java预期输出的值未显示

Question

我实例化了SpaceStation类并调用了addAstronaut方法，但在运行它时，名称、重量、高度和宇航员没有显示在我的输出中

空间站。爪哇：

public class SpaceStation {

   //private members
   private String name;
   private double stationWeight;
   private double altitude;

   private Astronaut[] Astronauts;
   private int totalAstronauts;

   //overloaded constructor
   public SpaceStation(String name, double 
   weight) {
      int altitude = 0;
      int totalAstronauts = 0; 
 }

  //method
  public void addAstronaut(String name, double 
     height, double weight) {
     Astronauts = new Astronaut[3];
     stationWeight = stationWeight + weight;
     totalAstronauts++;  
  }

  public double setAltitude(double altitude) { return this.altitude = altitude; }

  public String toString() {
     return "SpaceStation: " + name + "\n" +
            "Weight(kg): " + stationWeight + "\n" +
            "Altitude(km): " + (altitude) + "\n" +
            "Astronauts: " + (totalAstronauts);
  }

  public static void main(String[] args) {
     SpaceStation aa = new SpaceStation("ISS", 419700.0);
     System.out.println(aa);
     aa.addAstronaut("Eli", 167.64, 81.65);
     aa.addAstronaut("John", 185.43, 100.30);
     aa.addAstronaut("Joey", 175.38, 90.38);
     aa.setAltitude(400.0);
   
  }

 }

Answer 1

您的代码包含四个错误：

1-您没有在构造函数中设置实例变量name的内容。这会在打印输出时导致行SpaceStation: null。您需要在构造函数中设置名称。将构造函数更改为如下所示：

   public SpaceStation(String name, double 
   weight) {
      this.name = name;    // This line was added
      int altitude = 0;
      int totalAstronauts = 0; 
 }

2-在添加宇航员和设置高度之前，您正在打印ss的内容。当时没有增加宇航员，所以重量、宇航员人数和高度都是0是正常的。如果你在完成这些操作后打印出空间站的内容，它就会工作：

  public static void main(String[] args) {
     SpaceStation ss = new SpaceStation("ISS", 419700.0);
     
     ss.addAstronaut("Smith", 167.64, 81.65);
     ss.addAstronaut("John", 185.43, 100.30);
     ss.addAstronaut("Joey", 175.38, 90.38);
     ss.setAltitude(400.0);
     
     System.out.println(ss);    // This line was moved after the addAstronaut and setAltitude methods.
   
  }

3-正如@sagi标记的那样，在构造函数中，您正在声明和初始化另一个变量，而不是在类中，但名称相同。从技术上讲，double和integer已经用0.0初始化了，所以你不会真正注意到它，但是构造函数中的变量altitude和totalAstronauts实际上是无用的。建议如下更新构造函数：

   public SpaceStation(String name, double 
   weight) {
      this.name = name;
      altitude = 0;    // Removed "int"
      totalAstronauts = 0;  //Removed "int"
      Astronauts = new Astronaut[3];
 }

4-当@sagi标记时，每次添加宇航员时，都会初始化Astronaut数组。你需要在构造器中初始化它一次，并在每次添加一个时在阵列中设置宇航员

有了所有这些注释，SpaceStation类应该是这样的：

package gov.nasa.spacevehicles;

import gov.nasa.personnel.Astronaut;

public class SpaceStation {

   //private members
   private String name;
   private double stationWeight;
   private double altitude;

   private Astronaut[] Astronauts;
   private int totalAstronauts;

   //overloaded constructor
   public SpaceStation(String name, double 
   weight) {
      this.name = name;
      altitude = 0;
      totalAstronauts = 0; 
      Astronauts = new Astronaut[3];
 }


  //method
  public void addAstronaut(String name, double 
     height, double weight) {
     Astronauts[totalAstronauts] = new Astronaut(name, height, weight);
     stationWeight = stationWeight + weight;
     totalAstronauts++;  
  }

  public double setAltitude(double altitude) { return this.altitude = altitude; }

  public String toString() {
     return "SpaceStation: " + name + "\n" +
            "Weight(kg): " + stationWeight + "\n" +
            "Altitude(km): " + (altitude) + "\n" +
            "Astronauts: " + (totalAstronauts);
  }

  public static void main(String[] args) {
     SpaceStation ss = new SpaceStation("ISS", 419700.0);
     
     ss.addAstronaut("Smith", 167.64, 81.65);
     ss.addAstronaut("John", 185.43, 100.30);
     ss.addAstronaut("Joey", 175.38, 90.38);
     ss.setAltitude(400.0);
     
     System.out.println(ss);
   
  }

 }

Answer 2

（1）空间站的构造函数没有将传入的名称和权重值设置为空间站对象的参数。将this.name = name和this.stationWeight = weight添加到构造函数中。this关键字告诉Java，您所引用的变量是调用构造函数的对象的参数

