javamacros

Can I have macros in Java source files


In my program I'm reading integers form console many times. Every time, I need to type this line.

new Scanner(System.in).nextInt(); 

I'm used to C/C++ and I'm wondering if I can define something like

#define READINT Scanner(System.in).nextInt(); 

and then, in every place in my java program I can read form console as

int a = new READINT;

But I read form books Java does not support macros.

Someone please explain me why is it so, and can I do this in any other way.


Solution

  • You can but you shouldn't.

    ##The shouldn't part:

    You shouldn't because using the pre-processor in that way is considered bad practice to start with, and there are better and more Java-idiomatic ways to solve this use case.


    ##The can part: (*)

    Java itself doesn't support macros. On the other hand, you could pipe the source code through the C pre processor (CPP for short) just like the C/C++ compile chain does.

    Here's a demo:

    src/Test.java:

    #define READINT (new java.util.Scanner(System.in).nextInt())
    
    class Test {
        public static void main(String[] args) {
            int i = READINT;
        }
    }
    

    cpp command:

    $ cpp -P src/Test.java preprocessed/Test.java
    

    Result:

    class Test {
        public static void main(String[] args) {
            int i = (new java.util.Scanner(System.in).nextInt());
        }
    }
    

    Compile:

    $ javac preprocessed/Test.java
    

    ##A better workaround:

    You can write your own utility class with a static method instead:

    import java.util.Scanner;
    class StdinUtil {
        public final static Scanner STDIN = new Scanner(System.in);
        public static int readInt() {
            return STDIN.nextInt();
        }
    }
    

    And when you want to use it, you can statically import the readInt method:

    import static StdinUtil.readInt; 
    
    class Test {
        public static void main(String[] args) {
            int i = readInt();
        }
    }
    

    (or do static import StdinUtil.STDIN; and use STDIN.nextInt().)


    ##And finally, an anecdote

    I myself used the CPP preprocessing approach on Java code once! I was creating a programming assignment for a course. I wanted to be able to easily extract a code skeleton out of the reference solution. So I just used a few #ifdefs to filter out the "secret" parts of the solution. That way I could maintain the reference solution, and easily regenerate the code skeleton.


    (*) Since I hate answering questions with "you shouldn't". Besides, some future reader may have good reasons for wanting to use the cpp in conjunction with Java sources!