总体描述

ArrayList是基于数组实现的,并且支持自动扩容,相对于普通数组而言,由于他自动扩容的的特性,在日常开发过程中,使用的十分多。
源码解析:Oops ! JDK源码---集合类(一)之ArrayList源码_第1张图片

// ArrayList.java
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, java.io.Serializable
{
     

从类图和源码可知,ArrayList实现了四个接口:
java.util.List 接口,主要数组的接口类,提供数组的基本操作,如添加、删除、修改、遍历;
java.util.RandomAccess 接口,提供快速随机访问的能力。
java.io.Serializable 接口,表示ArrayList可序列化。
java.lang.Cloneable 表示其支持克隆
继承了java.util.AbstractList 抽象类,是List接口的基本实现,减少了具体实现类中的大部分共性代码的实现。

注:ArrayList重写了大部分AbstractList的方法实现,所以其对ArrayList作用不大,主要是其他减少了其他子类的实现。

0x01 源码阅读
属性

ArrayList的属性很少,只有两个,如下:

// ArrayList.java
/**
     * 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 == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added. 
     * 元素数组,可自动扩容
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     * 已使用的数组大小
     */
    private int size;

elementData 属性:元素数组,存储元素。
size属性:数组的大小。size表示的是elementData数组存储的元素数量,并且正好是元素添加到数组中的下标,ArrayList的真实大小是elementData的大小。

构造方法

ArrayList一共三个构造方法如下:
#ArrayList(int initialCapacity)

 /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     * 带构造方法,主要用于容量的初始化
     */
    public ArrayList(int initialCapacity) {
     
        // 初始容量大于0 创建Object数组
        if (initialCapacity > 0) {
     
            this.elementData = new Object[initialCapacity];
        // 等于0 使用其自身的final空数组EMPTY_ELEMENTDATA
        } else if (initialCapacity == 0) {
     
            this.elementData = EMPTY_ELEMENTDATA;
        // 小于0 则抛出参数不合法异常
        } else {
     
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

该构造方法可以根据传入的初始化容量大小,创建对应的ArrayList数组。对于我们已经明确知道容量大小的场景,可以使用这个构造方法,可以减少数组的扩容,提高系统的性能,合理使用内存。’

#ArrayList()
无参构造方法,是我们使用最多的构造方法。

// ArrayList.java
/**
     * Constructs an empty list with an initial capacity of ten.
     * 默认容量为10
     */
    public ArrayList() {
     
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

此处需要注意的是,创建的是为空的数组,为了节约内存,在初始化时为空数组,在添加第一个元素时,会真正初始化为10的数组。至于为什么要和EMPTY_ELEMENTDATA区分开来呢,是为区分起点,空数组是从0开始按照1.5呗倍扩容,而不是10,而DEFAULTCAPACITY_EMPTY_ELEMENTDATA首次扩容为10;
#ArrayList(Collection c)
从参数类型可以看出,传的参数为集合类型,作为elementData。

//ArrayList.java
/**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
     
    	// 转换为数组
        elementData = c.toArray();
        // 判断数组长度
        if ((size = elementData.length) != 0) {
     
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            // 转化为数组时,可能不是Object类型,则会创建新的Object数组,并赋值给elementData
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
     
            // replace with empty array.
            // 若数组大小等于0则直接赋值空数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
普通方法

① 扩容与缩容方法

#trimToSize()

数组的手动缩容方法,实现如下:

ArrayList.java
**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
     
    	// 记录修改次数
        modCount++;
        if (size < elementData.length) {
     
            elementData = (size == 0)
            // 大小为0则使用空数组
              ? EMPTY_ELEMENTDATA
              // 大于0则创建大小为size的新数组,将原数组复制进去
              : Arrays.copyOf(elementData, size);
        }
    }
#grow()

扩容方法,在扩容过程中,首先创建一个新的更大的数组,一般时1.5倍大小,将原数组复制到新数组中,最后返回新数组。代码如下:

ArrayList.java
/**
     * Increases the capacity of this ArrayList instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
     
