Introduction to Java

Main()

• Main method is where the program execution begins.

• There is only one main

• Displaying the results:System.out.println (“Hi there!”);

System.out.println(5+6);

Operators

• Relational operators• Result is either true or false

– Equal ==– Less than <– Less than or equal <=– Larger than >– Larger than or equal >=• 7 >= 8 is true• 6 == 9 is false

Operators

• Arithmetic operators

– Add +– Subtract -– Multiply *– Divide /– Remainder %

• System.out.println(5 + 9);• System.out.println (8 / 2);• System.out.println(8 % 3);• System.out.println(8 / 3);

variables

• Variables are used to store and recall results of computations.

a=5;b=7;C=4;System.out.println(a+b+c);

variables

• Variables are used to store and recall results of computations.

a=5;b=7;c=4;System.out.println(a+b+c);c=c+a;System.out.println(a+b+c);

variables

• Variables must be declared (defined)• Variables have types

– int (whole numbers)– double (decimal )

• int a;

• double b;

variables• Variable declaration maps the variable name to a unique memory

location– The value of the mapped memory location can be:

• changed (assignment)a=5;

• looked up

System.out.println(a);

b=a*0.10;

Strings

• A sequence of 0 or more charactersString s = "java";

• First character is at position 0;• Characters are accessed via charAt– s.charAt(1) returns 'a' //character 'a' not string

"a"

Control structures

if (condition) // if condition is true, then stmt1 is stmt1 // executed. If condition is not true, then

stmt2 // stmt1 not executed

condition is true: stmt1stmt2

condition is false: stmt2

Control structures

if ( a > b)System.out.writeln(“a is larger than b”);

System.out.println(“done!!”);

a=8;b=3;

a > b is true output: a is larger than bdone!!

a=5;b=7;

a > b is false output: done!!

Control structures

if (condition) // if condition is true, then stmt1 is stmt1 // executed and stmt2 is not executed.

else // If condition is not true (is false), then stmt2 // stmt1 is not executed and stmt2 is executed

stmt3

condition is true: stmt1stmt3

condition is false: stmt2stmt3

Control structures

if ( a > b)System.out.writeln(“a is larger than b”);

elseSystem.out.writeln(“a is less than b”);

System.out.println(“done!!”);

a=8;b=3;

a > b is true output: a is larger than bdone!!

a=5;b=7;

a > b is false output: a is less than bdone!!

Control structures

• More than one statement in the then-part?

If ( a > b ) {System.out.println(a);System.out.println(b);System.out.println(“a is larger than b”);

}

Control structures

• More than one statement in the else-part?

If ( a > b ) System.out.println(a-b);

else {System.out.println(a);System.out.println(b);System.out.println(“a is less than b”);

}

Control structures

• More than one statement in the then-part and else-part?

If (cond) {…

}else {

…}

Control structures

• Exercise:– Level of service indicates how happy we where

with the service we received in a restaurant ( 1 – 10)

– Bill is the amount of our bill;

– Tip is 10% if the service level was less than 5;– Tip is 15% if the service level is 5 or higher;

Control structures

level = 7;

amount = 23.97;

what are the types?

Complete the program

Control structures

int level = 7;double amount = 23.97;double topRate, total;if (level < 5)

tipRate=0.1;else

tipRate=0.15;total = amount + amount *tipRate;System.out.println(total);

input

• JOptionPane class

• JOptionPane.showInputDialog(null, “Enter the amount”);

• Returns the value typed in the text box (as String)

input

String s;

s=JOptionPane.showInputDialog(null, “Enter the amount”);

double amount;

amount=Double.parseDouble(s);

For Loops

• Loops are used to repeat a part of the program.

for (int i=0; i < 10; i=i+1)System.out.print(i);

Displays: 1 2 3 4 5 6 7 8 9

For Loops

for (initialization; condition; step)body

for ( int i=0; i < 10; i=i+1 )

System.out.println(i);

For Loops

for (initialization; condition; step)body

– initialization is executed only once when the loop starts

– Condition is evaluated.• If condition is true:

– body is executed– Step is executed after completion of body– Condition is evaluated as in above

