What mechanism does println use?

Question

I know the println method of the System.out object can be used to print any object, regardless of which class it belongs to. But can anyone tell me the mechanism that println uses to accomplish this task?

Answer 1

Calling System.out.println on an object uses Object 's toString() method - either the default implementation in Object class, or the implementations of classes that override this default.

System.out.println, eventually uses one of PrintStream 's various println methods, since System.out is a PrintStream instance.

If you pass a String to System.out.println , it will call println(String) . If you pass a char[] to it, it will call println(char[]) . If you pass any other type of Object , it will call println(Object) (unless the compiler decides to convert the Object to a String , using the object's toString() method, and then call println(String) instead).

println(Object x) converts the Object to a String by calling String.valueOf() , which returns either "null" (for null Objects) or obj.toString() .

Either way, the Object 's toString() method is used, unless the Object is null.

Answer 2

There is very little difference between the way PrintStream/PrintWriter.println(...) treats String 's and the way the compiler treats String 's. The print streams will automatically convert an Object to a String via the toString() method unless that object is null (in which case it prints null ). The difference is that you cannot set a String object to a general Object --but you can concatenate a String object with a general purpose Object which will be concatenated via the toString() method (unless the Object is null ). Frankly, this is normallly how you would use the toString() method. Here is a sample program that illustrates the differences:

public class PrintStreamTester {
    public static void main(String... args) {
        final MyObject o1 = null;
        final MyObject o2 = new MyObject("blah blah blah");

        String s1 = "o1.toString() failed";

        try {
            s1 = o1.toString();
        } catch (NullPointerException npe) {
            // who cares
        }
        final String s2 = "" + o1;
        final String s3 = o2.toString();
        final String s4 = "" + o2;

        System.out.println("o1 by itself (below):");
        System.out.println(o1);
        System.out.println("o1's toString(): " + s1);
        System.out.println("o1's \"\" + o1: " + s2);

        System.out.println("o2 by itself (below):");
        System.out.println(o2);
        System.out.println("o2's toString(): " + s3);
        System.out.println("o2's \"\" + o1: " + s4);
    }
}

class MyObject {
    private final String string;

    public MyObject(final String string) {
        this.string = string;
    }

    @Override
    public String toString() {
        return string;
    }
}

Here is the output from this program:

o1 by itself (below):
null
o1's toString(): o1.toString() failed
o1's "" + o1: null
o2 by itself (below):
blah blah blah
o2's toString(): blah blah blah
o2's "" + o1: blah blah blah