Thread: Help with null pointer exception

"Exception in thread "main" java.lang.NullPointerException
at Assignment6.Student.<init>(Student.java:21)

look at line 21 in the Student class. There is a variable there with a null value. Backtrack in your code to find out why and fix the code so that the variable has valid value.
If you can not see the variable, add a println statement just before line 21 to print out the values of all the variables on line 21.

Originally Posted by Norm

look at line 21 in the Student class. There is a variable there with a null value. Backtrack in your code to find out why and fix the code so that the variable has valid value.
If you can not see the variable, add a println statement just before line 21 to print out the values of all the variables on line 21.

Thanks for the suggestion!

But I still cant see how to fix it. When I print out the modules on line 21 it gives me the addresses of each module in the array. Which only confuses me more

Any other suggestions?

What variable is null?
Post the contents of line 21 if you need help finding the variables that are used on that line.

Originally Posted by Norm

What variable is null?
Post the contents of line 21 if you need help finding the variables that are used on that line.

When I modify the code to print out contents of line 21

	protected String name;
	int id;
	String programme;
 
	public Module[] modules;
 
 
public   Student(String name,int id,String programme, Module[] modules)
		{
			this.name= name;
			this.id= id;
			this.programme = programme;
 
			for(int i=0;i<modules.length;i++)
 
			{
				System.out.println(modules[i]);
				//this.modules[i] = modules[i];
			}
 
		}	
 
	public void display()
		{		
			System.out.println("Name: " + name);
			System.out.println("ID No.: " + id);
			System.out.println("Programme: " + programme);
 
			for(int i=0;i<modules.length;i++)
 
			{
				Module.display(modules[i]);
			}
 
 
		}
 
	public static void main(String[] args) {
 
	Module [] modules = new Module [4];	
 
	modules[0] = new Module("EE219","OOP",57.2f);
	modules[1] = new Module("EE203","Maths", 90.00f);
	modules[2] = new Module("EE223","Digital Electronics",75.8f);
	modules[3] = new Module("EE215","Solid State Electronics",45.7f);
 
	Student a = new Student("John Johnson",123456,"Electronic Engineering", modules);
	a.display();
 
}

I get the following output:

"Assignment6.Module@1be2d65
Assignment6.Module@9664a1
Assignment6.Module@1a8c4e7
Assignment6.Module@1172e08
Name: John Johnson
ID No.: 123456
Programme: Electronic Engineering
Exception in thread "main" java.lang.NullPointerException
at Assignment6.Student.display(Student.java:33)
at Assignment6.Student.main(Student.java:52)"

Exception in thread "main" java.lang.NullPointerException
at Assignment6.Student.display(Student.java:33)

The error has moved to line 33. What variable on line 33 is null?

You never posted line 21 so I can't help you determine which variable on that line is null.

Sorry this is line 21

this.modules[i] = modules[i];

and this is line 33

Module.display(modules[i]);

On line 21 You need to print out the value of this.modules and modules.
On line 33 you need to print out the value of Module and modules.

When I print out this.modules and modules on line 21 I get
"[LAssignment6.Module;@13e8d89
null"

And when I try to print out Module and modules on line 33 I get nothing and it wont let me print out Module as it is not a variable.

Thank you for you patience!

Which variable is null on line 21? Why is it null? Did you assign any value to that variable?

I get nothing

Please explain. Do you mean it prints spaces or blanks?
Change your println statement to have a String with the variable:
println("var=" + var + "<");

In my main of the student Class I create an array of modules with four modules, then the Student constructer is supposed to assign this array(modules) to the array Module [] modules that I declare at the top using

"this.modules[i] = modules[i];"

Heres the code I am trying to run at the moment to print out the Variables on line 33

package Assignment6;
 
public class Student {
 
	protected String name;
	int id;
	String programme;
 
	public Module[] modules;
 
 
public   Student(String name,int id,String programme, Module[] modules)
		{
			this.name= name;
			this.id= id;
			this.programme = programme;
 
			for(int i=0;i<modules.length;i++)
 
