1. Java Programming: Guided Learning with Early Objects
Chapter 3
Introduction to Objects and Classes

2. Objectives Learn about objects and reference variables
Explore how to use predefined methods in a program
Become familiar with the class String
Learn how to use the class Math
Java Programming: Guided Learning with Early Objects

3. Objectives (continued)
Explore the wrapper classes
Learn how to design your own classes
Explore how to write your own methods
Learn how to avoid bugs by preparing a class design document
Java Programming: Guided Learning with Early Objects

4. Objectives (continued)
Learn how to do a walk-through of a class design
Become familiar with the GUI components ContentPane, JFrame , JLabels , and JButtons
Learn how to use the classes Color and Font
Learn how to handle events in a program
Java Programming: Guided Learning with Early Objects

5. Objects and Reference Variables
new operator
System allocates memory space for specific data type
Stores data in memory space
Returns address of the memory space
Reference variable stores address of memory location
Object is an instance of a class
Garbage collection: Java automatically reclaims unused memory
Java Programming: Guided Learning with Early Objects

6. Figure 3-3 Variable str and object str
Java Programming: Guided Learning with Early Objects

7. Using Predefined Classes and Methods in a Program
Method: collection of programming statements to accomplish a task
Java includes many predefined classes
Predefined class includes predefined methods
Organized as class libraries
To use a predefined method, you must know:
Name of the class and package
Method name and parameters
Java Programming: Guided Learning with Early Objects

8. The Dot Between the Class (Object) Name and the Class Member: A Precaution
Member access operator: dot operator (.)
Dot separates reference variable from method
Methods distinguished from variables by parentheses
Methods with no parameters must have empty parentheses
Java Programming: Guided Learning with Early Objects

9. The class String Index of first character in string is 0
Length of string: number of characters in it
String variables are reference variables
String object an instance of class String
class String contains methods to process strings
String variable invokes String method using the dot operator, method names, arguments
Java Programming: Guided Learning with Early Objects

10. The class Math Method type is data type returned by the method
Contained in package java.lang
Java Programming: Guided Learning with Early Objects

11. Using Methods of the class Math in a Program
Java does not require package java.lang to be imported
Classes automatically imported by default
static method: method heading contains the word static
Non static method does not contain static
Actual parameters: parameters used in a method call
static import statements
import static packagename.ClassName.*
Java Programming: Guided Learning with Early Objects

12. Primitive Data Types and Wrapper Classes
Wrapper classes for primitive types:
class Integer for int
class Long for long
class Double for double
class Float for float
class Character for char
Wrapper classes are immutable
Autoboxing and auto-unboxing simplify wrapping and unwrapping of primitives
Java Programming: Guided Learning with Early Objects

13. The class Character (Optional)
Class Character manipulates characters
Contained in package java.lang
public static methods of class Character used like methods of class Math
Java Programming: Guided Learning with Early Objects

14. Introduction to Methods
Two categories of user-defined methods:
Value-returning methods
Void methods
To use a method in a program, you must know:
Name of method
Number, type, order of parameters
Data type of value returned
Code required to accomplish the task
Formal parameter: declared in method header
Actual parameter: listed in call to a method
Java Programming: Guided Learning with Early Objects

15. Syntax: Method Method syntax: modifier(s) returnType methodName
(formal parameter list) {
statement(s) }
modifier(s): visibility of method
returnType: type of value method returns
methodName: Java identifier
Java Programming: Guided Learning with Early Objects

16. Syntax: Formal Parameter List
Syntax of a formal parameter list:
dataType identifier, dataType identifier, …
Java Programming: Guided Learning with Early Objects

17. Method Call Method call syntax: methodName (actual parameter list);
Java Programming: Guided Learning with Early Objects

18. Syntax: Actual Parameter List
Actual parameter list syntax:
expression or variable, expression or variable,…
Call a method, use name together with parameters in parentheses
Formal parameter list may be empty
Parentheses still needed
Actual parameter list is empty if formal parameter list is also empty
Java Programming: Guided Learning with Early Objects

