MyArrayList.java

/*
 * created by Harry 2017-2-20 18:53:38
 * 实现简单的ArrayList,具有基本的增删改查功能
 */
package com.github.HarryHook.coding2017.basic;

import java.util.Arrays;
import java.util.NoSuchElementException;

public class MyArrayList implements List {
    private int size = 0; // 数组元素个数

    private Object[] elementData = new Object[10]; // 初始化数组大小为10

    // 将元素添加到数组尾部
    public void add(Object o) { // 需要判断数组空间是否够用
	ensureCapacity(size + 1);
	elementData[size++] = o;
    }

    // 在指定位置添加元素
    public void add(int index, Object o) {
	// 判断下标记是否越界
	if (index > size || index < 0)
	    throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
	ensureCapacity(size + 1);
	// 判断当前位置是否有元素，没有元素添加到当前位置；若有，当前元素及之后元素向右移
	if (elementData[index] == null) {
	    elementData[index] = o;
	} else {
	    for (int i = elementData.length - 1; i > index; i--) {
		elementData[i] = elementData[i - 1];
	    }
	    elementData[index] = o;
	}
	size++;

	/*
	 * //判断索引位置是否正确 if (index > size || index < 0) throw new
	 * IndexOutOfBoundsException( "Index: " + index + ", Size: " + size);
	 * //扩容检测 ensureCapacity(size+1); /* 对源数组进行复制处理（位移），从index +
	 * 1到size-index。 主要目的就是空出index位置供数据插入， 即向右移动当前位于该位置的元素以及所有后续元素。
	 * 
	 * System.arraycopy(elementData, index, elementData, index + 1, size -
	 * index); //在指定位置赋值 elementData[index] = 0; size++;
	 * 
	 */
    }

    public Object get(int index) {
	// 若index超出size应该抛出异常
	if (index >= size)
	    throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
	return elementData[index];

    }

    public Object remove(int index) { // 涉及到元素移位
	Object oldValue = elementData[index];
	for (int i = index; i < elementData.length - 1; i++)
	    elementData[i] = elementData[i + 1];
	elementData[--size] = null;

	return oldValue;
    }

    // 判断是否需要给数组扩容
    public void ensureCapacity(int minCapacity) {

	int oldCapacity = elementData.length;
	if (minCapacity > oldCapacity) {
	    // Object oldData[] = elementData;
	    // //防止copyof()执行的过程中新内存或者其他进程分配内存时占用旧内存
	    int newCapacity = (oldCapacity * 3) / 2 + 1; // 增加50%+1
	    if (newCapacity < minCapacity)
		newCapacity = minCapacity;
	    // minCapacity is usually close to size, so this is a win:
	    elementData = Arrays.copyOf(elementData, newCapacity);
	}
    }

    // 返回数组的大小
    public int size() {
	return size;
    }

    public Iterator iterator() {
	return new MyArrayListIterator();
    }

    private class MyArrayListIterator implements Iterator {
	private int cursor = 0; // 记录索引位置

	public boolean hasNext() {
	    return cursor != size;
	}

	public Object next() {
	    try {
		Object next = get(cursor);
		cursor++;
		return next;

	    } catch (IndexOutOfBoundsException e) {
		throw new NoSuchElementException();
	    }

	}
    }

    public static void main(String[] args) {
	MyArrayList myArrays = new MyArrayList();
	myArrays.add(3);
	myArrays.add(0, 11);
	myArrays.add(1, 2);
	myArrays.add(3, 5);
	myArrays.add(2, 1);
	myArrays.add(7);
	Print(myArrays);

	for (int i = 0; i < 19; i++)
	    myArrays.add(i, 55);

	System.out.println("获取指定位置元素： " + myArrays.get(2));
	System.out.println("删除指定位置元素： " + myArrays.remove(1));
	System.out.println("当前元素个数：" + myArrays.size());

	Print(myArrays);

    }

    public static void Print(MyArrayList myArrays) {
	Iterator it = myArrays.iterator();
	System.out.println("对链表中的元素进行打印：");
	while (it.hasNext())
	    System.out.print(it.next() + " ");
	System.out.println("");
	System.out.println("当前元素个数： " + myArrays.size());

    }

}