final variable initialization

[Harikrishna Gorrepati]

Hi, Why should we initialize final variable i.e., num1 where as we do not have to initialize non-final variable i.e., num2

class Question37
{
	final int num1;
	int num2;

	Question37(int i)	//Compile Error : The blank final field num1 may not have been initialized
	{
		num2 = num1 + i; //Compile Error : The blank final field num1 may not have been initialized
	}
	
	public static void main(String[] args)
	{
		Question37 t = new Question37(10);
		System.out.println(t.num2);
	}
}

[W. Joe Smith]

Look at it this way....you are trying to assign the value of num1 + i to num2. Say you pass in 3. So you are doing num2 = num1 + 3. How do you add 3 to num1 when you haven't given num1 a value yet?

[Harikrishna Gorrepati]

Let me change the code..With this, there is no error. I am getting the error in 2 cases when final variable is NOT initialized with any value

1. declared constructor
2. try to get value from final variable (It is not taking default value i.e., 0)

Why ??

class Question37
{
	//final int num1;
	int num2;

	Question37(int i)	
	{
		num2 = num2 + i; 
	}
	
	public static void main(String[] args)
	{
		Question37 t = new Question37(10);
		System.out.println(t.num2); //10
	}
}

[Abimaran Kugathasan]

Harikrishna Gorrepati wrote:Let me change the code..With this, there is no error. I am getting the error in 2 cases when final variable is NOT initialized with any value

1. declared constructor
2. try to get value from final variable (It is not taking default value i.e., 0)

Why ??

The final variable should be initialized in the same line, which were declared. Otherwise, you should initialize that variable in every Constructors or one of the instance initializer. Because, the final variable can be initialized only once. If it takes the default value, and because of the final, the value can't be changed in the future. So, this is non-sense. That's why, they don't get initialized with default value as like other non final instance variables.

[Prasad Kharkar]

this is from jls

Similarly, every blank final variable must be assigned at most once; it must be definitely unassigned when an assignment to it occurs. Such an assignment is defined to occur if and only if either the simple name of the variable, or its simple name qualified by this, occurs on the left hand side of an assignment operator. A Java compiler must carry out a specific conservative flow analysis to make sure that, for every assignment to a blank final variable, the variable is definitely unassigned before the assignment; otherwise a compile-time error must occur.

[Mohit G Gupta]

Refer this linkFinal for answer

[Harikrishna Gorrepati]

Thanks Mohit. This is a outstanding step by step tutorial.