java - 试图从 Arraylist 中删除一个对象 - 仍然可以访问对象

Question

我正在尝试添加函数以从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;
}

我不确定我是否理解正确，但如果我设置 delElem = 满足相同 id 的对象，删除我的对象并将其设置为 null，则该对象不应再引用。但是如果我在我的主要功能中测试它，例如 book.getTitle()，标题将被打印出来。我认为这是因为它只是我函数中的一个本地对象。如何在函数中全局删除对象并在成功删除对象时返回 boolean 值？

谢谢!

Answer 1

评论中已经说明您通常不会手动删除对象(通过将它们设置为空)。垃圾回收将在不再需要它们时完成它的工作。

代码中的奇怪之处在于媒体将自己添加到列表中。为什么不创建另一个代表书架的类。这样你就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());
    }

}