The System.out
object is defined in each Java program. It has methods for displaying text strings and numbers in plain text format on the system display, which is sometimes referred to as the "console". For example:
Program 3-2
public class PrintVar
{
public static void main(String[] args)
{
int number = 5;
char letter = 'E';
double average = 3.95;
boolean done = false;
System.out.println("number = " + number);
System.out.println("letter = " + letter);
System.out.println("average = " + average);
System.out.println("done = " + done);
System.out.print("The ");
System.out.println("End!");
}
}
Run output:
number = 5
letter = E
average = 3.95
done = false
The End!
Method System.out.println
displays (or prints) a line of text in the console window. When System.out.println
completes its task, it automatically positions the output cursor (the location where the next character will be displayed) to the beginning of the next line in the console window (this is similar to pressing the Enter key when typing in a text editor - the cursor is repositioned at the beginning of the next line in your file).
The expression
"number = " + number
from the statement
System.out.println("number = " + number);
uses the +
operator to "add" a string (the literal "number = "
) and number (the int
variable containing the number 5). Java has a version of the +
operator for String concatenation that enables a string and a value of another data type to be concatenated (added). The result of this operation is a new (and normally longer) string. String concatenation is discussed in more detail in the next lesson.
The lines
System.out.print("The ");
System.out.println("End!");
of Program 3-2 display one line in the console window. The first statement uses System.out
's method, print
, to display a string. Unlike println
, print
does not position the output cursor at the beginning of the next line in the console window after displaying its argument. The next character displayed in the console window appears immediately after the last character displayed with print
.
Note the distinction between sending a text constant, "number = "
, versus a variable, number
, to the System.out
object. A boolean
variable will be printed out as its representation of true
or false
.