B asic Concepts of Classes

1. Basic Concepts of Classes Chapter 3

2. Chapter 3 • 3.0 Basic Concepts of Classes • 3.1 Class Concept • 3.2 Class Defination • 3.3 Data Members • 3.4 Basic Types of Methods • 3.5 Methods Defination • 3.6 Static Fields • 3.7 Predefined classes and wrapper classes

3. Class Concept • The class is at the core of Java. Any concept you wish to implement in a Java program must be encapsulated within a class. • Object-oriented program consists of at least one class. A class consists of variables/data and methods. • Running an OO program requires a main method. If there are more than one class, there should be a(an) main/application class which includes the main method. • You will learn the basic elements of a class and how to create objects from the class.

4. Class Diagram

5. Class Defination General Form of a Class: A class is consists of data and code that operates on that data. The following is a general form of a class. class Classname { //declare instance variables type var1; type var2; //declare methods type method1(parameters){ //body of method } //end of method1   type method2(parameters){ //body of method } //end of method2 } //end of class Class declaration Variables declaration Methods

6. Class Defination Defining Classes: The syntax: <modifier> class <Classname> { //body of class } Example: public class Student { //body of class }

7. Access Modifier / Access Specifier • Encapsulation links data with the code (methods/operations) that manipulates it. • Through encapsulation, you can control what parts of a program can access the members of a class. It is to prevent misuse. • How a member of a class can be accessed is determined by the access modifier that modifies its declaration. • Java’s access modifiers are: • public • private • protected

8. Access Modifier / Access Specifier • Data (data members) are usually declared asprivate • While the behaviors of the instances is implemented by public methods

9. Defining a Class General syntax: <modifier> class <Classname> { …… //body of class } publicmodifier is commonly used first letter is uppercase

10. Variables privatemodifier is used • Instance variables • Syntax: <modifier> <primitive/reference data type><variable> ; • Class variables • Syntax: <modifier> static<primitive data type/reference data type><variable> ;

11. Methods public modifier is commonly used • General syntax : <modifier> <returntype> <nameOfMethod>( parameter) { …… //body of class } Primitive/reference The identifier starts with a lowercase letter

12. Common Methods in a Class

13. Example 1: Q: Write a program to input the length and width of a rectangle and calculate and print the perimeter and area of the rectangle. Step 1 : Identify all the relevant nouns. • Length • Width • Rectangle • Perimeter • Area

14. Example 1: continue… Step 2 : Identify the class(es). It is clear that: • Length – length for the rectangle • Width – width for the rectangle • Rectangle • Perimeter – perimeter for the rectangle • Area – area for the rectangle Therefore, Rectangle is suitable to be a class.

15. Example 1: continue.. Step 3 : Identify the data members for each of the classes. Determine the information that is essential to describe each class. • Perimeter – is not needed. It can be computed from length & width. • Area – is also not needed. It can be computed from length & width. • Length – is required . It is one of the data member • Width – is required. It is one of the data member

16. Example 1: continue.. Step 4 : Identify the operations for each of the classes. The operations can be determine by looking at the verbs. For example: input, calculate & print “Write a program to input the length and width of a rectangle and calculateand print the perimeter and area of the rectangle”

17. Example 1: continue.. Therefore, you need at least the following operations. • Set the Length • Set the Width • Calculate the Perimeter • Calculate the Area • Print the perimeter • Print the area

18. Example 1: continue.. Other operations that are customary to include: • Get the Length • Get the Width

19. Example 1: continue.. Class Diagram

20. Example 1: Defining a Class and Variables publicclass Rectangle { private double length; private double width; }

21. Example 1: Constructor public class Rectangle { private double length; private double width; public Rectangle ( ) { length=0.0; width=0.0; } } Set initial values for all variables

22. Example 1: Mutator methods //continue.. public void setLength( double len) { length=len; } public void setWidth(double wid) { width=wid; } Assign value len to length Assign value wid to width

23. Example 1: Accessor methods //continue.. public double getLength() { return length; } public double getWidth() { return width; } return value length return value width

