java - 如何限制 ArrayList 中的对象数量

Question

我只是在为我的 O.O.P. 做一些修改。下周就要考试了，我被一个问题困住了。这个问题基本上是举一个狗和跳蚤之间双向关联的例子。到目前为止，我有一只带跳蚤的狗。我坚持的部分是，“修改狗类，使狗对象最多只能容纳 5 个跳蚤对象(打印“你的狗有太多跳蚤!”如果跳蚤超过 5 个)。这是我的代码到目前为止:

狗.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 + ")";
    }

}

测试.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:

Freddy the Dog(5 岁)有跳蚤。他们是:[John(1 岁)、Patrick(3 岁)、Alan(7 岁)、Steven(2 岁)、Charles(5 岁)、Derek(1 岁)、Kevin(8 岁)]。

Answer 1

在此处添加检查您的条件:

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

不是在你的列表变量的定义中(就像你所做的那样)，因为你刚刚添加了 an instance initialization block .