如何限制ArrayList中的对象数量

Question

我只是在为下周的O.O.P.考试做些复习，我被一个问题困住了。这个问题基本上是给出狗和跳蚤之间双向关联的一个例子。到目前为止，我养了一只跳蚤狗。我坚持的部分是，“修改狗类，以便狗对象最多只能容纳5个跳蚤对象(打印”您的狗有太多跳蚤！“如果有超过5只跳蚤)。

Dog.java

import java.util.ArrayList;

public class Dog {

    private String name;
    private int age;
    private String address;

    ArrayList<Flea> fleas = new ArrayList<Flea>(); {
        if(fleas.size() > 5) {
             System.out.println("This dog has too many fleas!");
        }
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public void hostFlea(Flea flea) {
        fleas.add(flea);
    }

    public ArrayList<Flea> getDogFlea() {
         return fleas;
    }

    public String toString() {
        return name + " the Dog (aged " + age + ") has fleas. \nThey are: " + fleas + ".";
    }   

}

Flea.java

public class Flea {

    private String name;
    private int age; 

    public Flea (String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String toString() {
        return name + " (aged " + age + ")";
    }

}

Test.java

public class Test {

    public static void main(String[] args) {

        Dog dog = new Dog();
            dog.setName("Freddy");
            dog.setAddress("Cork");
            dog.setAge(5);

        Flea flea1 = new Flea("John", 1);
        dog.hostFlea(flea1);

        Flea flea2 = new Flea("Patrick", 3);        
        dog.hostFlea(flea2);

        Flea flea3 = new Flea("Alan", 7);
        dog.hostFlea(flea3);

        Flea flea4 = new Flea("Steven", 2);
        dog.hostFlea(flea4);

        Flea flea5 = new Flea("Charles", 5);
        dog.hostFlea(flea5);

        Flea flea6 = new Flea("Derek", 1);
        dog.hostFlea(flea6);

        Flea flea7 = new Flea("Kevin", 8);
        dog.hostFlea(flea7);

        System.out.println(dog);

    }

}

控制台：

狗弗雷迪(5岁)有跳蚤。他们是:约翰(1岁)、帕特里克(3岁)、艾伦(7岁)、史蒂文(2岁)、查尔斯(5岁)、德里克(1岁)、凯文(8岁)。

Answer 1

在这里添加检查您的条件：

public void hostFlea(Flea flea) {
    if(fleas.size() >= 5) {
        System.out.println("This dog has too many fleas!");
    } else {
        fleas.add(flea);
    }
}

而不是在列表变量的定义上(就像您所做的那样)，因为您刚刚添加了an instance initialization block。

Answer 2

我想你是被迫使用ArrayList的。因此，你必须确保没有人修改的名单，其他作为你的狗类。所以把清单写成私密

private ArrayList<Flea> fleas = new ArrayList<Flea>();

返回副本，而不公开getDogFlea方法中的原始列表

public ArrayList<Flea> getDogFlea() {
     return new ArrayList(fleas); //use copy constructor to not expose original list
}

并在hostFlea方法中添加检查。

public void hostFlea(Flea flea) {
    if(fleas.size() == 5 ) {
        System.out.println("This dog has too many fleas!");
        return; //so the 6th element is not added
    }
    fleas.add(flea);
}

也许最后一次考试就足够了，但是在一个真正的程序中，getter会成为一个问题，确保永远不会有超过5个元素。

Answer 3

狗应该添加跳蚤的公共方法是hostFlea，所以您需要修改它：

public void hostFlea(Flea flea) {
    // If the dog already has at least 5 fleas, you can't add another
    if (fleas.size() >= 5) {
        System.out.println("Your dog has too many fleas!");
    } else {
        fleas.add(flea);
    }
}

但是，getDogFlea()返回内部ArrayList，因此没有什么可以阻止确定的用户调用dog.getDogFlea().add(flea6)。为了防止这种行为，您可以复制数据：

public ArrayList<Flea> getDogFlea() {
     return new ArrayList<>(fleas);
}

或者，以放松API以返回List而不是ArrayList的“代价”，教科书解决方案将是使用Collections.unmodifiableList。

public List<Flea> getDogFlea() {
     return Collections.unmodifiableList(fleas);
}