The values from expected output are not showing

I instantiated the SpaceStation class and called addAstronaut method but the name, weight, altitude, and astronauts are not showing in my output when I run it.

SpaceStation.java:

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);
}
}

Your code contains four errors:

1- You are not setting the content of the instance variable name in the constructor. This causes the line SpaceStation: null when printing the output. You need to set the name in the constructor. Change the constructor to be like this:

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

2- You are printing the content of ss before adding your astronauts and setting the altitude. At that time, there are no astronauts added, so it's normal that the weight, number of astronauts and altitude is 0. If you print the content of the space station after doing those operations, it's going to work:

  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- As @sagi flagged, in the constructor, you're declaring and initializing another variable than in your class, but with the same name. Technically, doubles and integers are already initialized with 0.0 so you don't really notice it, but the variables altitude and totalAstronauts in your constructor are useless as they are. Suggesting to update the constructor like this:

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

4- As @sagi flagged, you're re-initializing the Astronaut array every time you add an astronaut. You need to initialize it in the constructor once, and presumably set the astronauts in the array every time you add one.

With all of those comments, the SpaceStation class should look like this:

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);
}
}

(1) The constructor of SpaceStation does not set the passed in values of name and weight to the parameters of the SpaceStation object. Add this.name = name and this.stationWeight = weight to the constructor. The this keyword tells Java that the variable you are referring to is the parameter of the object that the constructor is being called for.

The constructor also creates new variables named altitude and totalAstronauts that exist only inside of the constructor. To change the values of the parameters of the object the constructor is called for, add this.altitude = altitude and this.totalAstronauts = totalAstronauts to the constructor.

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

(2) Your main method prints ss before you perform operations on it. Place it after the other code in your main:

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) The Astronauts array is overwritten every time addAstronaut() is called. Fix this by creating and adding a new Astronaut instance to the Astronauts array in addAstronaut(). You should add a line to the constructor to initialize the array to an empty array of a size of 3 or greater so there is space to add Astronaut objects in addAstronaut().

//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++;  
  }

There are mainly 2 errors due to which name is been shown has null.<br>
1: You haven't set name in your constructor, Which resulted in name having a default value of null.<br>
So when you say this.name = name it tells that I have this in me, please look and set its value.

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

Also in the main function, you were trying to print it before setting the values for it.
You should have first given the values for it and then print it.

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
}

Additionally, I see you have made an Astronaut[] Astronauts array but never added any Astronauts in it. So what's the whole point of it?
When you were adding Astronauts it only stored the total numbers of astronauts but never an actual Astronauts Object.<br>

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

Main:

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);
}
}

Astronaut class:

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;
} 
}

Output:

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