Java自定义一个变长数组的思路与代码

public class ResizeableArray<E> {

    private static final int MIN_CAPACITY = 10;

    private static final int MAX_CAPACITY = Integer.MAX_VALUE - 8;

    // 当前实际数据大小
    private int size = 0;

    // 实际存放元素的数组，容量为 elements.length
    private Object[] elements;

    public ResizeableArray(){
        this(MIN_CAPACITY);
    }

    public ResizeableArray(int initCapacity){
        if(initCapacity < MIN_CAPACITY){
            initCapacity = MIN_CAPACITY;
        }else if(initCapacity > MAX_CAPACITY){
            initCapacity = MAX_CAPACITY;
        }
        this.elements = new Object[initCapacity];
    }

    // 数组的特性，根据下标获取元素
    public E get(int index){
        this.checkIndex(index);
        return (E)elements[index];
    }

    // 添加一个元素到最后
    public void add(E element){
        if(size == elements.length){
            // 需要扩容
            this.expandCapacity();
        }
        elements[size++] = element;
    }

    // 添加一个元素到指定位置
    public void add(int index, E element){

        if(index == size){
            this.add(element);
            return;
        }
        this.checkIndex(index);
        if(size == elements.length){
            // 需要扩容
            this.expandCapacity();
        }
        // 需要先将index和之后的所有元素向后移动一位
        for(int i = size - 1; i >= index; i--){
            elements[i + 1] = elements[i];
        }
        elements[index] = element;
        size++;

    }

    // 设置下标为index的元素
    private void set(int index, E element){
        this.checkIndex(index);
        elements[index] = element;
    }

    

    private void delete(int index){
        this.checkIndex(index);
        elements[index] = null;
        // 如果删除的不是最后一个元素，需要将后续元素往前移动一位
        if(index < size - 1){
            for(int i = index + 1; i < size; i++){
                elements[i - 1] = elements[i];
            }
        }
        size--;
        if(size <= 0.25 * elements.length){
            this.reduceCapacity();
        }
    }

    private void deleteLast(){
        this.delete(size - 1);
    }

    private void checkIndex(int index){
        if(index < 0 || index >= size){
            throw new IndexOutOfBoundsException(String.format("Out of bounds at: %s, size is: %d", index, size));
        }
    }

    private void expandCapacity(){
        if(MAX_CAPACITY == elements.length){
            // 容量达到最大限制，无法扩容。
            throw new RuntimeException("The capacity has reached its maximum limit and cannot be expanded.");
        }
        int newCapacity = Math.min(elements.length << 1, MAX_CAPACITY);
        Object[] newElements = new Object[newCapacity];
        System.arraycopy(elements, 0, newElements, 0, size);
        elements = newElements;
    }

    private void reduceCapacity(){
        if(elements.length == MIN_CAPACITY){
            return;
        }
        int newCapacity = Math.max(elements.length >> 1, MIN_CAPACITY);
        Object[] newElements = new Object[newCapacity];
        System.arraycopy(elements, 0, newElements, 0, size);
        elements = newElements;
    }



    public static void main(String[] args){
        ResizeableArray<Integer> resizeableArray = new ResizeableArray<>();
        System.out.printf("初始化后，size为: %d \n", resizeableArray.size);
        System.out.printf("初始化后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.println();

        for(int i = 0; i < 20; i++){
            resizeableArray.add(i);
        }

        System.out.printf("添加20个元素后，size为: %d \n", resizeableArray.size);
        System.out.printf("添加20个元素后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.printf("添加20个元素后，第5个元素是: %d \n", resizeableArray.get(4));
        System.out.println();

        resizeableArray.delete(4);

        System.out.printf("删除第五个元素后，size为: %d \n", resizeableArray.size);
        System.out.printf("删除第五个元素后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.printf("删除第五个元素后，第5个元素是: %d\n", resizeableArray.get(4));
        System.out.println();

        resizeableArray.add(4, 100);
        System.out.printf("在第五个位置插入元素后，size为: %d \n", resizeableArray.size);
        System.out.printf("在第五个位置插入元素后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.printf("在第五个位置插入元素后，第5个元素是: %d\n", resizeableArray.get(4));
        System.out.println();

        for(int i = 0; i < 15; i++){
            resizeableArray.deleteLast();
        }
        System.out.printf("删除后面15个元素后，size为: %d \n", resizeableArray.size);
        System.out.printf("删除后面15个元素后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.printf("删除后面15个元素后，第5个元素是: %d\n", resizeableArray.get(4));
        System.out.println();

        resizeableArray.set(4, 200);
        System.out.printf("修改第五个元素后，当前size为: %d \n", resizeableArray.size);
        System.out.printf("修改第五个元素后，capacity为: %d \n", resizeableArray.elements.length);
        System.out.printf("修改第五个元素后，第5个元素是: %d\n", resizeableArray.get(4));

    }
}