构造函数还创建名为altitude和totalAstronauts的新变量，这些变量仅存在于构造函数内部。要更改调用构造函数的对象的参数值，请向构造函数添加this.altitude = altitude和this.totalAstronauts = totalAstronauts

//overloaded constructor
public SpaceStation(String name, double weight) {
      this.altitude = 0;
      this.totalAstronauts = 0;
      this.name = name;
      this.stationWeight = weight;
 }

（2）主方法在对其执行操作之前打印ss。将其放在主代码中的其他代码之后：

public static void main(String[] args) {
     SpaceStation ss = new SpaceStation("ISS", 419700.0);
     
     ss.addAstronaut("Smith", 167.64, 81.65);
     ss.addAstronaut("John", 185.43, 100.30);
     ss.addAstronaut("Joey", 175.38, 90.38);
     ss.setAltitude(400.0);

     System.out.println(ss);
  }

（3）每次调用AddSpauton（）时，都会覆盖宇航员数组。通过创建一个新的宇航员实例并将其添加到AddSpauton（）中的宇航员数组中，可以解决这个问题。应该在构造函数中添加一行，以将数组初始化为大小为3或更大的空数组，这样就有空间在AddSpauton（）中添加宇航员对象

//overloaded constructor
public SpaceStation(String name, double weight) {
      this.altitude = 0;
      this.totalAstronauts = 0;
      this.name = name;
      this.stationWeight = weight;

      // Add this line
      Astronauts = new Astronaut[10];
 }

//method
  public void addAstronaut(String name, double height, double weight) {
     
     // Adds a new Astronaut object to the array at index totalAstronauts
     Astronauts[totalAstronauts] = new Astronaut(name, height, weight);
     
     stationWeight = stationWeight + weight;
     totalAstronauts++;  
  }

Answer 3

主要有两个错误导致name显示为空
1：您没有在构造函数中设置name，这导致name的默认值为null
所以当你说this.name = name它告诉我我有这个，请看并设置它的值

//overloaded constructor
       public SpaceStation(String name) {
          this.name = name; // changed
          int altitude = 0;
          Astronauts = new Astronaut[3];
          int totalAstronauts = 0; 
     }

同样在main函数中，您试图在设置值之前打印它。 你应该先给出它的值，然后再打印出来

public static void main(String[] args) {
     SpaceStation ss = new SpaceStation("ISS", 419700.0);
     
     ss.addAstronaut("Smith", 167.64, 81.65);
     ss.addAstronaut("John", 185.43, 100.30);
     ss.addAstronaut("Joey", 175.38, 90.38);
     ss.setAltitude(400.0);
     
     System.out.println(ss); // changed
   
  }

此外，我看到你制作了一个Astronaut[] Astronauts阵列，但从未在其中添加任何宇航员。那么它的全部意义是什么呢？ 添加宇航员时，它只存储astronauts的总数，而不存储实际的Astronauts Object

   //method
      public void addAstronaut(String name, double 
         height, double weight, Astronaut object) {
          Astronauts[totalAstronauts] = object; // changed
         stationWeight = stationWeight + weight;
         totalAstronauts++;  
      }

主要内容：

public static void main(String[] args) {
         SpaceStation ss = new SpaceStation("ISS");
         
         ss.addAstronaut("Smith", 167.64, 81.65, new Astronaut("Smith", 167.64, 81.65));
         ss.addAstronaut("John", 185.43, 100.30, new Astronaut("John", 185.43, 100.30));
         ss.addAstronaut("Joey", 175.38, 90.38, new Astronaut("Joey", 175.38, 90.38));
         ss.setAltitude(400.0);
         System.out.println(ss);
         
         // (Added) Printing astronauts from array
         for(Astronaut a : ss.Astronauts) {
             System.out.println(a);
         }

}

宇航员班：

public class Astronaut {
     //private fields
       private String name;
       private double height;
       private double weight;

       //defualt constructor
       public Astronaut() { }

       //overloaded constructor
      public Astronaut (String name, double height, double weight) {
         this.name = name;
         this.height = height;
         this.weight = weight;
     } 
      
        // Added 
      public String toString() {
          return "\nAstronaut Name: "+name+"\n"+name+" Height: "+height+"\n"+name+" Weight: "+weight;
      } 

}

输出：

SpaceStation: ISS
Weight(kg): 272.33
Altitude(km): 400.0
Astronauts: 3

Astronaut Name: Smith
Smith Height: 167.64
Smith Weight: 81.65

Astronaut Name: John
John Height: 185.43
John Weight: 100.3

Astronaut Name: Joey
Joey Height: 175.38
Joey Weight: 90.38