What happens when we pass an argument to System.out.println()?

Question

I'm a beginner and I am not able to understand how println() function actually works. When I pass primitive variable to it . It prints value of it. When I pass an object why does it print default toString() method definition? i.e.

 getClass().getName() + '@' + Integer.toHexString(hashCode())

So when I am trying to print out an array.

 System.out.println(Arrays.toString(array))

Am I overriding default method definition of toString of Object class or just converting Array to String using toString() method of Arrays class.

import java.util.Arrays;


public class RevString 
{
    public static void main(String[] args)
    {
        String str = "abcde";
        int x=2;
        String[] e =str.split("---");
        System.out.println(e);
        System.out.println(Arrays.toString(e)+ " " +"Primitive is " + " " + x);

    }

}

It does use `toString()` method. Unless it's overridden somewhere in the inheritance chain, the one from `Object` will be used. — PM 77-1, Jul 12 '14 at 21:59
Possible duplicate of [How to support println in a class?](http://stackoverflow.com/questions/27647567/how-to-support-println-in-a-class) — Raedwald, Mar 26 '16 at 14:13

score 3 · Accepted Answer · answered Jul 12 '14 at 22:01

toString() method if not overridden, will inherit from the object class. so you need to override it it in your class to be capable of showing the members o your object.

Current IDEs have this option automatic, so you just right click and generate toString method with the class members

look at this.

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

When I pass an object why does it print default toString() method definition?

Because it's defined in the Java specification Chapter 5 § 1.11:

Any type may be converted to type String by string conversion.

A value x of primitive type T is first converted to a reference value as if by giving it as an argument to an appropriate class instance creation expression (§15.9):

If T is boolean, then use new Boolean(x).

If T is char, then use new Character(x).

If T is byte, short, or int, then use new Integer(x).

If T is long, then use new Long(x).

If T is float, then use new Float(x).

If T is double, then use new Double(x).

This reference value is then converted to type String by string conversion.

Now only reference values need to be considered:

If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l).

Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.

score 0 · Answer 3 · edited May 23 '17 at 12:24

0

.toString takes the variable's value and changes it to a String. I do not believe it actually changes the libraries definition. This may help: Override valueof() and toString() in Java enum

edited May 23 '17 at 12:24

Community

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1

answered Jul 12 '14 at 22:01

NebulaeGuy

169
9

I like GingerHead's answer better, but if my one helped, let me know. – NebulaeGuy Jul 12 '14 at 22:02

score 0 · Answer 4 · answered Jul 12 '14 at 22:01

0

There is actually a println() method for every primitive and one for Objects. Primitives are printed as String.valueOf('primitive') whereas Objects are printed as 'object'.toString()

answered Jul 12 '14 at 22:01

succcubbus

874
1
7
24

What happens when we pass an argument to System.out.println()?

4 Answers4