Java Reference
In-Depth Information
By comparing the output that appears with the statements that produced it, it is easy to see that
a statement such as
System.out.println("# The BlueJ Line");
literally prints the string that appears between the matching pair of double-quote characters.
The basic form of a call to println is
System.out.println( something-we-want-to-print );
where something-we-want-to-print can be replaced by any arbitrary string, enclosed between a
pair of double-quote characters. For instance, there is nothing significant in the “#” character
that is in the string—it is simply one of the characters we wish to be printed.
All of the printing statements in the printTicket method are calls to the println method
of the System.out object that is built into the Java language, and what appears between the
round brackets is the parameter to each method call, as you might expect. However, in the
fourth statement, the actual parameter to println is a little more complicated and requires
some more explanation:
System.out.println("# " + price + " cents.");
What it does is print out the price of the ticket, with some extra characters on either side of the
amount. The two “+” operators are being used to construct a single actual parameter, in the
form of a string, from three separate components:
The method
println prints its
parameter to the
text terminal.
the string literal: "# " (note the space character after the hash);
the value of the price field (note that there are no quotes around the field name because we
want the field's value, not its name);
the string literal: " cents." (note the space character before the word "cents" ).
When used between a string and anything else, “+” is a string-concatenation operator (i.e., it concat-
enates or joins strings together to create a new string) rather than an arithmetic-addition operator. So
the numeric value of price is converted into a string and joined to its two surrounding strings.
Note that the final call to println contains no string parameter. This is allowed, and the result
of calling it will be to leave a blank line between this output and any that follows after. You will
easily see the blank line if you print a second ticket.
Exercise 2.36 Write down exactly what will be printed by the following statement:
System.out.println("My cat has green eyes.");
Exercise 2.37 Add a method called prompt to the TicketMachine class. This should
have a void return type and take no parameters. The body of the method should print the
following single line of output:
Please insert the correct amount of money.
Exercise 2.38 What do you think would be printed if you altered the fourth statement of
printTicket so that price also has quotes around it, as follows?
System.out.println("# " + "price" + " cents.");
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