Is System.out.println method calling toString() by default? A strange behavior.

[Marcus Jastrebowski]

I thought println() method would implicitly invoke toString() on an object when not explicitly stated. But here is an "inconsistent" behavior that I cannot explain to myself. Anybody has any idea what is happening here and why?

Case 1. Using toString() implicitly(?)

public class Test {
public static void main (String[] args) {
System.out.println(new Test());
}
 
public String toString() {
return null;
}
}

Output:

Exception in thread "main" java.lang.NullPointerException
        at java.io.Writer.write(Writer.java:126)
        at java.io.PrintStream.write(PrintStream.java:457)
        at java.io.PrintStream.print(PrintStream.java:616)
        at java.io.PrintStream.println(PrintStream.java:753)
        at Test.main(Test.java:3)

Case 2. Using toString() explicitly.

public class Test {
public static void main (String[] args) {
System.out.println(new Test().toString());
}
 
public String toString() {
return null;
}
}

Output:

null

[Kelvin Chenhao Lim]

This isn't really inconsistent. In Case 1, you're passing a valid object reference to println(), so println() will call that object's toString() method and expect it to return a String object. In Case 2, you're passing a null reference to println(), so println() simply prints "null" without ever trying to call a toString() method.

edit: I forgot to mention that you're also invoking two different overloaded versions of println() here. Case 1 invokes println(Object), whereas Case 2 invokes println(String), due to the "most specific method" disambiguation mechanism.
[ December 06, 2007: Message edited by: Kelvin Lim ]

[nico dotti]

Good point Kevin. I just did a javap -c to look at the bytecode and sure enough you can see that different methods are being called:

    
nico@toshiba:~/java/javaranch$ javap -c Test
Compiled from "Test.java"
class Test extends java.lang.Object{
Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>"  :(   )V
   4:   return
 
public static void main(java.lang.String[]);
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   new     #3; //class Test
   6:   dup
   7:   invokespecial   #4; //Method "<init>"  :(   )V
   10:  invokevirtual   #5; //Method [B]java/io/PrintStream.println  :(   Ljava/lang/Object  :wink:  V[/B]
   13:  return
 
public java.lang.String toString();
  Code:
   0:   aconst_null
   1:   areturn
 
}
 
nico@toshiba:~/java/javaranch$ javap -c Test
Compiled from "Test.java"
class Test extends java.lang.Object{
Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>"  :(   )V
   4:   return
 
public static void main(java.lang.String[]);
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   new     #3; //class Test
   6:   dup
   7:   invokespecial   #4; //Method "<init>"  :(   )V
   10:  invokevirtual   #5; //Method toString  :(   )Ljava/lang/String;
[B]   13:  invokevirtual   #6; //Method java/io/PrintStream.println  :(   Ljava/lang/String  :wink:  V[/B]
   16:  return
 
public java.lang.String toString();
  Code:
   0:   aconst_null
   1:   areturn
 
}
 

[ December 06, 2007: Message edited by: nico dotti ]

[Jesper de Jong]

No, System.out.println() does not call toString(). It calls String.valueOf(Object). The API doc of String.valueOf(Object) says:

Returns:
if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.

[Dinesh Tahiliani]

Hello,

Can you pleae little elaborate on this.

[Andry Dub]

I can't understand this behaviour

public class Test {
 
    public static void main(String[] args) {
        System.out.println(new Test());
    }
 
    public String toString() {
        return null;
    }
}

Output is: NullPointerException

public class Test {
 
    public static void main(String[] args) {
        System.out.println(String.valueOf(new Test()));
    }
 
    public String toString() {
        return null;
    }
}

Output is: null

[Ernest Friedman-Hill]

Originally posted by Andry Dub:
I can't understand this behaviour

It's the same thing as the original example. If you pass "null" to println() (and thus to String.valueOf()), it is properly handled. String.valueOf(null) returns "null", as a special case.

If, on the other hand, you pass an Object whose toString() method returns null, then valueOf() calls toString() on that object, gets a null value that it doesn't expect, and throws an NPE.

So to summarize: valueOf() can handle a null argument, but it can't handle an object that returns null from toString(). Writing toString() that returns null is an evil thing to do, in any case.

[Andry Dub]

Ernest Friedman-Hill, thank you very much for explanations.

[Marcus Jastrebowski]

However, I am a quite confused, since K&B book states that

"... toString() method is automatically called when you ask System.out.println() to print an object..."

(Chapter 7, page 605, Certification Summary).

So perhaps I'm missing some key point here, but in this exchange, Jesper and Ernest, you both seem to be saying that the statement above is not quite right? Would somebody please explain what exactly happens when System.out.println() method is called to print an object?

[Henry Wong]

So perhaps I'm missing some key point here, but in this exchange, Jesper and Ernest, you both seem to be saying that the statement above is not quite right? Would somebody please explain what exactly happens when System.out.println() method is called to print an object?

I think that you should reread the exchange again. Neither Jesper or Ernest said that the toString() method is not called -- they just said that it is not called directly. (going through the valueOf() method)

Henry

[Marcus Jastrebowski]

Thank you. That was the point I was missing.