集合类源码分析

川长思鸟来 2022-11-22 10:11 178阅读 0赞

# 1、ArrayList #

[https://blog.csdn.net/sihai12345/article/details/79382649][https_blog.csdn.net_sihai12345_article_details_79382649]

要点：

### 1.1 初始化大小 ###

默认初始大小为10

### 1.2 扩容 ###

添加元素时使用 `ensureCapacityInternal()` 方法来保证容量足够，如果不够时，需要使用 `grow()` 方法进行扩容，新容量的大小为 `oldCapacity + (oldCapacity >> 1)`，也就是旧容量的 **1.5 倍**。

扩容操作需要调用 `Arrays.copyOf()` 把原数组整个复制到新数组中，这个操作代价很高，因此最好在创建 ArrayList 对象时就指定大概的容量大小，减少扩容操作的次数。

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

[https_blog.csdn.net_sihai12345_article_details_79382649]: https://blog.csdn.net/sihai12345/article/details/79382649