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Java Programming
Objects
Java is an object-oriented programming
language
– use objects to define both the data type
and the operations that can be applied to
the data
 Objects have attributes and functionality
– attributes describe the state of the object
– the functionality of an object is the set of
actions the object can perform
 In Java, we define an object’s attributes using
variables and its functionality using methods
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Example: “Real-world” objects
Suppose we want to describe a car in terms
of its attributes and functionality
 Attributes:
– tires, steering wheel, doors, engine
– model, make, year, color
– attributes can have constant variables
• all cars have a steering wheel
 Actions:
– brake, stop, drive, reverse, park
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Java classes
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Java objects are created using classes
Encapsulation
– combining elements to create a new entity
A class encapsulates the variables and
methods that define an object
Instantiation
– the act of creating an object
– objects are called class instances
Java provides many predefined classes
You can (and will!) define your own classes
Java String class

The String class represents character
strings
String firstname = “Tammy”;
String lastname = “Bailey”;
Strings are basically an array of characters
 Strings can be concatenated (added
together) using the concatenation operator +
String fullname =
firstname + “ ” + lastname;
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Instantiation
Creating an object is called instantiation
– the new operator is used with class name
 Example: Create a TextField object
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TextField t = new TextField();
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Can create multiple instances of the same class
TextField t1 = new TextField();
TextField t2 = new TextField();
 Exception: The new operator is not required
when creating a String
Java TextField class
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The TextField class allows the editing and
display of a single line of text
TextField t = new TextField();
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Methods
– setText(String s)
• set the text of the field to the string s
– String getText()
• get the text of the field and assign it to a
variable of type string
Invoking an object’s methods
Once we create a text field, we can perform
actions on it using its methods
 The variables and methods of an object are
accessed using the dot operator
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TextField t = new TextField();
t.setText(“Hello”);
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Syntax
– object.verb( data );
– Perform verb on object using data
Interactive objects
User interaction determines the behavior of the
program
 Program receives user input through mouse
and keyboard and performs associated method
or action
 Text fields
– edit and display single line of text
 Buttons
– can specify action to occur when button is
clicked
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Action listeners
If we want a button to know when it is clicked,
we have to enable it to “listen” for user input
 This is done using the button method
addActionListener
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Button b = new Button(“click!”);
b.addActionListener(this);
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If we don’t invoke the addActionListener
method on a button, nothing will happen
when the button is clicked
Example
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We would like our applet to do the following:
– get text from text field t1 and display it in
text field t2 when button b is clicked
TextField t1 = new TextField();
TextField t2 = new TextField();
Button b = new Button(“copy text”);
b.addActionListener(this);
Actions
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We specify actions to occur when a button is
clicked in the actionPerformed method
public void actionPerformed(ActionEvent event)
{
Object cause = event.getSource();
if(cause == b)
{
t2.setText(t1.getText());
}
}
Numeric input
Suppose we want an applet that allows the
user to enter two integers and display the
minimum
– A text field contains a string
– If we want to perform numeric operations
on the input from a text field, we have to
convert the string to a numeric data type
– numbers are primitive data types, not
objects
 Can convert using Java type wrappers
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Type wrappers
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Convert primitive types into objects
Primitive Type
byte
short
int
long
float
double
char
boolean
Wrapper Type
Byte
Short
Integer
Long
Float
Double
Character
Boolean
String conversion
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Convert String str to byte b
byte b = Byte.parseByte(str);
Convert String str to short s
short s = Short.parseShort(str);
Convert String str to int i
int i = Integer.parseInt(str);
Convert String str to long l
long l = Long.parseLong(str);
Convert String str to float f
float f = Float.parseFloat(str);
Convert String str to double d
double d = Double.parseDouble(str);
Action performed for integer input
public void actionPerformed(ActionEvent event)
{
Object cause = event.getSource();
int x = Integer.parseInt(t1.getText());
int y = Integer.parseInt(t2.getText());
if(cause == b)
{
int min = minimum(x,y);
t3.setText(“The minimum is: ” + min);
}
}