java - 如何将ArrayList类转换为抽象类

我正在学习Java，不理解为什么这段代码会产生以下错误：“ ArrayListClass是抽象的；无法实例化。将不胜感激。

import java.util.*;

public class ArrayListClass {
    protected Object[] list;
    protected int maxSize;
    protected int length;

    public ArrayListClass() {
      maxSize = 100;
      length = 0;
      list = new Object[maxSize];
    }

    public ArrayListClass(int size) {
      maxSize = size;
      list = new Object[maxSize];
      length = 0;
    }

    public boolean isEmpty() {
      return length == 0;
    }

    public boolean isFull() {
      if (length == maxSize)
         return true;
      else
         return false;
    }

    public int listSize() {
      return length;
    }

    public int maxListSize(){
      return maxSize;
    }

    public void print() {
      System.out.print("The list contains:");
      for(int i = 0; i < length; i++)
         System.out.print(list[i] + " ");
      System.out.println();
    }

    public boolean isItemAtEqual(int location, Object item) {
      return (list[location].equals(item));
    }

    public void insertEnd(Object item) {
      if(!isFull())
         list[length++] = item;
    }

    public static void main(String [] args) {
        ArrayListClass dac = new ArrayListClass(5);
        dac.insertEnd(4);
        dac.insertEnd(5);
        dac.insertEnd(6);
        dac.print();
        System.out.println("dac.isItemAtEqual(0,9)"+dac.isItemAtEqual(0,9));
        System.out.println("dac.isItemAtEqual(1,9)"+dac.isItemAtEqual(1,9));
    }
}

最佳答案

您不能以任何编程语言实例化任何抽象类。抽象的基本构造是，它仅仅是蓝图，而不是真实的对象。它提供类的模板，并将类的形式或轮廓提供给实现类的具体类（“扩展”类...）

因此，您无法实例化ArrayListClass，因为这给出了一个蓝图。如果您扩展此类，例如DerievedArrayListClass扩展ArrayListClass，则可以实例化DerievedArrayListClass。

package com;

abstract  class ArrayListClass{
    protected Object [] list;
    protected int maxSize;
    protected int length;

    public ArrayListClass(){
        maxSize = 100;
        length = 0;
        list = new Object [maxSize];
    }
    public ArrayListClass(int size){
        maxSize=size;
        list=new Object [maxSize];
        length=0;
    }


    public boolean isEmpty(){
        return length==0;
    }
    public boolean isFull(){
        if(length==maxSize)
            return true;
        else
            return false;
    }


    public int listSize(){
        return length;
    }

    public int maxListSize(){
        return maxSize;
    }

    abstract void  print();

    public boolean isItemAtEqual(int location, Object item)
    {
        return (list[location].equals(item));
    }
    public void insertEnd(Object item){
        if(!isFull())
            list[length++] = item;
    }

}

public class ArrayListClassImpl extends ArrayListClass{

    public ArrayListClassImpl(int i) {
        super(i);
    }

    public void print(){
        System.out.print("The list contains:");
        for(int i = 0; i < length; i++)
            System.out.print(list[i] + " ");
        System.out.println();
    }

    public static void main(String [] args){
        ArrayListClass dac = new ArrayListClassImpl(5);
        dac.insertEnd(4);
        dac.insertEnd(5);
        dac.insertEnd(6);
        dac.print();
        System.out.println("dac.isItemAtEqual(0,9)"+dac.isItemAtEqual(0,9));
        System.out.println("dac.isItemAtEqual(1,9)"+dac.isItemAtEqual(1,9));
    }
}