手工實現ArrayList

ArrayList底層JDK源碼解讀 

 ArrayList底層是用數組實現的存儲。特點：查詢效率高。增刪效率低，線程不安全。不過一般情況還是使用ArrayList

package sxt.gaoqi.container;

/*
 * 增加泛型
 * 擴容
 * get/set方法
 * 索引越界RuntimeException
 * 增加remove
 */
public class JsArrayList<E>
{
	private Object[] elementData;
	private int size;
	
	private static final int DEFAULT_CAPACITY = 10;
	
	public JsArrayList()
	{
		elementData = new Object[DEFAULT_CAPACITY];
	}
	
	public JsArrayList(int capacity)
	{
		if(capacity < 0)
		{
			throw new RuntimeException("容器的容量不能爲負數: " + capacity);
		}
		else if(capacity == 0)
		{
			elementData = new Object[DEFAULT_CAPACITY];
		}
		else
		{
			elementData = new Object[capacity];
		}
	}
	
	public void add(E obj)
	{
		//什麼時候擴容？
		if(size == elementData.length)
		{
			//怎麼擴容？
			Object[] newArray = new Object[elementData.length + (elementData.length>>1)];
			System.arraycopy(elementData, 0, newArray, 0, elementData.length);
			elementData = newArray;
		}
		elementData[size++] = obj;
	}
	
	public E get(int index)
	{
		checkRange(index);
		return (E)elementData[index];
	}
	
	public void set(E element, int index)
	{
		checkRange(index);
		elementData[index] = element;
	}
	
	public void checkRange(int index)
	{
		if(index < 0 || index > size - 1)
			throw new RuntimeException("指針越界" + index);
	}
	
	public void remove(E element)
	{
		//element:將他和所有元素挨個比較，獲得第一個比較爲true的，返回
		for(int i = 0; i < size; i++)
		{
			if(element.equals(get(i)))
			{
				remove(i);
			}
		}
	}
	
	public void remove(int index)
	{
		//a.b.c.d.e.f.g
		//a.b.c.e.f.g
		int moveLength = elementData.length - index - 1;
		if(moveLength > 0)
		{
			System.arraycopy(elementData, index + 1, elementData, index, moveLength);
		}
		
		elementData[--size] = null;
	}
	
	public int size()
	{
		return size;
	}
	
	public boolean isEmpty()
	{
		return size == 0 ? true:false;
	}
	
	@Override
	public String toString()
	{
		StringBuilder sb = new StringBuilder();
		
		sb.append("[");
		for(int i =0; i < size; i ++)
		{
			sb.append(elementData[i] + ",");
		}
		sb.setCharAt(sb.length()-1, ']');
		
		return sb.toString();
	}
	
	public static void main(String[] args)
	{
		JsArrayList<String> s1 = new JsArrayList<>();
		for(int i = 0; i < 40; i++)
		{
			s1.add("aa" + i);
		}
		System.out.println(s1);
		s1.set("森", 10);  //測試set方法
		System.out.println(s1);
		System.out.println(s1.get(10)); //測試get方法
		s1.remove(2);  //測試remove方法
		s1.remove("aa4");
		System.out.println(s1);
	}
}