都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事

就用下面这段代码在jdk的三个版本运行看了下效果

import java.lang.reflect.Field;

import java.util.ArrayList;

public class ArrayListDemo {

    public static void main(String[] args) {

        ArrayList<Integer> list1 = new ArrayList<Integer>();

        Integer capacity = getCapacity(list1);// 获取容量

        int size = list1.size();

        System.out.println("list1的容量:" + capacity);

        System.out.println("list1的大小:" + size);

        System.out.println("----------------------------");

        ArrayList<Integer> list2 = new ArrayList<Integer>();

        list2.add(1);

        capacity = getCapacity(list2);// 获取容量,arraylist初始化容量是10

        size = list2.size();

        System.out.println("list2的容量:" + capacity);

        System.out.println("list2的大小:" + size);

        System.out.println("----------------------------");

        ArrayList<Integer> list3 = new ArrayList<Integer>();

        capacity = getCapacity(list3);// 获取容量,arraylist初始化容量是10

        for (int i = 0; i < 10; i++) {

            list3.add(i);

        }

        capacity = getCapacity(list3);

        size = list3.size();

        System.out.println("list3的容量:" + capacity);

        System.out.println("list3的大小:" + size);

        System.out.println("----------------------------");

        ArrayList<Integer> list4 = new ArrayList<Integer>();

        for (int i = 0; i < 11; i++) {

            list4.add(i);

        }

        capacity = getCapacity(list4);// 获取容量

        size = list4.size();

        System.out.println("list4的容量:" + capacity);

        System.out.println("list4的大小:" + size);

    }

    // 获取list容量

    public static Integer getCapacity(ArrayList list) {

        Integer length = null;

        Class clazz = list.getClass();

        Field field;

        try {

            field = clazz.getDeclaredField("elementData");

            field.setAccessible(true);

            Object[] object = (Object[]) field.get(list);

            length = object.length;

            return length;

        } catch (Exception e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        return length;

    }

}

1 jdk1.6

1.1 运行结果如下:

list1的容量:10

list1的大小:0

----------------------------

list2的容量:10

list2的大小:1

----------------------------

list3的容量:10

list3的大小:10

----------------------------

list4的容量:16

list4的大小:11

1.2 部分源代码

    public void ensureCapacity(int var1) {

        ++this.modCount;

        int var2 = this.elementData.length;

        if (var1 > var2) {

            Object[] var3 = this.elementData;

            int var4 = var2 * 3 / 2 + 1;

            if (var4 < var1) {

                var4 = var1;

            }

            this.elementData = Arrays.copyOf(this.elementData, var4);

        }

    }

2 jdk1.7

2.1 部分源代码

public boolean add(E e) {

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

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 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);
}

2.2 运行结果

list1的容量:0

list1的大小:0

----------------------------

list2的容量:10

list2的大小:1

----------------------------

list3的容量:10

list3的大小:10

----------------------------

list4的容量:15

list4的大小:11

3 jdk1.8

C:\Users\HP>java -version
java version "1.8.0_181"
Java(TM) SE Runtime Environment (build 1.8.0_181-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode)

3.1 部分源代码

public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, 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);
}

3.2 运行结果

list1的容量:0

list1的大小:0

----------------------------

list2的容量:10

list2的大小:1

----------------------------

list3的容量:10

list3的大小:10

----------------------------

list4的容量:15

list4的大小:11

4 总结

发现不同jdk是不一样的，关于（1.5倍+1)出现在jdk1.6，其他1.7和1.8都是（1.5倍扩容）。

关于详细代码和原理可以参考 3。参考1和2也是不错的！

参考 1 ArrayList扩容1.5倍

2 ArrayList源码解析

3 ArrayList初始默认容量（长度）

巴特西

都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事

都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事

1 jdk1.6

1.1 运行结果如下:

1.2 部分源代码

2 jdk1.7

2.1 部分源代码

2.2 运行结果

3 jdk1.8

3.1 部分源代码

3.2 运行结果

4 总结

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