Chapter 3. Input and Output
The programs you’ve looked at so far simply display messages, which doesn’t really involve that much computation. This chapter shows you how to read input from the keyboard, use that input to calculate a result, and then format that result for output.
The System Class
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 has additional methods (like
println) for displaying output.
In fact, we can use
System.out.println to display the value of
The result is shown here:
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 will likely get something else.
As shown in Figure 3-1,
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 ...