println隐式调用Java方法......为什么？

Question

I have a question about why the toString() method is implicitly called when the main method calls the printPersonPrinting() method.

public class PersonPrinting {

    private String name;
    private int age;

    public PersonPrinting(String aName, int anAge) {
        name = aName;
        age = anAge;
    }

    public String toString() {
        return name + " " + age;
    }

    public void printPersonPrinting() {
        System.out.println(this);
    }

    public static void main (String [] args) {
        PersonPrinting p = new PersonPrinting("Dan",10);
        //printPersonPrinting is called, but toString isn't. 
       //But the output is formatted by toString method.
        p.printPersonPrinting();
    }
}

The output of this code is --> Dan 10

Thanks!!

Answer 1

It's just the way Java works. By default, System.out.println() calls the toString() method on its param object to convert it to its string representation.

If the object's class is not overriding the toString() method, you may get as a result something like className@someHexadecimalNumber . (which is the result of the default implementation of the toString() method in the Object class)

Answer 2

Because System.out.println(this); invokes

String.valueOf(this)

which in turn calls

this.toString()

It's basically System.out.println(this.toString()); .

System.out is a PrintStream so you can see PrintStream.println(Object) which says (in part)

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() .

Answer 3

Java method implicitly called ... why?

One answer is (basically) that that is the way that println(Object) is specified to work.

Another answer is that that is the best (simple) design alternative. Consider:

• println(Object) is supposed to output a text rendering of the object

• there are two alternatives:

  • use a generic mechanism to render the object

  • use a rendering method (like toString() )

• considering the "generic mechanism", how would it decide what fields of the object to render, and how to render them? Unless the programmer can somehow tailor rendering, it is impossible for a generic mechanism to "get it right". And the tailoring, is liable to be more complicated than writing a toString() method.

• considering the toString() approach, the method can be overridden to render the object as the programmer wants ... or not.

Now admittedly, a simple toString() method is often not sufficiently flexible, but that can be addressed in a variety of ways; eg

• use PrintWriter.format(...) to do the formatting / writing, or

• use a formatter that generates a String and PrintWriter.print(String) .

(For situations where you need to render and output large amounts of text, you need to use a different approach; ie not a PrintWriter . That is a different topic.)