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一、ArrayList 的定义
//ListobjectName = new ArrayList<>();List list = new ArrayList<>();
E: 泛型数据类型,用于设置 objectName 的数据类型,只能为引用数据类型。objectName 是数组名。
这里定义一个String类型的ArrayList数组示例。
在定义的时候我们会调用ArrayList的构造方法,下面是ArrayList类构造函数的源码
public class ArrayListextends AbstractList implements List , RandomAccess, Cloneable, java.io.Serializable{ private static final long serialVersionUID = 8683452581122892189L; /** * Default initial capacity. */ private static final int DEFAULT_CAPACITY = 10; /** * Shared empty array instance used for empty instances. */ private static final Object[] EMPTY_ELEMENTDATA = { }; /** * Shared empty array instance used for default sized empty instances. We * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when * first element is added. */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = { }; /** * 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. */ transient Object[] elementData; // non-private to simplify nested class access /** * The size of the ArrayList (the number of elements it contains). * * @serial */ private int size; /** * 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 */ public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } } /** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
可以看到ArrayList的底层是一个Object[],添加到ArrayList中的数据保存在了elementData属性中。
Listlist = new ArrayList<>();
将一个空数组赋值给了elementData ,这时数组的长度size为0
Listlist = new ArrayList<>(10);
根据传入的长度,将一个长度为10的Object数组赋值给elementData
二、ArrayList 的使用
1.添加 add(E element)
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true;}
首先通过 ensureCapacityInternal(size + 1) 来保证底层Object[]数组有足够的空间存放添加的数据,然后将添加的数据存放到数组对应的位置上.
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);}
首先确定了Object[]足够存放添加数据的最小容量,然后通过grow方法进行扩容。
扩容规则为“数组当前足够的最小容量 + (数组当前足够的最小容量 / 2)”,即数组当前足够的最小容量 * 1.5
2.设置值 set(int index, E element)
public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue;}
通过传入的数字下标找到对应的位置,替换其中的元素,前提也需要首先判断传入的数组下标是否越界。
3.获取值 get(int index)
public E get(int index) { rangeCheck(index); return elementData(index);}E elementData(int index) { return (E) elementData[index];}
通过下标查找,同时需要进行了类型转换,因为数组为Object[],前提是需要判断传入的数组下标是否越界。
4.删除 remove(int index)
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;}
首先获取了待删除的元素,并最终返回了。其次计算了数组中需要移动的位数 size - index - 1,那么很明显我们可以得出待删除的是最后一个元素的话,移到位数为0,否则移动位数大于0,那么通过数组元素的拷贝来实现往前移动相应位数。
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