尝试从arraylist删除对象 - 仍然可以访问对象

Question

我正在尝试将函数添加到我的数组列表中删除对象(首先，我想为这些对象生成唯一的随机ID：

import java.util.ArrayList;
import java.util.ListIterator;
import java.util.Random;
public class Medium {
    public Medium(){
        this.id = generateID();
        System.out.println("ID: " + id);
        mlist.add(this);
    }
    protected int generateID() {
        Random random = new Random();
        int id;
        do {
            id = 100 + (random.nextInt(999-100));
        } while(idlist.contains(id));
         idlist.add(id);
        return id;
          }
    protected boolean delete(Medium delElem) {
        boolean isDeleted = false;
        ListIterator<Medium> it = mlist.listIterator(); 
                while(it.hasNext()) {
                it = mlist.listIterator();
                Medium next = it.next();
                if(delElem.getID() == next.getID()) {
                    delElem = next;
                    mlist.remove(delElem);
                    delElem = null;
                    if(delElem == null) {
                        System.out.println("Succesfull deleted");
                    }
                    isDeleted = true;
                    break;
                } 
            }
        return isDeleted;
        }
        protected int getID() {
            return id;
        }
        public String getTitle() {
            return title;
        }

    protected ArrayList<Medium> mlist = new ArrayList<Medium>();
    private ArrayList<Integer> idlist = new ArrayList<Integer>();
    protected String title;
    protected final int id;
}

我不确定我是否正确理解了这一点，但是如果我设置dellem =符合均等ID的对象，删除我的对象并将其设置为= null，则该对象不再引用。但是，如果我在主要功能中对其进行测试，例如book.gettitle((，标题将被打印。我认为这只是我功能中的本地对象。如果对象被成功地删除，我如何在函数中全球删除我的对象并返回布尔值？

谢谢！

Answer 1

在评论中已经说明了您通常不会手动删除对象(通过将它们设置为null(。当垃圾收集不再需要时，垃圾收集将完成工作。

您代码中的奇怪之处在于，媒体将自己添加到列表中。您为什么不创建代表书架的另一个类。这样，您可以在书架上实现方法来添加或删除书籍：

package com.example;
import java.util.ArrayList;
import java.util.Random;
public class BookShelf {
    protected ArrayList<Medium> mlist = new ArrayList<Medium>();
    private ArrayList<Integer> idlist = new ArrayList<Integer>();
    private class Medium {
        protected int id;
        protected String title;
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getTitle() {
            return title;
        }
        public void setTitle(String title) {
            this.title = title;
        }
    }
    public Medium addMedium(String title) {
        Medium medium = new Medium();
        medium.setId(this.generateID());
        medium.setTitle(title);
        this.mlist.add(medium);
        return medium;
    }
    protected int generateID() {
        Random random = new Random();
        int id;
        do {
            id = 100 + (random.nextInt(999 - 100));
        } while (idlist.contains(id));
        idlist.add(id);
        return id;
    }
    protected boolean delete(Medium delElem) {
        boolean isDeleted = false;
        for (Medium medium : mlist) {
            if (medium.getId() == delElem.getId()) {
                mlist.remove(medium);
                isDeleted = true;
                break;
            }
        }
        return isDeleted;
    }
    public ArrayList<Medium> getAllBooks() {
        return this.mlist;
    }
    @Override
    public String toString() {
        StringBuilder strBuilder = new StringBuilder();
        strBuilder.append("Books in the shelf: " + this.getAllBooks().size() + "n");
        for(Medium medium : this.getAllBooks()) {
            strBuilder.append("Title: " + medium.getTitle() + "n");
        }
        return strBuilder.toString();
    }
    public static void main(String[] args) {
        BookShelf bookShelf = new BookShelf();
        //Add two books to the shelf
        Medium medium1 = bookShelf.addMedium("Book 1");
        Medium medium2 = bookShelf.addMedium("Book 2");
        System.out.println(bookShelf.toString());
        //Delete one
        bookShelf.delete(medium1);
        System.out.println(bookShelf.toString());
    }
}