ArrayList部分源码实现

ArrayList内部实现是通过数组来完成的，由于数组在内存中是连续的储存单元，所以其添加元素和删除操作均通过System.arraycopy()实现。ArrayList在查找读取操作时效率高；当涉及到数据经常进行增删操作时应使用LinkedList的数据结构更合适。

public class ArrayList<E>{
    private Object[] element_data;
    private int size;

    private static final int DEFAULT_CAPACITY = 10;

    public ArrayList() {
        element_data = new Object[DEFAULT_CAPACITY];
    }

    public ArrayList(int capacity){
        element_data = new Object[capacity];
    }

    public void add(E element){
        if (element_data.length==size){
            Object[] new_data = new Object[element_data.length*2];
            System.arraycopy(element_data,0,new_data,0,element_data.length);
            element_data = new_data;
        }
        element_data[size++] = element;
    }

    public E get(int index){
        checkIndex(index);
        return (E) element_data[index];
    }

    public void set(int index,E element) {
        checkIndex(index);
        element_data[index] = element;
    }

    private void checkIndex(int index) {
         if (index<0 || index>size-1) {
             throw new RuntimeException("index Error! at: "+ index);
         }
    }

    //拷贝element_data自己
    public void remove(int index) {
        System.arraycopy(element_data,index+1,element_data,index,element_data.length-1-index);
        size--;
    }

    public void remove(E element) {
        for (int i=0;i<size;i++){
            if (element.equals(get(i))){
                remove(i);
            }
        }
    }

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

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        //[a,b,c]
        sb.append("[");
        for (int i = 0; i <size ; i++) {
            sb.append(element_data[i]);
            sb.append(",");
        }
        sb.setCharAt(sb.length()-1, ']');

        return sb.toString();
    }

    public static void main(String[] args){
        ArrayList<Object> my = new ArrayList<>(1);
        
        System.out.println(my.isEmpty());
        System.out.println(my);
    }
}