24. Example 1: Processor methods //continue.. public double calculatePerimeter() { double perimeter=(2*length)+2*width; return perimeter; } public double calculateArea() { return length*width; } Calculate & return value perimeter Calculate & return value area

25. Example : Printer method //continue.. public String toString() { String out= “Length = “ + length + “\nWidth = “ + width; return out; } Return all values as String

26. Methods -Math.sqrt() -Math.min() -ext java (package) lang (subpackage) (class) Math Methods -chatAt() -length() -indexOf() -equals() -toString() -ext (class) String (class) StringBuffer Methods -insert() -reverse() -charAt() -length() -ext

27. Predefined classes System.in • provides only a facility to input 1 byte at a time with its read method • need an input facility to process multiple bytes as a single unit. •  *associate a Scanner object to the System.in object. Scanner • Scanner object will allow us to read primitive data type values and strings. • Located in java.util.*. *other methods : nextByte(), nextDouble(), nextFloat(), nextLong(), nextShort(), next() – for String values

28. Predefined classes Math(refer Java API documentation) • contain class methods for commonly used mathematical functions • to call these class methods, you can use the following syntax: Math.<method()> • among others: • sqrt (a) – returns square root • min (a,b) – returns the smaller of a and b • max(a,b) – returns the larger of a and b • example : double punca= Math.sqrt(25); http://www.leepoint.net/notes-java/summaries/summary-math.html

29. Predefined classes Example: Math Class

30. Wrapper • Java wrappers are classes that wrap up primitive values in classes that offer utility methods to manipulate the values. • Utility class that has useful methods to be performs on primitive data. • One of the method is to perform type conversion. • Example: • parseDouble() method from Double class • parseInt() method from Integer class {Lab 2 Java Programming Basic[IOconsoleExample]}

31. Wrapper String • A string is a sequence of characters that is treated as a single value. •  As stated earlier, String is part of java.lang package which is automatically included during compilation. •  About 50 methods defined in the String class. Some of the predefined methods of String class that commonly used are : • substring( a, b ){Tutorial 2[Q2]} • a – specifies the position of the 1st character • b – specifies the value that is 1 more than the position of the last character. • this method returns a substring from a to (b-1) index of a string object.

32. Wrapper • length(){Tutorial 2[Q2]} • returns the number of characters of a sting. • indexOf(){Note 02 Example[Example 3]} • to locate the index of a substring within another string. • returns the index. • charAt(n) {Tutorial 2[Q2]} • to access an individual character of a string. n is the index /position of that character in a string. • toUpperCase(){Tutorial 2[Q2]} • converts every character in a string to uppercase

33. Wrapper • equals(){Tutorial 3[Worker]} • use to compare two string objects and returns TRUE if the two objects have the exact same sequence of characters. • Example : str1.equals(str2) • Returns TRUE if str1 equals to str2 and FALSE if they’re not equal. • equalsIgnoreCase(){Tutorial 3[Worker]} • has the same function as the equals() method but the comparison is done in a case-insensitive manner. • Example: str1=”hello”; • str1.equalsIgnoreCase(“Hello”); ----returns TRUE

34. Wrapper • compareTo() • compare two string objects. • Example : str1.compareTo(str2) • Will returns 0 if they are equal, a negative integer if str1 is less than str2, and a positive integer if str1 is greater than str2

35. Wrapper toString() {Tutorial 3[Worker]} • Creates a string that is the name of object’s class • Is a public value-returning method. • Not take any parameters and returns the address of a String object • The heading: • public String toString() • Can override the default definition of the method toString() to convert an object to desired string. • Is useful for outputting the values of the instance variables.

36. Exercises on methods of string class Exercise 1: ReverseCharacter • Write a program which requests a line of String from user. Store the String characters to an array. Then displays the character in reverse order with all words print in uppercase. Exercise 2: Egg • Write a program which requests a line of String from user. Then displays each words in reverse order. Convert it to a new string by placing egg in front of every vowel (a, e, u).

37. Exercises on methods of string class Exercise 3: ReverseWord • Write a program which requests a line of String from user. Then displays each words in reverse order. Exercise 4: ReverseWorldLine • Write a program which requests a line of String from user. Then displays each words in separate lines and in reverse order.