9

This is an example I was working on from my java tutorial. I have a Time1 class that does not have a constructor, and hence I was expecting it to be initialized with the default values to int, that is zero.

public class Time1 {
    private int hour; // expected to be initialised with zero
    private int minute; // expected to be initialised with zero
    private int second; // expected to be initialised with zero

    public void setTime(int hour, int minute, int second) {
        if (hour < 0 || hour >= 24 || minute < 0 || minute >= 60 || second < 0 || second >= 60) {
            throw new IllegalArgumentException("value out of range");
        }
        this.hour = hour;
        this.minute = minute;
        this.second = second;
    }

    public String toUniversalString() {
        return String.format("%02d:%02d:%02d", hour, minute, second);
    }

    public String toString() {
        return String.format("%d:%02d:%02d %s", ((hour == 0 || hour == 12) ? 12 : hour % 12), minute, second, (hour < 12 ? "AM" : "PM"));
    }
}

And now I have the main class

public class Time1test {
    public static void main(String[] args) {
        Time1 thistime = new Time1();
        System.out.println(thistime);
        thistime.setTime(13, 22, 33);
        System.out.println(thistime);
    }
}

I was expecting System.out.println(thistime); before using the setTime() method to return 00:00:00 because I haven't used any methods to reformat it, however I am getting the output as 12:00:00 AM, that is equal to calling toString() method. Why was this method being called by default when a new object is initialized, even without being called for?

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3
  • 3
    Why were you expecting 00:00:00? Did you expect Java to automatically call toUniversalString? Commented Nov 8, 2015 at 22:46
  • 2
    Basically, when you pass an Object to print, like this: System.out.println(Object), it really just does System.out.println(Object.toString()) Commented Nov 9, 2015 at 0:52
  • 1
    @user2357112, I was not aware what method is being called and why/how, and hence this question. And Thank you Zizouz212 Commented Nov 9, 2015 at 7:11

6 Answers 6

Reset to default
8

If you are using an ide like eclipse, you might have noticed a mark near your method toString() that says overrides java.lang.Object.toString. This is the method that is being called by default when you are trying to print an object. This looks like 

 * @return  a string representation of the object.
 */
public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

Since you have your own definitions inside the method by the same name, it is getting overridden. You can notice the difference if you rename your own toString method to something else, and the output would be something like Time1@2a139a55

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1 Comment

Well, I didn't notice that. Clicking on it opens that other class, and makes sense on why my own custom class is giving AM/PM whereas the default one is giving kind of a memory location.
8

The problem is in your toString method. What the following does

((hour==0 || hour==12)?12:hour%12)

is, whenever the value of hour is either 0 or 12, print 12, otherwise print hour % 12.

When you call:

System.out.println(thistime);

the print result will be the same as:

System.out.println(thistime.toString());

so this is why the toString method is invoked. You can write something like ((hour == 12) ? 12 : hour % 12) to fix it.

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1 Comment

It's not converted. PrintStream.println() is overloaded for Object, where String.valueOf() is called.
6

When you call System.out.println(thistime); for your object, your toString method is executed, and the String it returns is printed.

Your toString method explicitly returns 12 when hours == 0 :

return String.format("%d:%02d:%02d %s", ((hour==0 || hour==12)?12:hour%12),minute, second, (hour<12?"AM":"PM"));

                                          -------
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3 Comments

Thank you Eran, but I didn't call this method, so why was this method applied?
@scott When you attempt to print an object it calls toString.
Thank you. Answers my question. :)
4

System.out.println expects a String as parameter, so it calls your toString Method to convert your Time1 Object into a String. Due to the logic you have in there, you are seeing the "12:00:00 AM" output.

You can do

System.out.println(time1.toUniversalString());

to get the output you expect;

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4

I was expecting System.out.println(thistime); before using the setTime() method to return 00:00:00 because I haven't used any methods to reformat it, however I am getting the output as 12:00:00 AM, that is equal to calling toString() method. Why was this method being called by default when a new object is initialized, even without being called for?

When you print object(which you did with System.out.println(thistime)), its toString method is called(even without explicit calling) by default. Its as simple as that.

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1 Comment

This is wrong. PrintStream.println() is overloaded for Object, where String.valueOf() is explicitly called.
0

The print method is overloaded and calls .toString() for anything that is not primitive. The print method can format primitive values but calls toString() for any Object subclass.

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