自己實現 ArrayList

英文原文

ArrayList 類似於 Array 對象，但是當列表中的對象數量增加時，它提供了動態空間分配的功能。在 Array 對象中，我們需要在初始化時提供數組的大小，但這並不是 ArrayList 所必需的。實際上，當您初始化ArrayList時，它將自動將其容量分配爲10。

這裏我用 Array 對象實現ArrayList，並提供 get(index)，add(object) 和 remove(index) 等基本函數。

import java.util.ArrayList;

/**
 * <p></p>
 *
 * @author 三產
 * @version 1.0
 * @date 2017-03-24
 * @QQGroup 213732117
 * @website http://www.coderknock.com
 * @copyright Copyright 2017 拿客 coderknock.com  All rights reserved.
 * @since JDK 1.8
 */
public class MyArrayList {

        private static final int SIZE_FACTOR=5;

        private Object data[];

        private int index;

        private int size;

        public MyArrayList(){
            this.data=new Object[SIZE_FACTOR];
            this.size=SIZE_FACTOR;
        }

        public void add(Object obj){
            System.out.println("index:"+this.index+"size:"+this.size+"data size:"+this.data.length);
            if(this.index==this.size-1){
                //我們需要加大data[]的大小
                increaseSizeAndReallocate();
            }
            data[this.index]=obj;
            this.index++;

        }

        private void increaseSizeAndReallocate() {
            this.size=this.size+SIZE_FACTOR;
            Object newData[]=new Object[this.size];
            for(int i=0; i<data.length;i++){
                newData[i]=data[i];
            }
            this.data=newData;
            System.out.println("***index:"+this.index+"size:"+this.size+"data size:"+this.data.length);
        }

        public Object get(int i) throws Exception{
            if(i>this.index-1){
                throw new Exception("ArrayIndexOutOfBound");
            }
            if(i<0){
                throw new Exception("Negative Value");
            }
            return this.data[i];

        }

        public void remove(int i) throws Exception{
            if(i>this.index-1){
                throw new Exception("ArrayIndexOutOfBound");
            }
            if(i<0){
                throw new Exception("Negative Value");
            }
            System.out.println("Object getting removed:"+this.data[i]);
            for(int x=i; x<this.data.length-1;x++){
                data[x]=data[x+1];
            }
            this.index--;
        }

        public static void main(String[] args) throws Exception {
            MyArrayList mal = new MyArrayList();
            mal.add("0");
            mal.add("1");
            mal.add("2");
            mal.add("3");
            mal.add("4");
            mal.add("5");
            mal.add("6");
            mal.add("7");
            mal.add("8");
            mal.add("9");

            mal.remove(5);
            System.out.println(mal.get(7));
        }

}

輸出如下：

index:0size:5data size:5
index:1size:5data size:5
index:2size:5data size:5
index:3size:5data size:5
index:4size:5data size:5
***index:4size:10data size:10
index:5size:10data size:10
index:6size:10data size:10
index:7size:10data size:10
index:8size:10data size:10
index:9size:10data size:10
***index:9size:15data size:15
Object getting removed:5
8

這只是使用 Array 基本實現 ArrayList 的功能，只爲了解其原理。還請使用 JDK 中的 ArrayList。

原文中併爲實現泛型，我們就來補充下：

import java.util.ArrayList;

/**
 * <p></p>
 *
 * @author 三產
 * @version 1.0
 * @date 2017-03-24
 * @QQGroup 213732117
 * @website http://www.coderknock.com
 * @copyright Copyright 2017 拿客 coderknock.com  All rights reserved.
 * @since JDK 1.8
 */
public class MyArrayList<T> {

        private static final int SIZE_FACTOR=5;

        private Object data[];

        private int index;

        private int size;

        public MyArrayList(){
            this.data=new Object[SIZE_FACTOR];
            this.size=SIZE_FACTOR;
        }

        public void add(T obj){
            System.out.println("index:"+this.index+"size:"+this.size+"data size:"+this.data.length);
            if(this.index==this.size-1){
                //我們需要加大data[]的大小
                increaseSizeAndReallocate();
            }
            data[this.index]=obj;
            this.index++;

        }

        private void increaseSizeAndReallocate() {
            this.size=this.size+SIZE_FACTOR;
            Object newData[]=new Object[this.size];
            for(int i=0; i<data.length;i++){
                newData[i]=data[i];
            }
            this.data=newData;
            System.out.println("***index:"+this.index+"size:"+this.size+"data size:"+this.data.length);
        }

        public T get(int i) throws Exception{
            if(i>this.index-1){
                throw new Exception("ArrayIndexOutOfBound");
            }
            if(i<0){
                throw new Exception("Negative Value");
            }
            return (T) this.data[i];

        }

        public void remove(int i) throws Exception{
            if(i>this.index-1){
                throw new Exception("ArrayIndexOutOfBound");
            }
            if(i<0){
                throw new Exception("Negative Value");
            }
            System.out.println("Object getting removed:"+this.data[i]);
            for(int x=i; x<this.data.length-1;x++){
                data[x]=data[x+1];
            }
            this.index--;
        }

        public static void main(String[] args) throws Exception {
            MyArrayList<Integer> mal = new MyArrayList<Integer>();
            mal.add(0);
            mal.add(1);
            mal.add(2);
            mal.add(3);
            mal.add(4);
            mal.add(5);
            mal.add(6);
            mal.add(7);
            mal.add(8);
            mal.add(9);

            mal.remove(5);
            System.out.println(mal.get(7));
        }

}

這裏其實只是修改了 add(T)、get(index) 方法。