Java ArrayList，HashSet和HashMap中的内存重新分配问题

Question

Looked some unofficial references, and want to confirm here my understanding is correct. Suppose we are adding new (unique) elements from time to time,

1. ArrayList<T> will reallocate for memory since its memory needs to be continuous, when memory grow by newly inserted elements exceeds some threshold, reallocation of larger continuous memory space will happen, and existing elements will be moved to such newly allocated larger continuous memory space;
2. HashSet<T> and HashMap<T> has no such issue since memory of them does not require to be continuous?

BTW, if some good articles on these areas, appreciate for refer as well.

regards, Lin

Answer 1

If you checkout the sourcecode of add(E e) method in ArrayList<> in Java 8 (jre 1.8.0_71), it calls in for a method called ensureCapacityInternal(int minCapacity) . ie this method is called every time you add in an object to the ArrayList. This inturn calls in a series of methods and finally if the size of your ArrayList is smaller to hold the newly added element, it calls in a method called grow(int minCapacity) . This method is as shown below:

/**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    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);
    }

This will create a new array with size 1.5 times more than the initial one and copies all the elements from old array to the new one. This proves your point no. 1.

Coming back to your point no. 2, in case of HashMap<K,V> , they are a special type of array that hold key and value pairs. This array slots are called buckets . So, every object that you add into an HashMap, should override hashCode() and equals() method properly. When you call put(K key, V value) method, it inturn calls a method called putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) by calculating the #hash of the key hash(Object key) that you have passed. This hash indicates the bucket location where the Value object shoud go. Hence, the array here only indicates the address blocks where the objects goes in. This thread explains it in more detail. I hope this is what you were looking for.