Question

我正在学习Java，可以在这里使用一些帮助。

目前我正在进行多态，所以我的代码有一个Dog类，另一个类有一个方法可以将Dog对象添加到列表中（有点像ArrayList）。

    package chap8;

    /**
     * Created by user1 on 7/11/15.
     */

    class Dog {
        private String name;
        private int age;

        public int getAge() {
            return age;
        }

        public String getName() {
            return name;
        }

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

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

    public class MyDogList {

        //array to hold dog objects
        private Dog[] dogs = new Dog[5];

        //int to check index in dog array
        private int nextIndex = 0;

        //add method to add dog objects to dog array
        private void add(Dog d) {
            if(nextIndex < dogs.length) {
                dogs[nextIndex] = d;
                nextIndex++;
                System.out.println("Dog added at position: " + nextIndex);
                System.out.println("Space left for " + (dogs.length - nextIndex) + " more dogs.");
            }

            else {
                System.out.println("No more space to add dogs!");
            }
        }

   private void display() {
        for (int i = 0; i < dogs.length; i++) {
            System.out.println("Name of Dog: " + dogs[i].getName());

        }
    }

        public static void main(String[] args) {
            Dog obj1 = new Dog();
            obj1.setName("Corgi");
            obj1.setAge(2);

            MyDogList dogList = new MyDogList();
            dogList.add(obj1);
            dogList.display();

            Dog obj2 = new Dog();
            obj2.setName("Labrador");
            obj2.setAge(3);

            dogList.add(obj2);

            Dog obj3 = new Dog();
            obj3.setName("Golden Retriever");
            obj3.setAge(5);

            dogList.add(obj3);
        }
    }

虽然添加Dog对象的过程有效，但我想要的是另一种方法display，可能在class MyDogList中，它会显示dogs数组中所有Dog对象的名称。 MyDogList课程。但后来我意识到，如果该方法只接受一个Dog数组，那么就无法知道这个数组中各个dog对象的名称。

我该如何构建这样的方法？

修改

我从评论和答案中意识到，我可以使用getter获取Dog变量name来获取名称。但是当我修改代码时（上面用粗体显示），我得到一个Null Pointer Exception。什么地方出了错？在设置为某个位置之前，我是否尝试访问数组位置？

/usr/lib/jvm/java-8-oracle/bin/java -Didea.launcher.port=7535 -Didea.launcher.bin.path=/home/user1/Documents/idea-IC-141.177.4/bin -Dfile.encoding=UTF-8 -classpath "/home/user1/Dropbox/Development/Java/Head First/bin:/usr/lib/jvm/java-8-oracle/jre/lib/plugin.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jce.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jfr.jar:/usr/lib/jvm/java-8-oracle/jre/lib/rt.jar:/usr/lib/jvm/java-8-oracle/jre/lib/charsets.jar:/usr/lib/jvm/java-8-oracle/jre/lib/resources.jar:/usr/lib/jvm/java-8-oracle/jre/lib/javaws.jar:/usr/lib/jvm/java-8-oracle/jre/lib/management-agent.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jsse.jar:/usr/lib/jvm/java-8-oracle/jre/lib/jfxswt.jar:/usr/lib/jvm/java-8-oracle/jre/lib/deploy.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/jfxrt.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-8-oracle/jre/lib/ext/sunjce_provider.jar:/home/user1/Documents/idea-IC-141.177.4/lib/idea_rt.jar" com.intellij.rt.execution.application.AppMain chap8.MyDogList
Exception in thread "main" Dog added at position: 1
Space left for 4 more dogs.
Name of Dog: Corgi
java.lang.NullPointerException
    at chap8.MyDogList.display(MyDogList.java:52)
    at chap8.MyDogList.main(MyDogList.java:64)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

Process finished with exit code 1

Answer 1

这样的事可能吗？

public void diplay(){
  for (Dog dog : dogs){
    if (dog != null && dog.getName() != null){
      System.out.println("Name of Dog: " + dog.getName());
    }
  }
}

你的DogList类已经包含了狗的数组。无需将狗数组传递给显示方法。

另请注意检查null，因为您无法确定所有数组位置是否实际已填充。

Answer 2

据我所知，你在MyDogList类中需要这样的东西：

public void display(){
    for (int i = 0; i < dogs.length; i++) {
        System.out.println("Name of Dog: " + dogs[i].getName());
    }
}

Answer 3

我认为这就是你要找的东西。

public void display() {
   for(int i=0; i<nextIndex; i++) {
      if(null != this.dogs[i]) {
         System.out.println(this.dogs[i].getName());
      }
   }

迭代到最后添加的对象并在Dog对象中打印名称。你不需要遍历所有数组，你只需要设置nextIndex就可以了。

Answer 4

<强>解。

添加了显示方法。执行null检查，否则返回nullpointer 异常将发生。

private void display() {
    for (Dog d : dogs) {
        // null check
        if (d != null) {
            System.out.println("DogName:" + d.getName());
        }
    }
}

通过方法引用另一个类的类变量

4 个答案: