OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
Java Characters:
Normally, when we work with characters, we use primitive data types char.
Example:
char ch ='a';
Character Methods:
 isLetter( ) Determines whether the specified char value is a letter.
 isDigit( ) Determines whether the specified char value is a digit.
 isWhitespace( ) Determines whether the specified char value is white space.
 isUpperCase( ) Determines whether the specified char value is uppercase.
 isLowerCase( ) Determines whether the specified char value is lowercase.
 toUpperCase( ) Returns the uppercase form of the specified char value.
 toLowerCase( ) Returns the lowercase form of the specified char value.
 toString( ) Returns a String object representing the specified character value that is, a
one-character string.
All these Methods call after character.Method
public class Test {
public static void main(String args[]) {
System.out.println(Character.isLetter('c'));
System.out.println(Character.isLetter('5'));
}
}
This produces the following result:
true
false
public class Test {
public static void main(String args[]) {
System.out.println(Character.isDigit('c'));
System.out.println(Character.isDigit('5'));
}
}
This produces the following result:
false
true
public class Test{
public static void main(String args[]){
System.out.println(Character.isWhitespace('c'));
System.out.println(Character.isWhitespace(' '));
System.out.println(Character.isWhitespace('\n'));
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OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
System.out.println(Character.isWhitespace('\t'));
}
}
This produces the following result:
false
true
true
true
public class Test{
public static void main(String args[]){
System.out.println( Character.isUpperCase('c'));
System.out.println( Character.isUpperCase('C'));
System.out.println( Character.isUpperCase('\n'));
System.out.println( Character.isUpperCase('\t'));
}
}
This produces the following result:
false
true
false
false
public class Test{
public static void main(String args[]){
System.out.println(Character.isLowerCase('c'));
System.out.println(Character.isLowerCase('C'));
System.out.println(Character.isLowerCase('\n'));
System.out.println(Character.isLowerCase('\t'));
}
}
This produces the following result:
true
false
false
false
public class Test{
public static void main(String args[]){
System.out.println(Character.toUpperCase('c'));
System.out.println(Character.toUpperCase('C'));
}
}
This produces the following result:
C
C
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OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
public class Test{
public static void main(String args[]){
System.out.println(Character.toLowerCase('c'));
System.out.println(Character.toLowerCase('C'));
}
}
This produces the following result:
c
c
Java Strings:
Strings which are widely used in Java programming are a sequence of characters. In the Java
programming language, strings are objects.
Creating Strings:
The most direct way to create a string is to write:
String greeting ="Hello world!";
String Length:
length ( ) method returns the number of characters contained in the string object.
publicclassStringDemo{
public static void main(String args[]){
String s ="second class";
int len = s.length();
System.out.println("String Length is : "+ len );
}
}
This would produce the following result:
String Length is :12
String Methods:








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char charAt (int index) Returns the character at the specified index.
String concat(String str) Concatenates the specified string to the end of this string.
int indexOf(int ch) Returns the index within this string of the first occurrence of the specified
character.
int indexOf(String str) Returns the index within this string of the first occurrence of the specified
substring.
int length( ) Returns the length of this string.
String replace(char oldChar, char newChar) Returns a new string resulting from replacing all
occurrences of oldChar in this string with newChar.
String toLowerCase( ) Converts all of the characters in this String to lower case using the rules of the
default locale.
String toUpperCase() Converts all of the characters in this String to upper case using the rules of the
default locale.
OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
public class Test {
public static void main(String args[]) {
String s = "Strings are immutable";
char result = s.charAt(8);
System.out.println(result);
}
}
This produces the following result:
a
public class Test {
public static void main(String args[]) {
String s = "Strings are mutable";
s = s.concat(" all the time");
System.out.println(s);
}
}
This produces the following result:
Strings are mutable all the time
public class Test {
public static void main(String args[]) {
String Str = "Tutorials ";
System.out.print("Found Index :" );
System.out.println(Str.indexOf( 'o' ));
}
}
This produces the following result:
Found Index :4
public class Test{
public static void main(String args[]){
String Str = "Welcome to Tutorialspoint.com";
System.out.print("Return Value :" );
System.out.println(Str.replace('o', 'T'));
System.out.print("Return Value :" );
System.out.println(Str.replace('l', 'D'));
}
}
This produces the following result:
Return Value :WelcTme tT TutTrialspTint.cTm
Return Value :WeDcome to TutoriaDspoint.com
Page 37
OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
public class Test{
public static void main(String args[]){
String Str = "Welcome to Tutorialspoint.com";
System.out.print("Return Value :");
System.out.println(Str.toLowerCase());
}
}
This produces the following result:
Return Value :welcome to tutorialspoint.com
public class Test{
public static void main(String args[]){
String Str = new String("Welcome to Tutorialspoint.com");
System.out.print("Return Value :" );
System.out.println(Str.toUpperCase() );
}
}
This produces the following result:
Return Value :WELCOME TO TUTORIALSPOINT.COM
Object Oriented Concepts
1.1. Mutability
All objects either are immutable or mutable. the state of an immutable object never
changes, while the state of a mutable object can change.
String object are mutable , for example String methods have a concatenation operator
+, but is does not modify either of its argument, instead, it returns a new string whose
state is the concatenation of the states of its arguments. If we did the following
assignment to t with the state shown in Figure (b).
t=t + "g";
the results as shown in figure (c) is that t now refers to an a new String object whose
state is "abcdefg" and the object referred to by s is unaffected.
Mutable
object
Page 38
OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
On the other hand, arrays are mutable. The assignment
a[i]=e/// where e element is equal 6
causes the state of array a to change by replacing its ith element with the value
obtained by evaluating expression e ( the modification occurs only if i is in bounds for
a, an exception is thrown otherwise).
Discussion Note
If a mutable object is shared by two or more variables, modifications made through
one of the variables will be visible when the object is used through the other variable.
Example
Suppose the shared array in figure (b) is modified by
b[0]=i; // i= 6 before
this causes the zeroth element of the array to contain 6 ( instead of 1 it used to
contain ) , as shown in figure (c)
Mutable
object
1.2. Method Call Semantic
An attempt to call a method as in this form :
e. m (), // where e is a representative of a class and m is a method
first evaluates e to obtain the class of object whose method is being called .
Then the expression for the arguments are evaluated to obtain actual parameter values
‫المتغيرات الفعلية‬. This evaluation happen left to right.
Next an activation record is created for the call and pushed onto the stack; the
activation record contains room for the formal parameters ‫ المعامالت الشكلية‬of the
method ( the formals are the variables declared in the method header) and any other
local storage the method required.
Page 39
OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
When the actual parameter are assigned to the formals , this kind of parameters passing
is called "call by value" and Finally control is dispatched to the called method e.m.
Note: if the actual parameters value is a reference to an object, that reference is
assigned to the formal. this means that the called procedure shares object with its
caller. Furthermore, if these object are mutable, and the called procedure change their
state, these changes are visible to the caller when it returns.
Example 1
And in the main class, we define
And we call the method
The results in the Stack storage are:
After return
form
Method
Method Call
Page 40
After return form
multiples Method
OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
Example 2
Example 3
public class CallsByRefers {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
CallsByRefers cc=new CallsByRefers();
int [] b= {0,2,3,4};
int kl=9;
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OOP with Java
Dr. Ahmed M. Al-Salih
University of Babylon/ College of IT
class – First Semester- Department of Software
2nd
cc.change(b, kl);
for (int i=0; i<b.length;i++)
{
System.out.println(b[i]);
}
System.out.println("The Value of kl after return from
method "+kl);
}
public int change (int [] a, int j){
int i = j*2;
System.out.println("The value of i inside the methdo
"+i);
for (int k=0; k<a.length;k++)
{
a[k]=a[k]*2;
}
return j;
}// End Main Method
}// End CallsByRefers Class
The Output for Example 2 is :The value of i inside the method 18
0
4
6
8
The Value of kl after return from method 9
Quiz #2
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