实现ArrayList

迈不过友情╰ 2022-06-14 09:15 171阅读 0赞

ArrayList继承并实现了List，其源码实现也就是基于对数组的操作

package 学习;
    
    import java.util.ArrayList;
    
    class MyArrayList{
    	private Object[] elementData;
    	private int size;
    	//构造器
    	public MyArrayList(){
    		this(10); //默认数组大小
    	}
    	public MyArrayList(int initialCapacity){
    		if (initialCapacity < 0){
    			try {
    				throw new Exception();
    			} catch (Exception e) {
    				e.printStackTrace();
    			}
    		}
    		elementData = new Object[initialCapacity];
    	}
    	//添加元素
    	public void add(Object obj){
    		ensureCapacity();
    		elementData[size] = obj;
    		size++;
    	}
    	public void add(int index, Object obj){
    		rangeCheck(index);
    		ensureCapacity();
    		System.arraycopy(elementData, index, elementData, index+1, size-index);
    		size++;
    	}
    	
    	
    	public int size(){
    		return size;
    	}
    	//获取指定索引
    	public Object get(int index){
    		return elementData[index];
    	}
    	public void remove(int index){
    		rangeCheck(index);
    		//删除指定位置对象
    		int num = size-index-1;
    		if (num > 0){ //如果是最后一个就不用拷贝，把原来iNdex位置覆盖掉在数组最后设null;
    			System.arraycopy(elementData, index+1, elementData, index, num);
    		}
    		elementData[size--] = null;
    	}
    	public void remove(Object obj){//移除这个对象
    		for (int i=0; i<size; i++){
    			if (get(i).equals(obj)){ //找到相同的对象
    				remove(i);
    			}
    		} 
    	}
    	//设置指定索引值,返回值是原来的值
    	public Object set(int index, Object obj){
    		rangeCheck(index);
    		Object oldValue = elementData[index];
    		elementData[index] = obj;//设置新的值
    		return oldValue;
    	}
    	private void rangeCheck(int index){
    		if (index<0 || index>=size){
    			try {
    				throw new Exception();
    			} catch (Exception e) {
    				System.out.println("越界");
    			}
    		}
    	}
    	private void ensureCapacity(){
    		//数组扩容
    		if(size == elementData.length){
    		 Object[] newArrayList = new Object[size*2];
    		 System.arraycopy(elementData, 0, newArrayList, 0, newArrayList.length);
    		 elementData = newArrayList;
    		}
    	}
    }
    public class Main{
    	public static void main(String[] args) {
    		MyArrayList arr = new MyArrayList();
    		arr.add(123);
    		arr.add(222);
    		arr.add(333);
    		arr.add(444);
    		arr.remove(0);
    		System.out.println(arr.size());
    		arr.set(2, "替换掉");
    		for (int i=0; i<arr.size(); i++){
    			System.out.println(arr.get(i));
    		}
    	}
    }