Chapter 4 - Classes in Java

1
Classes
• class: reserved word; collection of a fixed
number of components
• Components: members of a class
• Members accessed by name
• Class categories/modifiers
– private
– protected
– public

2
Classes (continued)
• private: members of class are not accessible
outside class
• public: members of class are accessible
outside class
• Class members: can be methods or variables
• Variable members declared like any other
variables

3
Syntax
The general syntax for defining a class is:

4
Syntax (continued)
• If a member of a class is a named constant, you
declare it just like any other named constant
• If a member of a class is a variable, you declare it
just like any other variable
• If a member of a class is a method, you define it
just like any other method

5
Syntax (continued)
• If a member of a class is a method, it can
(directly) access any member of the class—data
members and methods
- Therefore, when you write the definition of a
method (of the class), you can directly access any
data member of the class (without passing it as a
parameter)

6 6
Syntax: Value-Returning Method

7 7
User-Defined Methods
• Value-returning methods
– Used in expressions
– Calculate and return a value
– Can save value for later calculation or print value
• modifiers: public, private, protected,
static, abstract, final
• returnType: type of the value that the method
calculates and returns (using return statement)
• methodName: Java identifier; name of method

8 8
Syntax
• Syntax: Formal Parameter List
-The syntax of the formal parameter list is:
• Method Call
-The syntax to call a value-returning method is:

9 9
Syntax (continued)
• Syntax: return Statement
-The return statement has the following syntax:
return expr;
• Syntax: Actual Parameter List
-The syntax of the actual parameter list is:

10 10
Equivalent Method Definitions
public static double larger(double x, double y)
{
double max;
if (x >= y)
max = x;
else
max = y;
return max;
}

11 11
(continued)
{
if (x >= y)
return x;
else
return y;
}

12 12
(continued)
{
if (x >= y)
return x;
return y;
}

13 13
Programming Example:
Palindrome Number
• Palindrome: integer or string that reads the
same forwards and backwards
• Input: integer or string
• Output: Boolean message indicating
whether integer string is a palindrome

14 14
Solution: isPalindrome Method
public static boolean isPalindrome(String str)
{
int len = str.length();
int i, j;
j = len - 1;
for (i = 0; i <= (len - 1) / 2; i++)
{
if (str.charAt(i) != str.charAt(j))
return false;
j--;
}
return true;
}

15 15
Sample Runs: Palindrome Number

16 16
Sample Runs: Palindrome Number
(continued)

17 17
Flow of Execution
• Execution always begins with the first statement
in the method main
• User-defined methods execute only when called
• Call to method transfers control from caller to
called method
• In method call statement, specify only actual
parameters, not data type or method type
• Control goes back to caller when method exits

18 18
Programming Example: Largest
Number
• Input: set of 10 numbers
• Output: largest of 10 numbers
• Solution
– Get numbers one at a time
– Method largest number: returns the larger of 2
numbers
– For loop: calls method largest number on each number
received and compares to current largest number

19 19
Solution: Largest Number
static Scanner console = new Scanner(System.in);
public static void main(String[] args)
{
double num;
double max;
int count;
System.out.println("Enter 10 numbers.");
num = console.nextDouble();
max = num;
for (count = 1; count < 10; count++)
{
num = console.nextDouble();
max = larger(max, num);
}
System.out.println("The largest number is "
+ max);
}

20 20
Sample Run: Largest Number
• Sample Run:
Enter 10 numbers:
10.5 56.34 73.3 42 22 67 88.55 26 62 11
The largest number is 88.55

21 21
Void Methods
• Similar in structure to value-returning
methods
• Call to method is always stand-alone
statement
• Can use return statement to exit method
early

22 22
Void Methods: Syntax
• Method Definition
-The general form (syntax) of a void method
without parameters is as follows:
modifier(s) void methodName()
{
statements
}
• Method Call (Within the Class)
-The method call has the following syntax:
methodName();

23 23
Void Methods with Parameters: Syntax

24 24
Void Methods with Parameters:
Syntax (continued)

