Question:Forward declarations in Java?

So what is Forward Declaration?

Ans:

 A forward declaration is a declaration of an identifier (denoting an entity such as a type, a variable, a constant, or a function) for which the programmer has not yet given a complete definition.

It is required for a compiler to know certain properties of an identifier (size for memory allocation, data type for type checking, such as type signature of functions), but not other details, like the particular value it holds (in case of variables or constants) or definition (in the case of functions). This is particularly useful for one-pass compilers and separate compilation.

Forward declaration is used in languages that require declaration before use; it is necessary for mutual recursion in such languages, as it is impossible to define such functions (or data structures) without a forward reference in one definition: one of the functions (respectively, data structures) must be defined first. It is also useful to allow flexible code organization, for example if one wishes to place the main body at the top, and called functions below it.

In other languages forward declarations are not necessary, which generally requires instead a multi-pass compiler and for some compilation to be deferred to link time. In these cases identifiers must be defined (variables initialized, functions defined) before they are used in execution, but do not need to be defined before they are used in source code for compilation or interpretation: identifiers do not need to be immediately resolved to an existing entity.

Example::

A basic example in C is:

void printThisInteger(int);

In C and C++, the line above represents a forward declaration of a function and is the function's prototype. After processing this declaration, the compiler would allow the program code to refer to the entity printThisInteger in the rest of the program. The definition for a function must be provided somewhere (same file or other, where it would be the responsibility of the linker to correctly match references to a particular function in one or several object files with the definition, which must be unique, in another):

void printThisInteger(int x)
{
   printf("%d\n", x);
}

Now,Come to JAVA,Let me give you an example:

class Test2
{    
    public static void main(String[] args)
{  
         Test1 t1 = new Test1();
         t1.fun(5);      
    }
}  
class Test1
{
    void fun(int x)
{
        System.out.println("fun() called: x = " + x);
    }
}

Predict the output: fun() called: x = 5

The Java program compiles and runs fine. Note that Test1 and fun() are not declared before their use. Unlike C++, we don’t need forward declarations in Java. Identifiers (class and method names) are recognized automatically from source files. Similarly, library methods are directly read from the libraries, and there is no need to create header files with declarations. Java uses naming scheme where package and public class names must follow directory and file names respectively. This naming scheme allows Java compiler to locate library files.