The Java Course provides a general introduction to programming in Java. It is based on A.B. Downey's book, How to Think Like a Computer Scientist. Click here for details.

Printing an Object

The output of this program is:


When Java prints the value of a user-defined object type, it prints the name of the type and a special hexadecimal (base 16) code that is unique for each object. This code is not meaningful in itself; in fact, it can vary from machine to machine and even from run to run. But it can be useful for debugging, in case you want to keep track of individual objects.

In order to print objects in a way that is more meaningful to users (as opposed to programmers), you usually want to write a method called something like printTime:

  public static void printTime (Time t) {
    System.out.println (t.hour + ":" + t.minute + ":" + t.second);

Compare this method to the version of printTime in Section 3.9.

The output of this method, if we pass either t1 or t2 as an argument, is 11:8:3.14159. Although this is recognizable as a time, it is not quite in the standard format. For example, if the number of minutes or seconds is less than 10, we expect a leading 0 as a place-keeper. Also, we might want to drop the decimal part of the seconds. In other words, we want something like 11:08:03.

In most languages, there are simple ways to control the output format for numbers. In Java there are no simple ways.

Java provides very powerful tools for printing formatted things like times and dates, and also for interpreting formatted input. Unfortunately, these tools are not very easy to use, so I am going to leave them out of this book. If you want, though, you can take a look at the documentation for the Date class in the java.util package.

Last Update: 2011-01-24