Java-ArrayList

huajiaohuixiang

2021-02-20

Java

JDK8版本

1 2	public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList 数组列表，分别实现了列表、随机访问、可克隆、序列化，继承了抽象列表,其关系如下所示：

1.几个参数

private static final long serialVersionUID = 8683452581122892189L;


private static final int DEFAULT_CAPACITY = 10;


private static final Object[] EMPTY_ELEMENTDATA = {};


private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};


transient Object[] elementData; // non-private to simplify nested class access


private int size;

2.主要方法

构造方法

如果是ArrayList arrayList=new ArrayList(20);只初始化了一次，算没扩容吧。

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);
    }
}

public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

传入另一个集合，先将collection转为数组，然后copy到elementData中

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // defend against c.toArray (incorrectly) not returning Object[]
        // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

3.trimToSize()修剪大小到size

public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

4. 增加容量（这个是给用户调用的，ArrayList内部没有调用）

如果要大批量插入数据，那么调用该函数主动增加容量

public void ensureCapacity(int minCapacity) {
    if (minCapacity > elementData.length
        && !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
             && minCapacity <= DEFAULT_CAPACITY)) {
        modCount++;
        grow(minCapacity);
    }
}
用到了grow() ,newCapacity(minCapacity) ,hugeCapacity(int minCapacity)方法

5.得到O地址

int indexOfRange(Object o, int start, int end) {
    Object[] es = elementData;
    if (o == null) {
        for (int i = start; i < end; i++) {
            if (es[i] == null) {
                return i;
            }
        }
    } else {
        for (int i = start; i < end; i++) {
            if (o.equals(es[i])) {
                return i;
            }
        }
    }
    return -1;
}

6.扩容

JDK11去掉了

add方法

/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return {@code true} (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    modCount++;
    add(e, elementData, size);
    return true;
}

/**
 * This helper method split out from add(E) to keep method
 * bytecode size under 35 (the -XX:MaxInlineSize default value),
 * which helps when add(E) is called in a C1-compiled loop.
 */
private void add(E e, Object[] elementData, int s) {
    if (s == elementData.length)
        elementData = grow();
    elementData[s] = e;
    size = s + 1;
}