Week_02_Lec_ Java Intro continueed..pptx

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

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)

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 }

Input from Keyboard
• Explained more in the Lab 2 manual
– Please refer to it

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;

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:

Auto type-conversion in expression
Arithmetic operators
convert byte or char or
short into integers
solution
Can not assign an int to byte

Type promotion
• int & char -> int
• int & float -> float
… …
• Example of multiple types together
(int + byte) * (float – double)
int * double
double

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);
……

Arithmetic Operators

Relational Operator

Boolean Logical Operators

Operator Precedence

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?

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

An example of for loop and if condition

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++;
}

break with Label

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 ) ;

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];

Array Example
Trying to access an array with negative index
or index value more than the size, a Java run-
time error will occur

2-D arrays
All rows contain same columns Each row contain different columns

Different array declaration syntax

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

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:

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);
}
}

• End of Lecture 2

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