how to make nested array of class

NickJG · November 5th, 2011, 08:36 PM

I'm trying to create a data structure where there are several players. Each player has several cities. Each city has several buildings. Sounds easy however I need the cities form different players to be able to share the same names and the buildings from different cities to also share names.
I have come up with the following code that would be perfect for my needs and it compiles ok however when I run it I get "Exception in thread "main" java.lang.NullPointerException" error. I really would somehow like to make a array of a class as shown in the following code. I also spent many hours trying to achieve the same thing with nested classes but also with no luck.

Code:

***********************************************
public class City {	
	String  cityName;                                   
	int[][] buildingList = new int[10][2];     
}
***********************************************
public class Player {	
             String playerName;                                    
             City[] cityList = new City[10]}
***********************************************	
public class TestCode {
              public static void main(String[] args) {
              Player player1 = new Player();		          
	
              player1.cityList[1].buildingList[1][1] = 5;


 /*  (player1.cityList[1].buildingList[1][1] = 5;) gives me the following error after trying run.
 Exception in thread "main" java.lang.NullPointerException at testCode.main(TestCode.java:9)  */


	}	
}
***********************************************

Actually I would like to make the player list a array also as in player[10] but for now player1, player2 will also work.

Please help. Thanks
Nick

ciderpunx · November 14th, 2011, 07:10 PM

Quote:

Originally Posted by NickJG

I'm trying to create a data structure where there are several players. Each player has several cities. Each city has several buildings. Sounds easy however I need the cities form different players to be able to share the same names and the buildings from different cities to also share names.
I have come up with the following code that would be perfect for my needs and it compiles ok however when I run it I get "Exception in thread "main" java.lang.NullPointerException" error. I really would somehow like to make a array of a class as shown in the following code. I also spent many hours trying to achieve the same thing with nested classes but also with no luck.

Code:

***********************************************
public class City {    
    String  cityName;                                   
    int[][] buildingList = new int[10][2];     
}
***********************************************
public class Player {    
             String playerName;                                    
             City[] cityList = new City[10]}
***********************************************    
public class TestCode {
              public static void main(String[] args) {
              Player player1 = new Player();                  
    
              player1.cityList[1].buildingList[1][1] = 5;


 /*  (player1.cityList[1].buildingList[1][1] = 5;) gives me the following error after trying run.
 Exception in thread "main" java.lang.NullPointerException at testCode.main(TestCode.java:9)  */


    }    
}
***********************************************

Actually I would like to make the player list a array also as in player[10] but for now player1, player2 will also work.

Please help. Thanks
Nick

Well, you need to put stuff in your array, otherwise you will be pointing at something null!

You can do this (explained inline):

Code:

charlie@localhost:~$ cat TestCode.java 
class City {    
    String  cityName;                                   
    int[][] buildingList = new int[10][2]; // creates array, but doesn't populate it
    public City(String name) {  // Now we have a constructor
        this.cityName=name;
                // Fill up the array "slots" with data
        for(int i=0;i<10;i++) {
            for(int j=0;j<2;j++) {
                this.buildingList[i][j]=i*j;
            }
        }
    }    
}
class Player {    
             String playerName;                                    
             City[] cityList = new City[10]; // creates but doesn't populate
}
public class TestCode {
              public static void main(String[] args) {
              Player player1 = new Player();                  
          player1.cityList[1]=new City("London"); // Have to "instantiate" your object
              player1.cityList[1].buildingList[1][1] = 5;
          System.out.println("Expect 5: " + player1.cityList[1].buildingList[1][1]); // Write a test ;-)
    }    
}
charlie@localhost:~$ javac TestCode.java 
charlie@localhost:~$ java TestCode 
Expect 5: 5

张松伟 · February 23rd, 2012, 10:50 AM

Quote:

Originally Posted by NickJG

I'm trying to create a data structure where there are several players. Each player has several cities. Each city has several buildings. Sounds easy however I need the cities form different players to be able to share the same names and the buildings from different cities to also share names.
I have come up with the following code that would be perfect for my needs and it compiles ok however when I run it I get "Exception in thread "main" java.lang.NullPointerException" error. I really would somehow like to make a array of a class as shown in the following code. I also spent many hours trying to achieve the same thing with nested classes but also with no luck.

Code:

***********************************************
public class City {	
	String  cityName;                                   
	int[][] buildingList = new int[10][2];     
}
***********************************************
public class Player {	
             String playerName;                                    
             City[] cityList = new City[10]}
***********************************************	
public class TestCode {
              public static void main(String[] args) {
              Player player1 = new Player();		          
	
              player1.cityList[1].buildingList[1][1] = 5;


 /*  (player1.cityList[1].buildingList[1][1] = 5;) gives me the following error after trying run.
 Exception in thread "main" java.lang.NullPointerException at testCode.main(TestCode.java:9)  */


	}	
}
***********************************************

Actually I would like to make the player list a array also as in player[10] but for now player1, player2 will also work.

Please help. Thanks
Nick

That is because you do not init player1.cityList[1],player1.cityList[1] is null