java容器手写简单ArrayList

梦里梦外; 2021-12-10 01:37 258阅读 0赞

> 之前分析记录了一下源码，现在模仿源码，手写ArrayList的功能，简单实现，加深记忆

/** * @author hao * @create 2019-07-15 ${TIM} */
    public class DiyCollection { 
    
        //元素个数
        private int size;
    
        //数组存储
        private Object elementData [];
    
        //默认初始容量
        private final static int  DEFAULT_CAPACITY = 10;
    
        @Override
        public String toString() { 
            StringBuilder stringBuilder = new StringBuilder();
            stringBuilder.append("[");
            for (int i = 0; i < this.size; i++) { 
                stringBuilder.append(elementData[i]+",");
            }
            stringBuilder.setCharAt(stringBuilder.length()-1,']');
    
            return stringBuilder.toString();
        }
    
    
        public DiyCollection(){ 
            this.elementData = new Object [DEFAULT_CAPACITY];
        }
    
             // add方法
        public void add(Object o){ 
            ensureCapacity(this.size+1);  // 检查扩容
            elementData[size++] = o;    //添加元素
        }
    
        /** * 移除元素操作 * @param index */
        public void remove(int index){ 
             int movestart = index + 1;
             int movenums = elementData.length - movestart;
             if (movenums >0)
                System.arraycopy(elementData,movestart,elementData,index,movenums);
             //移除
            elementData[--size] = null;
        }
        //get方法
        public  Object get(int index){ 
            if(index >= size) throw new ArrayIndexOutOfBoundsException("越界");
            return elementData[index];
        }
    
        private void ensureCapacity(int size){ 
            if(size > this.elementData.length ){  //判断预期容量是否足够
                //扩容操作
                capacity();
            }
        }
    
    
        private void capacity(){ 
            //新长度
            int newLength = this.elementData.length + (this.elementData.length>>1);
            //底层还是使用 System.arraycopy方法
            elementData = Arrays.copyOf(this.elementData, newLength);
        }