Muthu Ganapathy Nathan Muthu Ganapathy Nathan - 1 month ago 15
Java Question

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.

Answer

You can but you shouldn't.

The shouldn't part:

You shouldn't because using the pre-processor is considered bad practice to start with, and the need for it has vanished in modern languages.


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.


This post has been rewritten as an article here.


(*) 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!