Java - Custom type nested in class

I am once again asking for technical support.

I need to define a custom type inside a class, I've done it like this:

public class MainClass {
    private class CustomType {
        public byte[] varA;
        public int varB;
		
        public CustomType() {
            varA = new byte[3];   
            varB = 13;
        }
    }
	
    private CustomType[] myArray;


    public MainClass() {
		myArray = new CustomType[1024]
		System.out.println(this.CustomType[0].varB);
    }
}

When I run it throws a NullPointerException at System.out.println(this.CustomType[0].varB);

I've tested if myArray gets properly initialized with 1024 elements and it does, however I can't seem to access them.

I just moved from C++ to Java so I'm still getting used to it, am I missing something blatant?.

You only create an array without any objects, so this.CustomType[0] is null.

You should add the objects to the array:

public MainClass() {
    myArray = new CustomType[1024]
    for (int i =0; i<myArray.length;i++ {
      myArray[i] = new CustomType();
    }
    System.out.println(this.myArray[0].varB);
}

Also you should make the member of CustomType private and access it via getter and setter.

Two things,

• You must instantiate CustomType.
• CustomType does not need access to MainClass.this so you can make it static.

So

public class MainClass {
    private static class CustomType {
        public byte[] varA;
        public int varB;

        public CustomType() {
            varA = new byte[3];   
            varB = 13;
        }
    }

    private CustomType[] myArray;


    public MainClass() {
        myArray = new CustomType[1024];
        for (int i = 0; i < myArray.length; ++i) {
            this.CustomType[i] = new CustomType();
        }
        // Or
        Arrays.setAll(myArray, CustomType::new);
        System.out.println(this.CustomType[0].varB);
    }
}

Not making it static stores a MainClass.this in every CustomType instance which is unnecessary overhead.

Arrays in java are objects. The following line of the code you posted creates an array of 1024 elements where each and every element is null.

myArray = new CustomType[1024];

If you want to place actual objects in the array, named myArray, you need to create instances of class CustomType and assign them to elements of the array, for example:

CustomType instance = new CustomType();
myArray[0] = instance;

Then you can execute the following line of code and it will not throw NullPointerException.

System.out.println(myArray[0].varB);

Here is the full code to get the value of varB. In which you can avoid declaring CustomType[] myArray

public class Test 
{
	private static class CustomType 
	{
		public byte[] varA;
		public int varB;

		public CustomType() {
			varA = new byte[3];   
			varB = 13;
		}
	}


	public static void main(String... args) 
	{		
		System.out.println(new CustomType().varB);
	}
}

The solution is to add some elements to that array. See the below steps for more information.

1. constructor will be invoked, when you create the object of that class

2. And then you created an empty array of CustomType with size 1024 and trying to access the first element which does not exist(default is null) and trying to perform operations on that null reference. So you are getting the NullPointerException.