In a Java interface, when must variables (fields) be initialized, considering that they are implicitly public, static, and final constants?

Introduction / Context:
Variables declared inside a Java interface behave differently from typical instance variables in a class. This question checks whether you know that interface variables are actually constants and must be initialized at the point of declaration. Understanding this rule is important when designing interfaces that expose configuration values or shared constants.

Given Data / Assumptions:

• We are working with a standard Java interface definition.
• Any variable declared inside the interface is implicitly public, static, and final.
• Final variables must receive a value exactly once before they are used.
• Interface definitions cannot contain constructors to initialize fields later.

Concept / Approach:
Because interface variables are implicitly final, they must be initialized when they are declared. There is no constructor for an interface, and no instance of the interface is created to populate the fields. Instead, these variables behave like constants that belong to the interface type itself. This means you cannot assign to them later inside methods or implementation classes, and you cannot leave them uninitialized. The only safe and legal moment to provide a value is at the declaration line inside the interface.

Step-by-Step Solution:
Step 1: Consider an interface Config with a field int TIMEOUT.Step 2: If you write interface Config { int TIMEOUT; } the code will not compile because a final variable has no initializer.Step 3: The correct way is interface Config { int TIMEOUT = 30; } which provides a value at the point of declaration.Step 4: You cannot later write Config.TIMEOUT = 60; from some class, because the field is final and assignment is not allowed after initialization.Step 5: Therefore variables in an interface must be initialized during declaration, which matches option D.

Verification / Alternative check:
To verify, compile an interface that declares a variable without an initializer and observe that the compiler reports an error about a missing initializer for a final variable. Then add an initializer at the declaration and see that the error disappears. Attempting to reassign the variable elsewhere results in another compilation error, confirming that it is a constant fixed at declaration time.

Why Other Options Are Wrong:
Option A suggests initialization inside an implementing class, but the interface field is already fixed before any class uses it. Option B proposes lazy initialization just before use, which is not possible for final interface fields. Option C refers to function bodies in the interface, which historically contained only abstract methods and even with default methods cannot be used to set final interface fields after declaration.

Common Pitfalls:
A common mistake is treating interface variables as if they were instance variables and expecting each implementing class to have its own copy. In reality, there is a single constant associated with the interface type. Another pitfall is using many constants in interfaces purely as a way to share values, which can lead to the anti pattern known as constant interface. A better pattern is to use a dedicated constants class or an enum where appropriate.

Final Answer:
Interface variables must be initialized during the time of declaration inside the interface, because they are implicitly public, static, and final constants.