19. return Statement
Value-returning methods use return statement to return a value
Pass value back when method completes
Syntax: return expr;
expr is a variable, constant, or expression
return is a reserved word
Method immediately terminates and control goes back to caller after return statement
Java Programming: Guided Learning with Early Objects

20. Value-Returning Method
Local variable: variable is local to a method
Example:
public static double square(double num) {
return num * num; }
Java Programming: Guided Learning with Early Objects

21. Void Method Value-returning method returns one value
May want to return no value or many values
Void method returns no value:
public static void printNum(double x) {
System.out.print(x); }
Java Programming: Guided Learning with Early Objects

22. Flow of (Method) Execution
Methods can appear in any order
Method main executes first
Other methods execute when called
Method call transfers control to first statement in method body
After method finishes, control passed back to caller
After value-returning method executes, return value replaces method call statement in caller
Java Programming: Guided Learning with Early Objects

23. Classes Class: a collection of components called members
Syntax for defining a class:
modifier(s) class ClassName modifier(s) {
classMembers }
private: cannot be accessed directly outside the class
public: can be accessed directly outside class
Java Programming: Guided Learning with Early Objects

24. Classes (continued)
Members accessed directly from outside the class should be public
Members that should not be accessed directly should be private
User should never be given direct access to data members
Instance variables: variables declared without using the modifier static
Non static methods of a class are instance methods
Java Programming: Guided Learning with Early Objects

25. Constructors Constructor has same name as the class
Executes automatically when object is created
Used to guarantee instance variables are initialized
Two types of constructors:
With parameters
Without parameters (default constructor)
Has no return type
Class can have more than one, but must have same name, different parameters
Java Programming: Guided Learning with Early Objects

26. Variable Declaration and Object Instantiation
Declaring a reference variable of class type does not allocate memory
Syntax for new operator:
new className();
new className(arg1, arg2, …,argN);
Number of arguments and types match formal parameters
If type of arguments does not match, Java tries to find the best match
Java Programming: Guided Learning with Early Objects

27. Defining Methods and Constructors
Method definition:
public void setCurrentTemp(int cTemp) {
currentTemp = cTemp; }
Constructor definitions:
public Thermometer() {
currentTemp = 0;
public Thermometer (int cTemp) {
Java Programming: Guided Learning with Early Objects

28. The Method toString Definition of method toString:
public String toString() {
return “Current temp: “ + currentTemp; }
Suppose myTemp is a Thermostat object
System.out.println(myTemp) is:
Current temperature: 40
When object reference parameter to print, println, printf, method toString called
Java Programming: Guided Learning with Early Objects

29. The Definition of the class Thermometer
public class Thermometer {
private int currentTemp;
public Thermometer() {
currentTemp = 0; }
public Thermometer (int cTemp) {
currentTemp = cTemp;
public void setCurrentTemp (int cTemp) { …
Java Programming: Guided Learning with Early Objects

30. Avoiding Bugs: Designing a Class and Documenting the Design
Good programmers determine how to solve a problem completely before writing code
Design written in enough detail that another programmer with same skill can produce code
Algorithm describes means for achieving objective
Usually described using pseudocode
Pseudocode resembles code without formal syntax
Java Programming: Guided Learning with Early Objects

31. Debugging: Design Walk-Throughs
Code walk-throughs find and remove bugs
Design walk-throughs follow same principle
Bugs may creep in at design time
Design should be prepared carefully before presented to someone else
Do not shortchange the design phase
Java Programming: Guided Learning with Early Objects

32. GUI JFrame, JLabel, Colors, Fonts, JButton, and Handling an Event (Optional)
Output dialog box an example of a graphical user interface (GUI)
GUI components are placed in the content pane of the window
Inner window area below title bar, inside border
Clicking a button causes an event to occur
Java system listens for events and reacts
Java Programming: Guided Learning with Early Objects

