java基础-数组扩容详解

  public boolean add(E e) {
       //扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

   private static int calculateCapacity(Object[] elementData, int minCapacity) {
        //如果传入的是个空数组则最小容量取默认容量与minCapacity之间的最大值
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

 private void ensureExplicitCapacity(int minCapacity) {
          modCount++;
          // 如果最小需要空间比elementData的内存空间要大，则需要扩容
          if (minCapacity - elementData.length > 0)
              //扩容
              grow(minCapacity);
      }

 private void grow(int minCapacity) {
          // 获取到ArrayList中elementData数组的内存空间长度
          int oldCapacity = elementData.length;
         // 扩容至原来的1.5倍
         int newCapacity = oldCapacity + (oldCapacity >> 1);
         // 再判断一下新数组的容量够不够，够了就直接使用这个长度创建新数组，
          // 不够就将数组长度设置为需要的长度
         if (newCapacity - minCapacity < 0)
             newCapacity = minCapacity;
         //若预设值大于默认的最大值检查是否溢出
         if (newCapacity - MAX_ARRAY_SIZE > 0)
             newCapacity = hugeCapacity(minCapacity);
         // 调用Arrays.copyOf方法将elementData数组指向新的内存空间时newCapacity的连续空间
         // 并将elementData的数据复制到新的内存空间
         elementData = Arrays.copyOf(elementData, newCapacity);
     }
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//节点
public class node {
	Node previous;//前继，指向前一个Node
	Object data;//节点数据
	Node next;//后继，指向后一个Node
	public Node() {
	}
	public Node(Node previous, Object data, Node next) {
		super();
		this.previous = previous;
		this.data = data;
		this.next = next;
	} 
}

public class MyLinkedList {
	private Node first;//首节点
	private Node last;//尾节点
	private int size;//链表大小
	public MyLinkedList() {
		first = null;
		last = null;
		size = 0;
	}
}

public void add(Object obj){
		Node node = new Node(null,null,null);
		if(first==null){//first=null,说明LinkedList中没有一个节点
			node.data = obj;
			first = node;
			last = node;//第一个节点和最后一个节点都是node
			size++;
		}else{
			node.data = obj;
			last.next = node;//和最后一个连接起来
			node.previous = last;
			last = node;//当前节点变为末尾节点
			size++;
		}

public Node get(int index) {
		RangeCheck(index);
		Node temp = null;
		if(index < (size>>1)){//改进的遍历方法，右移运算符的巧用
			temp = first;
			for(int i=0;i<index;i++){
				temp = temp.next;
			}
		}else {
			temp = last;
			for(int i=size-1;i>index;i--){
				temp = temp.previous;
			}
		}
		return temp;
	}

public void add(int index,Object obj) {
		RangeCheck(index);//对传入的索引必须进行检查，判断是否越界
		Node node = new Node(null,null,null);
		node.data = obj;
		Node node2=first;
		for(int i=0;i<index-1;i++){
			node2 = node2.next;
		}
		node.next = node2.next;
		node2.next.previous=node;
		node2.next = node;
		node.previous=node2;
		size++;
	}

private void RangeCheck(int index) {
		if(index<0||index >= size){
			throw new IndexOutOfBoundsException("IndexOutOfBounds"+index);//不合法则抛出异常
		}
	}

public boolean remove(Object obj) {
		Node node = first;
		if(obj==null){
			while(node!=null){
				if(node.data==null){
					removefast(node);
					return true;
				}
				node = node.next;
			}
		}else {
			while(node!=null){
				if(obj.equals(node.data)){
					removefast(node);
					return true;
				}
				node = node.next;
			}
		}
		return false;
	}
	private void removefast(Node node){
		node.previous.next=node.next;
		size--;
		node.data=null;
		node.previous = node.next = null;
	}

public Object set(int index,Object obj) {
		Node node = get(index);
		Object oldObject=node.data;
		node.data = obj;
		return oldObject;
	}