Week_02_Lec_ Java Intro continueed..pptx

1. School of Computing
COMP 155: Object Oriented Programming
Dr Ziaul Hossain
Lec 2: JAVA constructs

2. Date School of Computing Slide 2
Input From Keyboard
• We use the Scanner class for that purpose
• Have to import java.util.Scanner
• nextInt() function for Integer type
• nextDouble() for Double type
• nextLine() for a Line (a string before you press Enter)

3. Date School of Computing Slide 3
Sample code for Integer Input
1 import java.util.Scanner;
2
3 public class InputTest {
4 public static void main(String[] args) {
5 Scanner input = new Scanner(System.in);
6
7 System.out.println("Please enter an integer");
8 int userGivenInteger = input.nextInt();
9
10 System.out.println("You entered: " +
userGivenInteger);
11 }
12 }

4. Date School of Computing Slide 4
Input from Keyboard
• Explained more in the Lab 2 manual
– Please refer to it

5. Date School of Computing Slide 5
Type Conversion
• Java does automatic type conversion
– Types are compatible
– Destination type is larger size than source type
– byte -> short -> char -> int -> long -> float -> double
– Also known as Widening Casting
int i = 20; char ch = ‘Q’;
double d = i; long l = ch;

6. Date School of Computing Slide 6
Explicit Casting
• Manual or explicit casting
– Converts larger sized data types into smaller ones
– May loose value or precision
– Also known as Narrowing Casting
Output:

7. Date School of Computing Slide 7
Auto type-conversion in expression
Arithmetic operators
convert byte or char or
short into integers
solution
Can not assign an int to byte

8. Date School of Computing Slide 8
Type promotion
• int & char -> int
• int & float -> float
… …
• Example of multiple types together
(int + byte) * (float – double)
int * double
double

9. Date School of Computing Slide 9
Demonstration of println()
• println() or print() function are capable of printing
multiple variables together
• Some examples you should try to find the output
and understand the reasoning behind:
int a=20, b=30;
System.out.println(“a”);
…..println(a);
…..println(a+b);
……println(“a”+b);
……println(“Sum=” + a+b);
……

10. Date School of Computing Slide 10
Arithmetic Operators

11. Date School of Computing Slide 11
Relational Operator

12. Date School of Computing Slide 12
Boolean Logical Operators

13. Date School of Computing Slide 13
Operator Precedence

14. Date School of Computing Slide 14
If-else example
int i=10, j=15, k=70;
int a=0, b=30, c=40, d=20;
if(i == d/2) {
if(j < 20) a = b;
if(k > 100) c = d;
else a = c;
}
else a = d;
What is the value of a?

15. Date School of Computing Slide 15
While Loop
int i=100, j=200;
while ( ++i < --j) ;
System.out.println(
“Midpoint: ” + i);
Another Example

16. Date School of Computing Slide 16
An example of for loop and if condition

17. Date School of Computing Slide 17
Another for Loop Example - condition
boolean done = false;
int i = 0;
for( ; !done; ) {
System.out.println("i is " + i);
if(i == 10) done = true;
i++;
}

18. Date School of Computing Slide 18
break with Label

19. Date School of Computing Slide 19
Ternary Operator - ? :
if ( a > b )
max = a;
else
max = b;
max = (a>b)? a : b;
max= (a>b)? ( (a>c)?a:c ) : ( (b>c)?b:c ) ;

20. Date School of Computing Slide 20
Arrays
• In C, just declaration was enough
int arr [10];
• In Java, declaration and allocation with new
keyword required
int months[];
months = new int [12]; int months[] = new int [12];

21. Date School of Computing Slide 21
Array Example
Trying to access an array with negative index
or index value more than the size, a Java run-
time error will occur

22. Date School of Computing Slide 22
2-D arrays
All rows contain same columns Each row contain different columns

23. Date School of Computing Slide 23
Different array declaration syntax

24. Date School of Computing Slide 24
A new format in for loop
int nums[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int sum = 0;
for(int i=0; i < 10; i++) sum += nums[i];
int nums[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int sum = 0;
for(int x: nums) sum += x;
New Format allowed in Java
* You can use the break statement if want to finish the loop early

25. Date School of Computing Slide 25
Explanation of for loop new format
public class JavaApplication3 {
public static void main(String[] args) {
int nums[] = { 1, 2, 3, 4, 5, 6,
7, 8, 9, 10 };
for(int x : nums) {
System.out.print(x + " ");
x = x * 10; // no effect on nums
System.out.println(x);
}
}
}
1 10
2 20
3 30
4 40
5 50
6 60
7 70
8 80
9 90
10 100
Output:

26. Date School of Computing Slide 26
For-each loop in 2D array
public class JavaApplication3 {
public static void main(String[] args) {
int sum = 0;
int nums[][] = new int[3][5];
for(int i = 0; i < 3; i++)
for(int j=0; j < 5; j++)
nums[i][j] = (i+1)*(j+1);
for(int x[] : nums) {
for(int y : x) {
System.out.println("Value is: " + y);
sum += y;
}
}
System.out.println("Summation: " + sum);
}
}

27. Date School of Computing Slide 27
• End of Lecture 2