Blank Final Variable

A blank final is a final variable whose declaration lacks an initializer.

public class BlankVariableEx {
    final int var;
}

In the above class variable 'var' is blank final variable, since it is not initialized.

A blank final variable can by static or non static.

Blank final static variable

Blank final static variable must be initialized in static block, else compiler error thrown.

class Rectangle{
 final static int width;
}

When you tries to compile the above program, below error thrown.

Rectangle.java:2: error: variable width not initialized in the default constructor
        final static int width;
                         ^
1 error

To make the above program compiles fine, initialize the variable in static block.

class Rectangle{
 final static int width;
 
 static{
  width = 100;
 }
}

Blank final Instance variable

A blank final instance variable must be definitely assigned at the end of every constructor of the class in which it is declared, otherwise a compile-time error occurs.

class Rectangle{
 final int width;
}

When you tries to compile the above program, compiler throws below error, since the final blank variable is not initialized.

Rectangle.java:2: error: variable width not initialized in the default constructor
        final int width;
                  ^
1 error

class Rectangle{
 final int width;
 
 Rectangle(){
  width = 100;
 }
 
 Rectangle(int width){
 
 }
}

Above program also won't compile, since width is not initialized in parameterized constructor. Since a blank final instance variable must be definitely assigned at the end of every constructor of the class in which it is declared. To make the program compile fine, initialize the variable width in All constructors like below.

class Rectangle{
 final int width;
 
 Rectangle(){
  width = 100;
 }
 
 Rectangle(int width){
  this.width = width;
 }
}

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