We have been using
System.out.println for a while, but you might not have thought about what it means.
System is a class that provides methods related to the “system” or environment where programs run. It also provides
System.out, which is a special value that provides methods for displaying output, including
In fact, we can use
System.out.println to display the value of
The result is:
This output indicates that
System.out is a
PrintStream, which is defined in a package called
java.io. A package is a collection of related classes;
java.io contains classes for “I/O” which stands for input and output.
The numbers and letters after the @ sign are the address of
System.out, represented as a hexadecimal (base 16) number. The address of a value is its location in the computer’s memory, which might be different on different computers. In this example the address is
685d72cd, but if you run the same code you might get something different.
As shown in Figure 3.1.1,
System is defined in a file called System.java, and
PrintStream is defined in PrintStream.java. These files are part of the Java library, which is an extensive collection of classes you can use in your programs.