Chapter 4 1

Defining Classes and Methods

Chapter 4

Chapter 4 2

Reminders

• Project 2 was due last night• Project 3 released: due Sept 29 @ 10:30 pm

- No Late Submissions - 2 weeks b/c of exam

• Follow the newsgroup for project information and questions – newsgroup postings from GTAs are official. Even if you don’t follow the newsgroup you are responsible for all updates/changes to any projects mentioned there.

Chapter 4 3

Reminders 2

• Make sure to match given output exactly in your projects. You will lose points if your output does not match exactly. We give you sample inputs and outputs for this reason.

• Project submissions use your lab section, not your recitation section (this is clearly written on all project outlines) turnin –c cs180secXXXX –p project1 …

Chapter 4 4

Exam 1 Reminder• Tuesday, September 20• 7:00 pm – 8:00 pm• Physics 112• Covers chapters 1 – 4• More details towards end of recitation

Chapter 4 5

Using Methods

• two kinds of methods:– methods that return a single value (e.g. nextInt)

– methods that perform some action other than returning a single value (e.g println), called void methods

Chapter 4 6

Defining Methods That Return a Value

• examplepublic int fiveFactorial();{

int factorial = 5*4*3*2*1;return factorial;

}• As before, the method definition consists of

the method heading and the method body.– The return type replaces void.

Chapter 4 7

Methods That Return a Value, cont.• The body of the method definition must contain

return Expression;– This is called a return statement.– The Expression must produce a value of the

type specified in the heading.• The body can contain multiple return statements,

but a single return statement makes for better code.

• void methods can have return statements, but must not return anything.

return;

Chapter 4 8

Naming Methods• Use a verb to name a void method.

– void methods typically perform some action(s). (e.g. getAge)

• Use a noun to name a method that returns a value.– Methods that return a value are used like a

value. (e.g. fiveFactorial)• Observe the convention of starting the

method name with a lowercase letter.

Chapter 4 9

Variables• Variables of a class type name objects, which

is different from how primitive variables store values.

• All variables are implemented as memory locations.– The value of a variable of a primitive type

is stored in the assigned location.– The value of a variable of a class type is

the address where a named object of that class is stored.

Chapter 4 10

Variables, cont.• A value of any particular primitive type always

requires the same amount of storage.– example: a variable of type int always

requires 4 bytes.• An object of a class type might be arbitrarily

large.– An object of type String might be empty, or

might contain 1, 120, 5280, or more characters.

Chapter 4 11

Variables, cont.

• However, there is always a limit on the size of an address.

• The memory address where an object is stored is called a reference to the object.

• Variables of a class type behave differently from variables of a primitive type.

Chapter 4 12

Chapter 4 13

Allocating Memory for a Reference and an Object

• A declaration such asSpeciesFourthTry s;

creates a variable s that can hold a memory address.

• A statement such ass = new SpeciesFourthTry();

allocates memory for an object of type SpeciesFourthTry.

Chapter 4 14

== with Variables of a Class Type

Chapter 4 15

== with Variables of a Class Type, cont.

• When used with variables of a class type, == tests if the variables are aliases of each other, not if they reference objects with identical data.

• To test for equality of objects in the intuitive sense, define and use an appropriate equals method.

Chapter 4 16

== with Variables of a Class Type• class Species

Chapter 4 17

== with Variables of a Class Type• class SpeciesEqualsDemo

Chapter 4 18

Method equals• The definition of method equals depends on

the circumstances.– In some cases, two objects may be “equal”

when the values of only one particular instance variable match.

– In other cases, two objects may be “equal” only when the values of all instance variables match.

• Always name the method equals.

Chapter 4 19

Boolean-Valued Methods

• A method that returns a value of type boolean is called a boolean-valued method.

• Method equals produces and returns a value of type boolean.

• The invocation of a boolean-valued method can be used as the condition of an if-else statement, a while statement, etc.

Chapter 4 20

Boolean-Valued Methods, cont.

• The value returned by a boolean-valued method can be stored in a variableboolean areEqual = s1.equals(s2);

• Any method that returns a boolean value can be used in the same way.

Chapter 4 21

Class Parameters

• Recall– When the assignment operator is used with

objects of a class type, a memory address is copied, creating an alias.