			{
				System.out.println(modules);
				System.out.println(this.modules);
				this.modules[i] = modules[i];
			}
 
		}	
 
	public void display()
		{		
			System.out.println("Name: " + name);
			System.out.println("ID No.: " + id);
			System.out.println("Programme: " + programme);
 
			for(int i=0;i<4;i++)
 
			{
				System.out.println("var=" + modules  + "<");
				System.out.println("var=" + Module + "<");
			Module.display(modules[i]);
			}
 
 
		}
 
	public static void main(String[] args) {
 
	Module [] modules = new Module [4];	
 
	modules[0] = new Module("EE219","OOP",57.2f);
	modules[1] = new Module("EE203","Maths", 90.00f);
	modules[2] = new Module("EE223","Digital Electronics",75.8f);
	modules[3] = new Module("EE215","Solid State Electronics",45.7f);
 
	Student a = new Student("John Johnson",123456,"Electronic Engineering", modules);
	a.display();
 
}

The result I get is

"
Exception in thread "main" java.lang.NullPointerException
at Assignment6.Student.<init>(Student.java:21)
at Assignment6.Student.main(Student.java:54)
[LAssignment6.Module;@13e8d89
null
"

the code on line 54 is
"
Student a = new Student("John Johnson",123456,"Electronic Engineering", modules);

"

}

In your printout, What variable is null? Your print out does not have an id String with the null line.

Exception in thread "main" java.lang.NullPointerException
at Assignment6.Student.<init>(Student.java:21)
at Assignment6.Student.main(Student.java:54)

You read the stack trace from the bottom up.
At line 54 in main() the code calls the Constructor<init> for the Student class.
At line 21 in the constructor there is a null variable (that we are still looking for. What variable is it?

I believe the null variable is "this.modules"

I think it is null as the constructor does not seem to assign it the array that I pass to it.

Where do you assign a value to the "this.modules" variable? You define it but never give it a value.

"this.modules[i] = modules[i];" was supposed to assign it?

Yes that assigns a value to one of the slots in an existing array, but you need to define it first.
What if modules[] had 2 slots, would you be able to assign values to the 10th slot?
Before you use an array you must define it with a specific size.

Does this not happen when in my main at

"Module [] modules = new Module [4];" ?

That would define a variable: modules known only in the main method. The class variable with the same name would still not have a value.
If you want to assign a value to the modules variable leave off the definition part of that statement:
modules = new Module [4];

Thank you that has sorted my Null Pointer Exception error.

The programm now compiles and runs ok but is not outputing the correct information.

package Assignment6;
 
public class Module {
 
	protected static 
	 String code;
	 static String title;
	 static float result;
 
 
	public  Module(String acode,String atitle, float aresult)
 
		{
			code = acode;
			title = atitle;
			result = aresult;
		}
 
	public static  void display(Module a)
	{
		System.out.println("Module Code:" + code);
		System.out.println("Moule Title:" + title);
		System.out.println("Result:" + result + "%");
	}
 
	public static void main(String[] args) {
 
	}
 
}

package Assignment6;
 
public class Student {
 
	protected String name;
	int id;
	String programme;
	public static Module[] modules;
 
 
 
public   Student(String name,int id,String programme, Module[] modules)
		{
			this.name= name;
			this.id= id;
			this.programme = programme;
 
 
			for(int i=0;i<modules.length;i++)
 
			{
				this.modules[i] = modules[i];
			}	
		}
 
	public void display()
		{		
			System.out.println("Name: " + name);
			System.out.println("ID No.: " + id);
			System.out.println("Programme: " + programme);
 
			for(int i=0;i<4;i++)
 
			{
				Module.display(modules[i]);
			}
 
 
		}
 
	public static void main(String[] args) {
	modules = new Module [4];
 
	modules[0] = new Module("EE219","OOP",57.2f);
	modules[1] = new Module("EE203","Maths", 90.00f);
	modules[2] = new Module("EE223","Digital Electronics",75.8f);
	modules[3] = new Module("EE215","Solid State Electronics",45.7f);
 
