Change output without modifying code

Question

This question was recently asked in an interview that i attended.

public class MagicOutput { 
   public static void main(final String argv[]) { 
      System.out.println("Hello, World!"); 
   } 
}

I was asked to preserve both method main signature (name, number and type of parameters, return type) and implementation (body of method),

and to make this program to output to standard console message "Magic Output !"

I took around 2 minutes to answer. My solution was to put a static block there and output the required string.

static{
   System.out.println("Magic Output !");
}

This does work but , it prints both Magic Output ! and Hello, World!

How can i make it to output only magic string ?

assylias · Accepted Answer · 2014-04-21 12:55:24Z

9

static{
   System.out.println("Magic Output !");
   System.exit(0);
}

or funnier (depends on String implementation - works with openjdk 8):

static {
  System.out.println("Magic Output!");
  try {
    Field f = String.class.getDeclaredField("value");
    f.setAccessible(true);
    f.set("Hello, World!", new char[0]);
    f.set(System.lineSeparator(), new char[0]);
  } catch (Exception ignore) { }
}

edited Apr 21, 2014 at 12:55

answered Apr 21, 2014 at 12:43

assylias

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1 Comment

Mohsen Kamrani Over a year ago

Nice answer in a short time! I want to up vote 10 times.

Alexis C. · Accepted Answer · 2014-04-21 18:05:25Z

1

You could also use static nested classes to achieve that.

public class MagicOutput {   
    public static void main(final String argv[]) { 
        System.out.println("Hello, World!"); 
    }   
    static class System {
        static Out out = new Out();
    }
    static class Out {
        void println(String s){
            java.lang.System.out.println("Magic Output !");
        }
    }
}

edited Apr 21, 2014 at 18:05

answered Apr 21, 2014 at 18:00

Alexis C.

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Comments

Andres · Accepted Answer · 2014-04-21 12:47:57Z

0

Redirect System.out to an instance of PrintStream using System.setOut() on the static block.

answered Apr 21, 2014 at 12:47

Andres

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