How to implement a custom ArrayList class in Java

1. Overall implementation idea

When customizing the ArrayList class, you need to think about many aspects in advance

1. The ArrayList needs to inherit the parent interface List

2 , need to rewrite the methods in the parent interface

3. The properties and methods that need to be used: the array elementData that specifically stores data. Since the data type of the data is not determined, the data type of the array is defined here as Object. , a member variable count is needed to record the number of elements in the array.

4. Provide a parameterless constructor and a parameterized constructor

5. Modify the content of the method according to needs

2. Add method

There are two methods, one is a method with only one formal parameter, the other is a method with two formal parameters

The add method with only one formal parameter

This method Plays the role of adding, adding the passed in elements to the already defined array

@Override
    public boolean add(Object o) {
    // 将传入的数据o放入数组中，该count是指代元素的个数
        elementData[count] = o;
    // 添加完成后需要将count加1
        count++;
        return true;
    }

There are two add methods with formal parameters

The first parameter of this method needs to be inserted The position of The count defined at the beginning is related to

@Override
    public void add(int index, Object element) {
        // todo 在指定位置插入元素
        for (int i = count; i >index; i--) {
            elementData[i]=elementData[i-1];
        }
        elementData[index]=element;
    }

4. isEmpty method

This method only needs to judge the count. If the count is 0, it means the array is empty

@Override
    public int size() {
        return count;
    }

5. Construction method

Initialize the array in the construction method and define the length of the array

@Override
    public boolean isEmpty() {
        return count==0;
    }

6. ArrayList overall code

    public ArrayList(Object[] eleArr) {
        this.elementData = eleArr;
    }

    public ArrayList() {
        // todo 初始化数组的长度
        elementData = new Object[10];
    }

7. Test test class

import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

public class ArrayList implements List {
    /*
     * 自定义实现ArrayList
     * 1、要实现List接口
     * 2、思考该类抽象出哪些属性-->数组用于保存元素的值elementData，数据类型为Object，用于计数的全局变量count
     * 3、思考方法--一个全参的构造方法，一个不带参的构造方法
     * 4、根据具体需求来实现具体的方法
     * */
    Object elementData[];
    int count;  // TODO  数组中的元素个数


    @Override
    public void add(int index, Object element) {
        // todo 在指定位置插入元素
        for (int i = count; i >index; i--) {
            elementData[i]=elementData[i-1];
        }
        elementData[index]=element;
    }

    @Override
    public boolean add(Object o) {
        elementData[count] = o;
        count++;
        return true;
    }

    public ArrayList(Object[] eleArr) {
        this.elementData = eleArr;
    }

    public ArrayList() {
        // todo 初始化数组的长度
        elementData = new Object[10];
    }

    @Override
    public int size() {
        return count;
    }

    @Override
    public boolean isEmpty() {
        return count==0;
    }

    @Override
    public boolean contains(Object o) {
        return false;
    }

    @Override
    public Iterator iterator() {
        return null;
    }

    @Override
    public Object[] toArray() {
        return new Object[0];
    }


    @Override
    public boolean remove(Object o) {
        return false;
    }

    @Override
    public boolean addAll(Collection c) {
        return false;
    }

    @Override
    public boolean addAll(int index, Collection c) {
        return false;
    }

    @Override
    public void clear() {

    }

    @Override
    public Object get(int index) {
        return elementData[index];
    }

    @Override
    public Object set(int index, Object element) {
        return null;
    }



    @Override
    public Object remove(int index) {
        return null;
    }

    @Override
    public int indexOf(Object o) {
        return 0;
    }

    @Override
    public int lastIndexOf(Object o) {
        return 0;
    }

    @Override
    public ListIterator listIterator() {
        return null;
    }

    @Override
    public ListIterator listIterator(int index) {
        return null;
    }

    @Override
    public List subList(int fromIndex, int toIndex) {
        return null;
    }

    @Override
    public boolean retainAll(Collection c) {
        return false;
    }

    @Override
    public boolean removeAll(Collection c) {
        return false;
    }

    @Override
    public boolean containsAll(Collection c) {
        return false;
    }

    @Override
    public Object[] toArray(Object[] a) {
        return new Object[0];
    }

}

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