» » What happens When System.out.println(null)?

  • Compilation Error -This is because you can pass an Object or a String or char[]. Since null can fit in both, the compiler doesn't know which method to use, leading to compile error.

  • Method Overloading:
        1.public void prinltln(String str) { }
        2.public void prinltln(char[] ch){ }
        3.public void prinltln(Object ch){ 
  •  It seems the call System.out.print(null) is ambiguous to compiler because print(null) here will find the two best specific matches i.e. print(String) and print(char[]) . So compiler is unable to determine which method to call here .
  • Compilation Error:
       System.out.println(null)

Program #1: what will happen when we print System.out.println(null)


System out prinln null

  • Compile Fine:
         System.out.println((String)null);//null
         System.out.println((char[])null);
         System.out.println((Object)null);//null
  • It's the compiler type-checking the parameters of the method call.
  • But here we need to know one more thing  System.out.println((char[])null); will compile fine but at run time will throw runtime exception.

 Program #2: what will happen when we print System.out.println(null) by type casting
  

  1. package com.systemoutprintn;
  2. public class Test {
  3.  
  4.     /**
  5.      * @Website: www.instanceofjava.com
  6.      * @category: System.out.println(null)
  7.      */
  8.  
  9. public static void main(String[] args) {
  10.  
  11.          System.out.println((String)null);//null
  12.          System.out.println((Object)null);//null
  13.          System.out.println((char[])null);
  14.             
  15.  
  16. }
  17.  
  18. }

 Output:
 

  1. null
  2. null
  3. Exception in thread "main" java.lang.NullPointerException
  4.     at java.io.Writer.write(Unknown Source)
  5.     at java.io.PrintStream.write(Unknown Source)
  6.     at java.io.PrintStream.print(Unknown Source)
  7.     at java.io.PrintStream.println(Unknown Source)
  8.     at com.systemoutprintn.Test.main(Test.java:13)


Instance Of Java

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4 comments for What happens When System.out.println(null)?

  1. System.out.println((char[])null);

    This statement will not work. Because according to the javadocs, the char type will not handle null pointer which leads to a null pointer exception. String and Object works fine as you said.

    -Regards,
    Karthik Byggari

    ReplyDelete
  2. You are telling the compiler which implementation of System.out.println(*) to use, but when trying to use it execution fails... Interesting... Testing should lead you to validate that output...

    ReplyDelete
  3. class J
    {

    public static void main(String [] args)
    {

    System.out.println((char[])null);

    }

    }


    this code is compile fine but runtime error like

    Exception in thread "main" java.lang.NullPointerException
    at java.io.Writer.write(Writer.java:127)
    at java.io.PrintStream.write(PrintStream.java:503)
    at java.io.PrintStream.print(PrintStream.java:653)
    at java.io.PrintStream.println(PrintStream.java:792)
    at logicalquestion.J.main(J.java:9)

    ReplyDelete
    Replies
    1. This is correct and exactly what is mentioned in the last line of the post:
      "But here we need to know one more thing System.out.println((char[])null); will compile fine but at run time will throw runtime exception."

      Delete

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