Which keyword is used to declare a constant variable in Java?

The keyword used to declare a constant variable in Java is 'final'. When a variable is declared as final, its value cannot be modified once it has been assigned. This is crucial in programming because it helps to prevent accidental changes to values that are intended to remain constant throughout the program's execution.

Using 'final' provides both clarity and security in code; it indicates to anyone reading the code that this variable is not supposed to change, enhancing maintainability. For example, if you declare a variable as final int MAX_VALUE = 100;, any attempt to change MAX_VALUE later in the code would result in a compile-time error.

Other keywords such as 'static', 'constant', and 'immutable' do not serve the same purpose in Java. 'Static' is used for variables that belong to the class, rather than instances of the class. 'Constant' is not a recognized keyword in Java, and 'immutable' is a concept rather than a keyword, generally associated with classes or data structures that cannot be changed after they are created, but it does not apply directly to variable declarations in the same way as 'final'.