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如何限制ArrayList中的对象数量

bidirectional arraylist java

Jaywin · 技术社区 · 9 年前

我正在为下周即将到来的O.O.P.考试做一些复习,我被一个问题卡住了。问题基本上是给出一个狗和跳蚤之间双向关联的例子。到目前为止,我有一只小狗和跳蚤。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(如果跳蚤超过5个,请打印“您的狗有太多跳蚤!”)。以下是我的代码:

Dog.java公司

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 + ".";
    }   

}

跳蚤.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);

    }

}

慰问:

狗弗雷迪(5岁)身上有跳蚤。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)]。

4 回复 | 直到 9 年前

Community CDub 8 年前

在此处添加检查条件:

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

不在列表变量的定义上(正如您所做的那样),因为您刚刚添加了 an instance initialization block .

mszalbach 9 年前

我猜你是被迫使用ArrayList的。因此,你必须确保除了你的狗类之外,没有人修改列表。因此,将列表设为私有

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

返回一份副本,不要在 getDogFlea 方法

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

并在您的 hostFlea 方法

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);
}

也许最后一个对你的考试来说已经足够了,但在一个真正的程序中,吸气者会遇到一个问题,确保永远不会有超过5个元素。

Mureinik 9 年前

狗的公共方法 想象上的 添加跳蚤是 hostFlea ,因此您需要更改:

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);
    }
}

然而 getDogFlea() 返回内部 ArrayList ,所以没有什么可以阻止确定的用户调用 dog.getDogFlea().add(flea6) 。为了防止此类行为,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 阵列列表 ,教科书解决方案将使用 Collections.unmodifiableList :

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

Amit Bhati 9 年前

在hostFlea方法中:-

在将Flea对象添加到Dog类中的跳蚤ArrayList之前,只需检查这个数组列表的大小。

喜欢:-

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