What is the difference between declaration and definition in Java?

I’m very confused between the two terms. I checked on stackoverflow and there’s a similar question for C++ but not for java.

Can someone explain the difference between the two terms for java?


The conceptual difference is simple:

  • Declaration: You are declaring that something exists, such as a class, function or variable. You don’t say anything about what that class or function looks like, you just say that it exists.

  • Definition: You define how something is implemented, such as a class, function or variable, i.e. you say what it actually is.

In Java, there is little difference between the two, and formally speaking, a declaration includes not only the identifier, but also it’s definition. Here is how I personally interpret the terms in detail:

  • Classes: Java doesn’t really separate declarations and definitions as C/C++ does (in header and cpp files). You define them at the point where you declare them.

  • Functions: When you’re writing an interface (or an abstract class), you could say that you’re declaring a function, without defining it. Ordinary functions however, are always defined right where they are declared. See the body of the function as its definition if you like.

  • Variables: A variable declaration could look like this:

    int x;

    (you’re declaring that a variable x exists and has type int) either if it’s a local variable or member field. In Java, there’s no information left about x to define, except possible what values it shall hold, which is determined by the assignments to it.

Here’s a rough summary of how I use the terms:

abstract class SomeClass {                // class decl.
    int x;                                // variable decl.            |
                                          //                           |
    public abstract void someMethod();    // function decl.            |
                                          //                           |
    public int someOtherMethod() {        // function decl.            |
                                          //                           | class
        if (Math.random() > .5)           //                          | def.
            return x;                     //  |  function definition   |
        else                              //  |                        |
            return -x;                    // /                         |
                                          //                           |
    }                                     //                           |
}                                         //                          /