ArrayList source code相关方法阅读

本文主要是介绍ArrayList source code相关方法阅读，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1、新增一个对象

/*** Appends the specified element to the end of this list.** @param e element to be appended to this list* @return <tt>true</tt> (as specified by {@link Collection#add})*/public boolean add(E e) {ensureCapacity(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}

 /*** Increases the capacity of this <tt>ArrayList</tt> 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) {modCount++;int oldCapacity = elementData.length;if (minCapacity > oldCapacity) {Object oldData[] = elementData;int newCapacity = (oldCapacity * 3)/2 + 1;if (newCapacity < minCapacity)newCapacity = minCapacity;// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);}}

源码里面elementData对象是这样定义：private transient Object[] elementData;

在之前的开发中从来没用过transient 关键字，特意测试了一下，发现transient就是persistent的对立，当用到字节流、网络传输的时候一旦属性定义为transient将会出现获取值为null的情况。

详情可参考：What is transient keyword in Java?

还有一个需要了解知识点的地方就是Arrays.copyOf(elementData, newCapacity)方法，它是将elementData数组的数据按照newCapacity长度建立一个新的数组的同时，将原数组elementData数组里面的数据拷贝到新的数组中。

2、移除一个对象

/*** Removes the first occurrence of the specified element from this list,* if it is present.  If the list does not contain the element, it is* unchanged.  More formally, removes the element with the lowest index* <tt>i</tt> such that* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>* (if such an element exists).  Returns <tt>true</tt> if this list* contained the specified element (or equivalently, if this list* changed as a result of the call).** @param o element to be removed from this list, if present* @return <tt>true</tt> if this list contained the specified element*/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; // Let gc do its work}

这里面关键点就是fastRemove方法中的System.arraycopy(elementData, index+1, elementData, index, numMoved);

这个方法的含义是将elementData数组从index+1位置到numMoved位置的数据拷贝到数组elementData的index位置，依次往后填充

详情可参考：老紫竹

以上两点我们可以看出：list其实的存储其实就是数组；数组都是按位来存储，所以在插入或者是移除的时候都是依次顺序往后推移，所以我们的面试题经常会提到ArrayList查询效率要高于新增和删除的效率

这里暂时只列出两个方法，其他今后有时间研究再这里列出来，待续....

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