Why does this Java code implicitly calls toString() method?

Question

Why is the output? : Sphere 0

Somehow it's calling the toString() method implicitly? How does this work ?

class BerylliumSphere {
    private static long counter = 0;
    private final long id = counter++;
    public String toString() { 
        return "Sphere " + id;
    }
}

public class Test {
    public static void main (String[] args) {
        BerylliumSphere spheres = new BerylliumSphere();
        System.out.println(spheres);
    }
}

// output: Sphere 0

There is no magic and it's not "Java" that does it, but the `println` method. You can implement such a method yourself. http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/java/io/PrintStream.java#820 — Marko Topolnik, Oct 04 '16 at 08:27
Passing object in println() always gives the toString representation — Kahn, Oct 04 '16 at 08:28
This is not "implicit", it is "explicit", just look what `println` does. — Tom, Oct 04 '16 at 08:30
@Tom Hey tom it is implicitly calling because I never called the toString() method in the main method so it happens internally. — Timothy, Oct 04 '16 at 15:56
@TimothyKimman Just because you don't call it, it means that this is implicit. The call of `toString()` is explicitly done in the source of `String`. — Tom, Oct 04 '16 at 16:46
Implicit is stuff defined by the JLS, like abstract methods in interfaces are *implicitly* `public` even if one doesn't add an access modifier, or that `super()` will *implicitly* called in a subclass constructor if not done explicitly. — Tom, Oct 04 '16 at 16:53

score 5 · Accepted Answer · edited Oct 04 '16 at 16:47

5

System.out is a PrintStream instance that is a static member of System. The PrintStream class has a function println() that accepts an argument of type Object. That function, in Open JDK, looks like the following:

public void println(Object x) {
     String s = String.valueOf(x);
     synchronized (this) {
         print(s);
         newLine();
     }
}

And if you look at String.valueOf(), which accepts an argument of type Object, you can see:

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

There is no magic. It's just a Java class that calls toString on objects.

Further Reading

Take a look at the docs for the PrintStream class: https://docs.oracle.com/javase/7/docs/api/java/io/PrintStream.html

edited Oct 04 '16 at 16:47

Tom

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answered Oct 04 '16 at 08:31

christopher

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But I don't get why it is using the toString() method I wrote in the BerylliumSphere class since I never called it? – Timothy Oct 04 '16 at 16:27
1

You need to learn about Polymorphism my friend! By creating your own toString(), you have overridden the implementation of the same function in all superclasses. – christopher Oct 04 '16 at 16:29

score 3 · Answer 2 · answered Oct 04 '16 at 08:30

Here is what System.out.println does: https://docs.oracle.com/javase/7/docs/api/java/io/PrintStream.html#println%28java.lang.Object%29

It says the following:

This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println().

And here is what String.valueOf does: https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#valueOf%28java.lang.Object%29

And it says:

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

In short, printing an object will result in calling its toString method and printing what it returns.

But I don't get why it is using the toString() method I wrote in the BerylliumSphere class since I never called it? — Timothy, Oct 04 '16 at 16:15

score 1 · Answer 3 · answered Oct 04 '16 at 08:30

When you try to System.out.println(spheres) it looks like following:

public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

And this is valueOf(Object obj) method:

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

Why does this Java code implicitly calls toString() method?

3 Answers3