Java - Class Structure

Java Class Structure
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam 1

2
Classes
 A class describes a set of objects
 The objects are called instances of the class
 A class describes:
 Fields (instance variables)that hold the data for each object
 Constructors that tell how to create a new object of this class
 Methods that describe the actions the object can perform
 In addition, a class can have data and methods of its own
(not part of the objects)
 For example, it can keep a count of the number of objects it has
created
 Such data and methods are called static
 We are avoiding static data and methods for the time being
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

3
Defining a class
 Here is the simplest syntax for defining a class:
class ClassName {
// the fields (variables) of the object
// the constructors for the object
// the methods of the object
}
 You can put public, protected, or private before the word
class
 Things in a class can be in any order (I recommend the
above order)

4
Defining fields
 An object’s data is stored in fields (also called
instance variables)
 The fields describe the state of the object
 Fields are defined with ordinary variable declarations:
 String name;
Double health;
int age = 0;
 Instance variables are available throughout the entire
class that declares them

Defining a method
 A method has the syntax:
return-type method-name(parameters) {
method-variables
code
}
 Example:
boolean isAdult(int age) {
int magicAge = 21;
return age >= magicAge;
}
 Example:
double average(int a, int b) {
return (a + b) / 2.0;
}
5Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

6
Methods may have local variables
 A method may have local (method) variables
 Formal parameters are a kind of local variable
 int add(int m, int n) {
int sum = m + n;
return sum;
}
 m, n, and sum are all local variables
 The scope of m, n, and sum is the method
 These variables can only be used in the method, nowhere else
 The names can be re-used elsewhere, for other variables

7
Declarations in a method
 The scope of formal parameters is the entire method
 The scope of a variable in a block starts where you
define it and extends to the end of the block
if (x > y) {
int larger = x;
}
else {
int larger = y;
}
return larger;
larger
scope of larger
larger
scope of a
different larger
Illegal: not declared in current scope

8
Nested scopes
int fibonacci(int limit) {
int first = 1;
int second = 1;
while (first < 1000) {
System.out.print(first + " ");
int next = first + second;
first = second;
second = next;
}
System.out.println( );
} limitfirst
next
second

Returning a result from a method
 If a method is to return a result, it must specify the type
of the result:
 boolean isAdult ( …
 You must use a return statement to exit the method
with a result of the correct type:
 return age >= magicAge;

Returning no result from a method
 The keyword void is used to indicate that a method
doesn’t return a value
 The return statement must not specify a value
 Example:
 void printAge(String name, int age) {
System.out.println(name + " is " + age + " years old.");
return;
}
 There are two ways to return from a void method:
 Execute a return statement
 Reach the closing brace of the method

11
Sending messages to objects
 We don’t perform operations on objects, we “talk” to them
 This is called sending a message to the object
 A message looks like this:
object.method(extra information)
• The object is the thing we are talking to
• The method is a name of the action we want the object to take
• The extra information is anything required by the method in order
to do its job
 Examples:
g.setColor(Color.pink);
amountOfRed = Color.pink.getRed( );

12
Putting it all together
class Person {
// fields
String name;
int age;
// constructor
Person(String name) {
this.name = name;
age = 0;
}
// methods
String getName() {
return name;
}
void birthday() {
age = age + 1;
System.out.println(
"Happy birthday!");
}
}

13
Using our new class
Person john;
john = new Person("John Smith");
System.out.print (john.getName());
System.out.println(" is having a birthday!");
john.birthday();
 Of course, this code must also be inside a class!

14
Diagram of program structure
 A program consists of
one or more classes
 Typically, each class is
in a separate .java file
Program
File File
File
File
Class
Variables
Constructors
Methods
Variables
Variables
Statements
Statements

End…

Java - Class Structure

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