25 25
Primitive Data Type Variables as
Parameters
• A formal parameter receives a copy of its
corresponding actual parameter
• If a formal parameter is a variable of a
primitive data type:
– Value of actual parameter is directly stored
– Cannot pass information outside the method
– Provides only a one-way link between actual
parameters and formal parameters

26 26
Reference Variables as Parameters
• If a formal parameter is a reference variable:
– Copies value of corresponding actual parameter
– Value of actual parameter is address of the object
where actual data is stored
– Both formal and actual parameter refer to same
object

27 27
Uses of Reference Variables as
Parameters
• Can return more than one value from a
method
• Can change the value of the actual object
• When passing address, would save memory
space and time, relative to copying large
amount of data

28 28
Reference Variables as Parameters:
type String

29 29
Example 7-11
public class Example7_11
{
public static void main(String[] args)
{
int num1; //Line 1
IntClass num2 = new IntClass(); //Line 2
char ch; //Line 3
StringBuffer str; //Line 4
num1 = 10; //Line 5
num2.setNum(15); //Line 6
ch = 'A'; //Line 7
str = new StringBuffer("Sunny"); //Line 8
System.out.println("Line 9: Inside main: "
+ "num1 = " + num1
+ ", num2 = "
+ num2.getNum()
+ ", ch = " + ch
+ ", and str = "
+ str); //Line 9
type String (continued)

30 30
funcOne(num1, num2, ch, str); //Line 10
System.out.println("Line 11: After funcOne: "
+ "num1 = " + num1
+ ", num2 = "
+ num2.getNum()
+ ", ch = " + ch
+ ", and str = "
+ str); //Line 11
}

31 31
public static void funcOne(int a, IntClass b,
char v,
StringBuffer pStr)
{
int num; //Line 12
int len; //Line 13
num = b.getNum(); //Line 14
a++; //Line 15
b.addToNum(12); //Line 16
v = 'B'; //Line 17
len = pStr.length(); //Line 18
pStr.delete(0, len); //Line 19
pStr.append("Warm"); //Line 20
System.out.println("Line 21: Inside funcOne: n"
+ " a = " + a
+ ", b = " + b.getNum()
+ ", v = " + v
+ ", pStr = " + pStr
+ ", len = " + len
+ ", and num = " + num); //Line 21
}
}

32 32

33 33
num1 = 10; //Line 5
num2.setNum(15);
//Line 6 ch = 'A';
//Line 7
str = new StringBuffer("Sunny"); //Line 8

34 34
System.out.println("Line 9: Inside main: "
+ "num1 = " + num1
+ ", num2 = "
+ num2.getNum()
+ ", ch = " + ch
+ ", and str = "
+ str);
//Line 9

35 35
int num; //Line 12
int len;
//Line 13 num = b.getNum();
//Line 14

36 36
num = b.getNum(); //Line 14

37 37
a++; //Line 15

38 38
b.addToNum(12); //Line 16

39 39
v = 'B'; //Line 17

40 40
len = pStr.length(); //Line 18

41 41
pStr.delete(0, len); //Line 19

42 42
pStr.append("Warm"); //Line 20

43 43Java Programming: From Problem Analysis to Program Design, 3e
System.out.println("Line 21: Inside funcOne: n"
+ " a = " + a
+ ", b = " + b.getNum()
+ ", v = " + v
+ ", pStr = " + pStr
+ ", len = " + len
+ ", and num = " + num); //Line 21

44 44Java Programming: From Problem Analysis to Program Design, 3e

45 45
System.out.println("Line 11: After funcOne: "
+ "num1 = " + num1
+ ", num2 = "
+ num2.getNum()
+ ", ch = " + ch
+ ", and str = "
+ str); //Line 11

46
Accessing Objects
• Referencing the object’s data:
objectRefVar.data
e.g., myCircle.radius
• Invoking the object’s method:
objectRefVar.methodName(arguments)
e.g., myCircle.getArea()

Chapter 4 - Classes in Java

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Chapter 4 - Classes in Java