动态数组

1. 手动实现版本

public class Array<E> {

    private E[] data;
    private int size;

    public Array(int capacity) {
        data = (E[]) new Object[capacity];
        size = 0;
    }

    public Array() {
        this(10);
    }

    public void add(int index, E e) {
        if (index < 0 || index > size) {
            throw new RuntimeException("Illegal index, index should in[0, size]!");
        }
        //动态扩容
        if (size >= data.length) {
            resize(2 * data.length);
        }
        for (int i = size - 1; i >= index; i--) {
            data[i + 1] = data[i];
        }
        data[index] = e;
        size++;
    }

    public void addFirst(E e) {
        add(0, e);
    }

    public void addLast(E e) {
        add(size, e);
    }

    public int find(E e) {
        for (int i = 0; i < size; i++) {
            if (data[i].equals(e)) {
                return i;
            }
        }
        return -1;
    }

    public boolean contains(E e) {
        for (int i = 0; i < size; i++) {
            if (data[i].equals(e)) {
                return true;
            }
        }
        return false;
    }

    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new RuntimeException("Illegal index, index should in[0, size)!");
        }
        return data[index];
    }

    public E getFirst() {
        return data[0];
    }

    public E getLast() {
        return data[size - 1];
    }

    public void set(int index, E e) {
        if (index < 0 || index >= size) {
            throw new RuntimeException("Illegal index, index should in[0, size)!");
        }
        data[index] = e;
    }

    public E remove(int index) {
        if (index < 0 || index >= size) {
            throw new RuntimeException("Illegal index, index should in[0, size)!");
        }
        E ret = data[index];
        for (int i = index + 1; i < size; i++) {
            data[i - 1] = data[i];
        }
        size--;
        // azy缩容
        if (size == data.length / 4 && data.length / 2 != 0) {
            resize(data.length / 2);
        }
        return ret;
    }

    public E removeFirst() {
        return remove(0);
    }

    public E removeLast() {
        return remove(size - 1);
    }

    public void remove(E e) {
        int index = find(e);
        if (index != -1) {
            remove(index);
        }
    }

    public int getCapacity() {
        return data.length;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public int getSize() {
        return size;
    }

    private void resize(int newCapacity) {
        if (newCapacity < size) {
            throw new RuntimeException("Illegal newCapacity, newCapacity should >= size");
        }
        E[] newData = (E[]) new Object[newCapacity];

        for (int i = 0; i < size; i++) {
            newData[i] = data[i];
        }
        data = newData;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Array size:" + size + ", capacity:" + data.length + "\n");
        res.append("data[");
        for (int i = 0; i < size; i++) {
            res.append(data[i] + " ");
        }
        res.append("]");
        return res.toString();
    }
}

2. JDK版本分析

2.1 需要扩容时，变为原来的1.5倍

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);
    }

2.2 通过垃圾回收，动态缩容

    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;
    }

总结：还是JDK写的漂亮啊🤣