33. Figure 3-9 Java GUI components
Java Programming: Guided Learning with Early Objects

34. Figure 3-10 GUI of Figure 3-8 after clicking the Color button
Java Programming: Guided Learning with Early Objects

35. Figure 3-11 GUI of Figure 3-8 after clicking the Font button
Java Programming: Guided Learning with Early Objects

36. Creating a Window GUI components are objects
Instances of a particular class
Attributes associated with a window:
Every window has a title
Every window has width and height
Java Programming: Guided Learning with Early Objects

37. JFrame class JFrame provides methods to control attributes of a window
Two ways to create a window:
Declare an object of type JFrame
Extend class JFrame
Inheritance: new class derived from existing class
Use the modifier extends
Java Programming: Guided Learning with Early Objects

38. JFrame (continued)
Subclass of superclass inherits all properties of the superclass
All public members of superclass directly accessed by the subclass
Set window height in pixels
Pixel is smallest unit of space on the screen
If window created by using a JFrame object, properties have to be specified
If window created by extending JFrame, it inherits JFrame properties
Java Programming: Guided Learning with Early Objects

39. Getting Access to the Content Pane
Content pane is the inner area of window
Below title, inside border
Method getContentPane allows access to window content pane
Must declare reference variable of Container type to use getContentPane
Container pane;
pane = getContentPane();
class Container in package java.awt
Java Programming: Guided Learning with Early Objects

40. Table 3-6 Commonly Used Methods of the class Container
Java Programming: Guided Learning with Early Objects

41. JLabel Labels are objects of class JLabel
Instantiate JLabel objects to create labels
JLabel in package javax.swing
Labels have the attributes title, width, height
Java Programming: Guided Learning with Early Objects

42. class Color Text black by default
class Color contained in package java.awt
Java uses RGB color scheme
Create color by mixing red, green, blue hues
Color redColor = new Color(255,0,0);
class Color defines standard colors as constants
Java Programming: Guided Learning with Early Objects

43. class Font Show text in different fonts with class Font
Contained in java.awt
Constructor of class Font takes three arguments:
String specifying font name
int value specifying font style
int value specifying font size
Expressed in points
Guaranteed fonts: Serif, SanSerif , Times New Roman , Monospaced , Dialog , DialogInput
Java Programming: Guided Learning with Early Objects

44. JButton Creates a button private JButton colorB, fontB;
colorB = new JButton(“Color”);
fontB = new JButton(“Font”);
Modify GridLayout to accommodate two rows and two columns, then add components
pane.setLayout(new GridLayout(2,2));
pane.add(colorB);
pane.add(fontB);
Java Programming: Guided Learning with Early Objects

45. Handling an Event Clicking a JButton generates an action event
Sends message to action listener
Sending an event to a listener invokes a method with the event as the argument
Invocation happens automatically
Specify two items:
Corresponding listener object
Register the listener
Method to be invoked when event is sent
Java Programming: Guided Learning with Early Objects

46. Figure 3-15 Processing an action event
Java Programming: Guided Learning with Early Objects

47. Summary Reference variables store a memory location
Objects are instances of a class
Use the new operator to instantiate an object
Unused memory collected and freed using Java garbage collector
Predefined classes organized as collections of packages called class libraries
Package must be imported to be used
Java Programming: Guided Learning with Early Objects

48. Summary (continued)
All primitive data types have corresponding wrapper class
Wrapping and unwrapping facilitated by autoboxing and auto-unboxing
Two types of methods:
Value-returning methods
Void methods
Two types of parameters:
Actual parameters
Formal parameters
Java Programming: Guided Learning with Early Objects

49. Summary (continued) Encapsulation: combining data and operations
Class members: components of a class
Data members are fields
Non static class methods are instance methods
Constructor executes when object created
Algorithm describes means for achieving objective
Described with pseudocode
Java Programming: Guided Learning with Early Objects

50. Summary (continued) Design walk-throughs help prevent bugs
Same process as code walk-throughs
Create GUIs with JFrame, JLabel, JButton
Clicking a button generates an action event
When event is generated, message is sent to action listener
Action listener invokes a method to perform some action
Java Programming: Guided Learning with Early Objects