	Student a = new Student("John Johnson",123456,"Electronic Engineering", modules);
	a.display();
 
}
 
 
 
}

Output
"
Name: John Johnson
ID No.: 123456
Programme: Electronic Engineering
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%
"

Any ideas why this is happening, I just cant get my head around this

Thanks again for all your help

The output im looking for is

Name: John Johnson
ID No.: 123456
Programme: Electronic Engineering
Module Code:EE219
Moule Title:OOP
Result:57.2%
Module Code:EE203
Moule Title:Maths
Result:90.00%
Module Code:EE223
Moule Title: Digital Electronics
Result:75.8f%
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%
"
The problem I am having is that the outputed Modules are all the same
"
Module Code:EE215
Moule Title:Solid State Electronics
Result:45.7%"
"

I had thought it might have been a problem with my "public static Module[] modules; " but when I remove the static from that I get an error(Cannot make a static reference to the non-static field modules) in my main where I am assigning the modules in the array.

You should get rid of all static variables.

the outputed Modules are all the same

Only the value of the last data assigned is present in all the instances because they are sharing one static variable instead of having their own variables.

Change the code in the main() method so you do not need any static variables. Put the code in main in a constructor for the Student class. Have main pass the size of the array to the constructor.
Add a method to the Student class to add Module objects to the array and call that method from main.

The modules variable is not your problem. Continue looking and removing static

Thank you for all your suggestions.

Here is my code as it stands now

package Assignment6;
 
public class Module {
 
	  protected
	   String code;
	   String title;
	   float result;
 
 
	public  Module(String acode,String atitle, float aresult)
 
		{
			code = acode;
			title = atitle;
			result = aresult;
		}
 
	public void display(Module a)
	{
		System.out.println("Module Code:" + code);
		System.out.println("Moule Title:" + title);
		System.out.println("Result:" + result + "%");
	}
 
	public static void main(String[] args) {
 
 
	}
 
}

package Assignment6;
 
public class Student {
 
	protected String name;
	int id;
	String programme;
	public  Module[] modules;
 
 
 
public   Student(String name,int id,String programme, int i)
		{
			this.name= name;
			this.id= id;
			this.programme = programme;
 
			modules = new Module [i];
			modules[0] = new Module("EE219","OOP",57.2f);
			modules[1] = new Module("EE203","Maths", 90.00f);
			modules[2] = new Module("EE223","Digital Electronics",75.8f);
			modules[3] = new Module("EE215","Solid State Electronics",45.7f);
		}
 
	public void array()
	{
		for(int i=0;i<modules.length;i++)
 
		{
 
			this.modules[i] = modules[i];
		}	
 
	}
	public void display()
		{		
			System.out.println("Name: " + name);
			System.out.println("ID No.: " + id);
			System.out.println("Programme: " + programme);
 
			for(int i=0;i<4;i++)
 
			{
				Module.display(modules[i]);
			}
 
 
		}
 
	public static void main(String[] args) {
 
	Student a = new Student("John Johnson",123456,"Electronic Engineering", 4);
	a.array();
	a.display();
 
}
 
 
 
}

My problem now is an error with "Module.display(modules[i]);" in the student class.
(Cannot make a static reference to the non-static method display(Module) from the type Module)

Then if I make the display method in the Module class static, all the variables have to be static and I'm back to the same problem of it outputting the same last module 4 times.

You haven't followed my suggestions (which I see were not clear). You should NOT put your testing code into the Student class's constructor.
Have main pass the size of the array to the Student class's constructor.
Add a method to the Student class to add Module objects to the array and call that method 4 times from main.

Module.display(modules[i]);

The syntax of this statement is backwards.
display( ) is a method of the Module class. It has access to all the data in its instance of the class.
modules[i] is an instance of the Module class. Use that instance to call the display method.
Look at the code for the display method, what does it do with the parameter a?