To solve this question, the user needs to understand the syntax and rules for declaring methods within an interface in Java.
In Java, an interface is a collection of abstract methods, which means the methods declared within an interface do not have a body. The purpose of an interface is to define a contract that classes can implement, specifying the methods they must provide.
Now, let's go through each option and explain why it is right or wrong:
A. public double methoda();
This option is a valid declaration within an interface definition. It declares a public method named "methoda" that returns a double value. Since interfaces only contain abstract methods, there is no need to provide a method body.
B. public final double methoda();
This option is not a valid declaration within an interface definition. The "final" keyword cannot be used to modify a method declaration within an interface. The "final" keyword is used to indicate that a method cannot be overridden by a subclass.
C. static void methoda(double d1);
This option is not a valid declaration within an interface definition. The "static" keyword cannot be used to modify a method declaration within an interface. Static methods belong to the class itself, not an instance of the class.
D. protected void methoda(double d1);
This option is not a valid declaration within an interface definition. The "protected" keyword cannot be used to modify a method declaration within an interface. Protected methods are accessible within the same package and by subclasses.
Therefore, the valid declaration within an interface definition is:
The Answer is: A. public double methoda();