深入理解Java HashSet：自定义对象移除机制与常见陷阱

public Penny removePenny(){
    if (myset.size() > 0){
        myset.remove(Penny); // 错误：Penny是类型名，不是对象实例
    } else {
        return null;
    }
}

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

// 假设Penny类有某种可区分的状态，例如一个ID
public class Penny {
    private final int id;

    public Penny(int id) {
        this.id = id;
    }

    public int getId() {
        return id;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Penny penny = (Penny) o;
        return id == penny.id; // 基于ID判断相等
    }

    @Override
    public int hashCode() {
        return Objects.hash(id); // 基于ID生成哈希码
    }

    @Override
    public String toString() {
        return "Penny{id=" + id + '}';
    }
}

public class PocketWithHashSet {
    private Set<Penny> myset;

    public PocketWithHashSet(int pocketSize) {
        myset = new HashSet<>();
        for (int i = 0; i < pocketSize; i++) {
            myset.add(new Penny(i)); // 添加具有不同ID的Penny
        }
    }

    public int pennyCount() {
        return myset.size();
    }

    // 正确的移除方法：需要指定要移除的Penny实例
    public boolean removeSpecificPenny(Penny pennyToRemove) {
        if (pennyToRemove == null) {
            return false;
        }
        return myset.remove(pennyToRemove);
    }

    // 如果想移除“任意一个”Penny，HashSet本身没有直接提供这个功能
    // 可以通过迭代器实现，但这不是Set设计的初衷
    public Penny removeAnyPenny() {
        if (myset.isEmpty()) {
            return null;
        }
        // 获取一个迭代器，移除第一个元素
        java.util.Iterator<Penny> iterator = myset.iterator();
        if (iterator.hasNext()) {
            Penny removedPenny = iterator.next();
            iterator.remove(); // 使用迭代器的remove方法移除当前元素
            return removedPenny;
        }
        return null; // 理论上不会执行到这里
    }

    public static void main(String[] args) {
        PocketWithHashSet pocket = new PocketWithHashSet(5);
        System.out.println("初始硬币数量: " + pocket.pennyCount()); // 5

        Penny penny3 = new Penny(3); // 创建一个与集合中ID为3的硬币相等的对象
        boolean removed = pocket.removeSpecificPenny(penny3);
        System.out.println("移除ID为3的硬币是否成功: " + removed); // true
        System.out.println("移除后硬币数量: " + pocket.pennyCount()); // 4

        Penny nonExistentPenny = new Penny(99); // 一个不存在的硬币
        removed = pocket.removeSpecificPenny(nonExistentPenny);
        System.out.println("移除ID为99的硬币是否成功: " + removed); // false
        System.out.println("移除后硬币数量: " + pocket.pennyCount()); // 4

        // 移除任意一个硬币
        Penny anyRemoved = pocket.removeAnyPenny();
        if (anyRemoved != null) {
            System.out.println("移除了任意一个硬币: " + anyRemoved);
        }
        System.out.println("再次移除后硬币数量: " + pocket.pennyCount()); // 3
    }
}

import java.util.ArrayList;
import java.util.List;

public class PocketWithList {
    private List<Penny> myset; // 改用List

    public PocketWithList(int pocketSize) {
        myset = new ArrayList<>();
        for (int i = 0; i < pocketSize; i++) {
            myset.add(new Penny(i)); // 假设Penny有ID，但ArrayList允许重复
        }
    }

    public int pennyCount() {
        return myset.size();
    }

    public Penny removePenny() {
        if (myset.size() > 0) {
            // 移除最后一个元素（或myset.remove(0)移除第一个）
            return myset.remove(myset.size() - 1);
        } else {
            return null;
        }
    }

    public static void main(String[] args) {
        PocketWithList pocket = new PocketWithList(5);
        System.out.println("初始硬币数量: " + pocket.pennyCount()); // 5

        Penny removedPenny = pocket.removePenny();
        if (removedPenny != null) {
            System.out.println("移除了一个硬币: " + removedPenny); // 移除最后一个
        }
        System.out.println("移除后硬币数量: " + pocket.pennyCount()); // 4
    }
}

import java.util.ArrayDeque;
import java.util.Queue;

public class PocketWithQueue {
    private Queue<Penny> myset; // 改用Queue

    public PocketWithQueue(int pocketSize) {
        myset = new ArrayDeque<>();
        for (int i = 0; i < pocketSize; i++) {
            myset.add(new Penny(i)); // 假设Penny有ID
        }
    }

    public int pennyCount() {
        return myset.size();
    }

    public Penny removePenny() {
        return myset.poll(); // 移除并返回队列头部元素，如果队列为空则返回null
    }

    public static void main(String[] args) {
        PocketWithQueue pocket = new PocketWithQueue(5);
        System.out.println("初始硬币数量: " + pocket.pennyCount()); // 5

        Penny removedPenny = pocket.removePenny();
        if (removedPenny != null) {
            System.out.println("移除了一个硬币: " + removedPenny); // 移除第一个
        }
        System.out.println("移除后硬币数量: " + pocket.pennyCount()); // 4
    }
}