Difference between print(null.toString()) and print(null)

This post demonstrates difference between print(null.tostring()) and print(null) in Java programming.

The difference is print(null.tostring()) throws null pointer exception but print(null) doesn't. Why?

Consider the below sample code to understand the concept clearly.

public class NPETest {
    public static void main(String[] args) {
        Object obj1 = null;
        Object obj2 = null;
        System.out.print(obj1.toString()); //throws Null Pointer Exception
        System.out.print(obj2); // prints null
    }
}

 

The statement System.out.print(obj1.toString()) causes following runtime exception.

Exception in thread "main" java.lang.NullPointerException
at com.org.s20150310072124635.NPETest.main(NPETest.java:5)

 

The statement System.out.println(obj2) runs fine and prints a text "null".

Why an exception is caused?

Look at the java bytecode of the program.

> javap -classpath target\test-classes -c NPETest

Compiled from "NPETest.java"
public class NPETest extends java.lang.Object{
public NPETest();
  Code:
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   aconst_null
   1:   astore_1
   2:   aconst_null
   3:   astore_2
   4:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   7:   aload_1
   8:   invokevirtual   #23; //Method java/lang/Object.toString:()Ljava/lang/String;
   11:  invokevirtual   #27; //Method java/io/PrintStream.print:(Ljava/lang/String;)V
   14:  getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   17:  aload_2
   18:  invokevirtual   #33; //Method java/io/PrintStream.print:(Ljava/lang/Object;)V
   21:  return

}

 In code column, line #8 shows the bytecode representation of calling obj1.toString(). Here obj1 is null and so any attempt on a method invocation on null results in a NullPointerException.

Why print(null) works fine?

Consider the bytecode, in code column line #18 shows the bytecode representation of calling print(obj2). Here object is passed as a parameter to the PrintStream.print() method. According to the Java documentation, the source code will tell you that why this does not result in NullPointerException.

public void print(Object obj) {
    write(String.valueOf(obj));
}

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

 
 The documentation tells that the null reference is handled if null reference is passed. If argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned. So this statement does not result in NullPointerException.

Summary -

print(null.tostring()) causes Null Pointer Exception. print(null) prints text "null".

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Karthik Byggari

Author & Editor

Computer Science graduate, Techie, Founder of logicallyproven, Love to Share and Read About pprogramming related things.