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Using Objects, Part I

PrintStream Objects
We have already seen how to use the predefined object System.out
- recall that System.out is an instance of the class PrintStream, which defines the behavior for objects like System.out, or, in other words, the messages that can be sent to the objects.
- recall that one of those behaviors was called println
PrintStream objects have behaviors other than printing a line of text (i.e., can send other kinds of messages):
- print ("something to display") - prints the text on the screen, but doesn't cause subsequent text to be written on the next line
- PrintStream provides another version of println, that requires no additional information.
Thus, classes may define more than one behavior.
Exercise
Write two different sequences (each containing three statements) that display a single line, Fly me to the moon And let me play among the stars . How would you change the program so that each phrase was printed on a different line, double spaced?
Java terminology
- References - ways of referring to objects, e.g., System.out
- In Java, a method is a section of Java code within a class that provides a particular behavior.
- An identifier indicates the name of the methods, the method-name.
- The additional information sent in a message is called a method's arguments.
- Thus, a message, in Java terminology, is: method-name (arguments).
- When a message is sent to an object, the sender of the message suspends until the receiver gets the message and acts on it (i.e., executes it). This is called invoking the method.
- Java has certain conventions for identifiers:
Optional exercise
Identify the object references, method-names and arguments in the Java statements below:
```
System.out.print ("CLEMSON!");
car54.ask ("Where are my fish?");
smart.get ();
x.y ("a", "b");
```

The String class

Another predefined Java class is String, which models a sequence of characters.

any group of characters in double quotes is a string e.g., "CLEMSON!"
this is called a String reference
we have seen String references already

References can also be used to send messages to the objects to which they refer. System.out is a reference to to a PrintStream object.

If we can have references to String objects, what behaviors does the String class provide?

One method is toUpperCase.

Using the form reference . methodname ( arguments ) we can work out how to send a message to the String object referenced by "sc".

"sc".toUpperCase()

The above message does not change "sc", rather it creates a reference to a new String object that contains "SC".

This new reference is returned as a result of invoking the method toUpperCase.

What do we already know that we can do references?

send messages to them
use them as arguments in messages to other objects

Let's look at the methods we know so far:

Class Method Return value Arguments

PrintStream println none none

PrintStream println none String reference

PrintStream print none String reference

String toUpperCase String reference none

Some more terminology:

the signature of a method is its name and a description of its arguments
the prototype of a method is the description of its return value with its signature

Reference variables
A reference variable is an identifier that can be given a value that is a reference. The given value can vary.
Suppose we had a variable called chant that could contain a reference to a String object, containing "Go, tigers go!"
We can use this variable in our program, thus:
```
System.out.println (chant);
System.out.println (chant);
System.out.println (chant);
System.out.println ("Go, go, go!!");
```
Because chant is a String reference, we can even send it messages, e.g.,
```
System.out.println (chant.toUpperCase ());
```
Declaring reference variables and saving references
To introduce a reference variable into a Java program, it must first be declared. A declaration consists of the class of the object to be referenced followed by the name (a valid Java identifier) of our choice. E.g.,
```
String      chant;
PrintStream output;
```
If you do not declare a variable, but use it in your program, an error will be issued at compile time.
To give a variable a value, we use an assignment statement:
```
chant = "Go tigers, go!"
```
We can then use chant as we did above.
The general form of an assignment statement is:
variable = value
Some other examples of assignment and use:

Assignment Use

output = System.out output.println ("Hello");

loudChant = chant.toUpperCase (); output.println (loudChant);
Summary
In this section, we have looked at the following topics:
- Using PrintStream objects
- An example of using String object
- Reference variables and assigning values
Last modified: 1/12/99

System.out.print ("Hello"); System.out.print ("World");	Hello World
System.out.println ("Hello"); System.out.println ("World");	Hello World

Class	Method	Return value	Arguments
PrintStream	println	none	none
PrintStream	println	none	String reference
PrintStream	print	none	String reference
String	toUpperCase	String reference	none

Assignment	Use
output = System.out	output.println ("Hello");
loudChant = chant.toUpperCase ();	output.println (loudChant);

Using Objects, Part I

PrintStream Objects

Exercise

Java terminology

Optional exercise

The String class

Reference variables

Declaring reference variables and saving references

Summary