– When the assignment operator is used with a primitive type, a copy of the primitive type is created.

Chapter 4 22

Class Parameters, cont.

– When a parameter in a method invocation is a primitive type, the corresponding formal parameter is a copy of the primitive type.

Chapter 4 23

Class Parameters, cont.

• When a parameter in a method invocation is a reference to a class type (i.e. a named object), the corresponding formal parameter is a copy of that reference (i.e. an identically valued reference to the same memory location).

Chapter 4 24

Class Parameters, cont.

• Exampleif (s1.equals(s2))…public boolean equals (Species otherObject)

causes otherObject to become an alias of s2, referring to the same memory location, which is equivalent tootherObject = s2;

Chapter 4 25

Class Parameters, cont.

• Any changes made to the object named otherObject will be done to the object named s2, and vice versa, because they are the same object.– If otherObject is a formal parameter of a

method, the otherObject name exists only as long as the method is active.

Chapter 4 26

Comparing Class Parameters and Primitive-Type

Parameters• A method cannot change the value of a

variable of a primitive type passed into the method.

• A method can change the value(s) of the instance variable(s) of a class type passed into the method.

Chapter 4 27

The Graphics Class

• An object of the class Graphics represents an area of the screen.

• The class Graphics also has methods that allow it do draw figures and text in the area of the screen it represents.

Chapter 4 28

Chapter 4 29

The Graphics Class, cont.

• A Graphics object has instance variables that specify an area of the screen

• In examples seen previously, the Graphics object represented the area corresponding to the inside of an applet.

Chapter 4 30

The Graphics Class, cont.

• When an applet is run, a suitable Graphics object is created automatically and is used as an argument to the applet’s paint method when the paint method is (automatically) invoked.– The applet library code does all this for us.– To add this library code to an applet

definition, useextends JApplet

Chapter 4 31

Chapter 4 32

Chapter 4 33

Programming Example, cont.

Chapter 4 34

The init Method

• An init method can be defined when an applet is written.

• The method init (like the method paint) is called automatically when the applet is run.– The paint method is used only for things

like drawing.– All other actions in an applet (adding

labels, buttons, etc.) either occur or start in the init method.

Chapter 4 35

Adding Labels to an Applet

• A label is another way to add text to an applet.

Chapter 4 36

Adding Labels to an Applet,

Chapter 4 37

The Content Pane

• Think of the content pane as inside of the applet.Container contentPane = getContentPane();

• When components are added to an applet, they are added to its content pane.

• The content pane is an object of type Container.

Chapter 4 38

The Content Pane, cont.

• A named content pane can do things such as setting colorcontentPane.setBackground(Color.WHITE);or specifying how the components are arrangedcontentPane.setLayout

(new FlowLayout());

Chapter 4 39

The Content Pane, cont.

or adding labelsJLabel label1 = new JLabel(“Hello”);contentPane.add(label1);

Chapter 4 40

Summary• You have become familiar with the concept of

a class and an object that instantiates the class.

• You have learned how to define classes in Java.

• You have learned how to define and use methods in Java.

• You have learned how to create objects in Java

Chapter 4 41

Summary, cont.• You have learned how parameters work in

Java.• You have learned about information hiding

and encapsulation.• You have become familiar with the notion of a

reference (to understand class variables and class parameters).

• (optional) You have learned more about applets.

Chapter 4 42

Exam 1 Information

• Time/Location– Tuesday, Sept 20 7-8pm, Physics 112

• Material Covered– Chapters 1 - 4

• Format:– 20 MC questions (2 pts each)– 5 short programming questions (3 x 10pts, 2 x

15pts) • Old exams on course website

Chapter 4 43

Exam 1 Information

• Topics– Encapsulation, polymorphism, information

hiding– Accessor and mutator methods– Objects, classes– Public and private modifiers– Java naming conventions– Primitive types vs. class types

Chapter 4 44

Exam 1 Information

• Topics, cont– Defining classes and methods (including the

main method)– Void methods vs. methods that return a value– Looping structures (while, do-while, for)– If-else, switch– Primitive types vs. class types (and memory

representation)– Scanner class for input, System.out for output

Chapter 4 45

Exam 1 Information

• Topics, cont– Arithmetic expressions– Boolean variables– String methods– == vs. equals methods– Basic graphics methods