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
     
            ensureExplicitCapacity(minCapacity);
        }
    }
    private void ensureExplicitCapacity(int minCapacity) {
     
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 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
     *
     */
    private void grow(int minCapacity) {
     
        // overflow-conscious code
        int oldCapacity = elementData.length;
        // 1.5的大小 扩容
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        // 扩容后的大小比所需要的容量小
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
         // 扩容后比最大的数组大小要大
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            // 则进行容量的相关判断
            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;
    }

② 查找指定元素对应的下标

#indexOf(Object o)
ArrayList.java
/**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index i such that
     * (o==null ? get(i)==null : o.equals(get(i))),
     * or -1 if there is no such index
     */
    public int indexOf(Object o) {
     
    	// 判断元素是否为null
        if (o == null) {
     
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
     
        	// 非null使用equals判断全等,比较的是对象是否相等
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

③ 克隆方法

#clone()
// ArrayList.java
/**
     * Returns a shallow copy of this ArrayList instance.  (The
     * elements themselves are not copied.)
     *
     * @return a clone of this ArrayList instance
     */
    public Object clone() {
     
        try {
     
        	// 调用父类的克隆方法
            ArrayList<?> v = (ArrayList<?>) super.clone();
            // 将原数组拷贝进新数组
            // 此处的数组是新拷贝出来的,避免和原数组共享引用
            v.elementData = Arrays.copyOf(elementData, size);
            // 设置修改次数为0
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
     
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

④ 转换为数组

#toArray() and #toArray(T[] a)
// ArrayList.java
 /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * 

The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * *

This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list in * proper sequence */ // 此方法返回是Object[] public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * Returns an array containing all of the elements in this list in proper * sequence (from first to last element); the runtime type of the returned * array is that of the specified array. If the list fits in the * specified array, it is returned therein. Otherwise, a new array is * allocated with the runtime type of the specified array and the size of * this list. * *

If the list fits in the specified array with room to spare * (i.e., the array has more elements than the list), the element in * the array immediately following the end of the collection is set to * null. (This is useful in determining the length of the * list only if the caller knows that the list does not contain * any null elements.) * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ // public <T> T[] toArray(T[] a) { // 若传入的数组大小小于size的大小 if (a.length < size) // Make a new array of a's runtime type, but my contents: // 复制返回一个新数组 return (T[]) Arrays.copyOf(elementData, size, a.getClass()); // 将elementData复制到a System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; // 返回a return a; }

由于可能返回新的数组,所以这里建议使用时还是这么写:a = list.toArray(a)

⑤ 获取指定位置元素

#get(int index)
// ArrayList.java
	E elementData(int index) {
     
        return (E) elementData[index];
    }

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
     
		// 下标检查 index 不能超过size
        rangeCheck(index);
        return elementData(index);
    }
    /**
     * 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));
    }

⑥ 设置指定位置元素

#set(int index, E element)
// ArrayList.java
/**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
     
    	// 下标检验
        rangeCheck(index);
		// 获取下标的原数据
        E oldValue = elementData(index);
        // 对应下标设置新元素
        elementData[index] = element;
        // 返回老值
        return oldValue;
    }

⑦ 添加单个元素

#add(E e)

顺序添加某个元素

// ArrayList.java

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return true (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
     
    	// 扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 将新建的元素添加到末尾
        elementData[size++] = e;
        return true;
    }
	private void ensureCapacityInternal(int minCapacity) {
     
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
	private void ensureExplicitCapacity(int minCapacity) {
     
		// 增加数组的修改次数
        modCount++;

        // overflow-conscious code
        // 数组扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
     
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
     
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
    
#add(int index, E element)

在某个确定位置添加元素

// ArrayList.java
/**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
     
        // 范围检查
        rangeCheckForAdd(index);
        // 数组扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        // 数组元素拷贝 空出index位置
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        // 对应位置赋值
        elementData[index] = element;
        // size增加1
        size++;
    }

⑧ 移除元素

#remove(int index)
// ArrayList.java
/**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    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);
        // clear to let GC do its work
        elementData[--size] = null;

        return oldValue;
    }

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