• If condition is false– body is not executed – execution is continued right after the body

For Loops

for (int i=0; i < 3; i=i+1)System.out.println(i);

System.out.println(“done”);

1: allocate an integer variable, i, and assign 0 to it2: evaluate the condition i < 3;

i < 3 is true;execute loop body

output: 0 3: execute i=i+1; I becomes 1 4: evaluate the condition i < 3;

i < 3 is true;execute loop body

output: 1

For Loops

for (int i=0; i < 3; i=i+1)System.out.println(i);

System.out.println(“done”);

5: execute i=i+1; i becomes 22: evaluate the condition i < 3;

i < 3 is true;execute loop body

output: 2 3: execute i=i+1; I becomes 3 4: evaluate the condition i < 3;

i < 3 is false;continue execution right after the loop body

output: “done!”

loops

• Write a for loop that displays 1, 3, 5, … 99 for (int i = 1; i < 100; i = i + 2) System.out.println(i);

• Write a for loop that displays 99, 97, 95, … 1 for (int i = 99; i > 0; i = i - 2) System.out.println(i);

loops

• Read a sequence of integers until zero is entered and display the sum of all the numbers that were read

– Read and keep a running total until a zero is read!!

Scanner in = new Scanner (System.in); int sum=0; System.out.println("Enter the next value ");

for (int next= in.nextInt(); next > 0; next = in.nextInt()) { sum = sum+next;

System.out.println("Enter the next value "); }

Who understands this?

Scanner in = new Scanner (System.in); int sum=0; int next;

System.out.println("Enter the next value"); next = in.nextInt(); while (next > 0) { sum = sum + next; System.out.println("Enter the next value"); next = in.nextInt(); } System.out.println("Sum = " + sum);

Who understands this?

while loop

while (condition)body

1. Condition is evaluated.• If condition is true:

– body is executed, and then step 1 is repeated.

• If condition is false– body is not executed – execution is continued right after the

body

• Write a while loop that displays 1, 3, 5, … 99 int value=1;

while (value <= 99) {System.out.println(value); value = value +2

}

while loop

• Write a while loop that displays 99, 97, 95, … 1

int value=99;

while (value >= 1) {System.out.println(value); value = value - 2;

}

while loop

• Write a while loop that displays 1, 3, 5, … 99, except the numbers that are divisible by 5 int value = 1; while (value <= 99) { if (value % 5 != 0) System.out.println(value); value = value + 1; }

while loop

loops

• Let s be a String

• Write a program that• 1:– displays number of occurrences of letter a in s that

appear before letter x in s– in case s does not contain letter x, the program must

display 0.• 2:– Displays s in reverse order

String s = "abbcdbz"; char x = 'f'; char a = 'd';

int i = 0; int count = 0; while (i < s.length()) { if (s.charAt(i) == a) { count = count + 1; } if (s.charAt(i) == x) break; // exit the loop i = i + 1; } if (i == s.length()) { // came out of the loop because we didn't see x count = 0; } System.out.println(count); for (i=s.length()-1; i>=0; i=i-1) System.out.print(s.charAt(i)); System.out.println();

Break

• Break statement exits the loop

while (condition1) {…if (condition2)

break; // continue execution from right // after the end of the loop (stmt1)

…}stmt1

break• Write all prime numbers between 10 and 1000

boolean prime; int i, j;

for (i=10; i<1000; i=i+1) { //pick all numbers between 10 and1000 prime=true; for (j=2; j<i; j=j+1) if (i%j == 0) { prime=false; break; // i is not a prime number exit the loop } if (prime) System.out.println(j); } }

Write a method:

int getGCD(int m, int n)

that returns the largest integer that divides both m and n

Public static int getGCD(int m, int n) { boolean found = false;

if (m > n) { gcd = n; } else { gcd = m; }

while (!found) { if (m % gcd == 0 && n % gcd == 0) { found = true; } else { gcd = gcd - 1; } }

return gcd;

}

Write a boolean method that given an integer m returns true if m is a prime number;

Test:Scanner inp = new Scanner(System.in);

int x = inp.nextInt(); if (isPrime(x)) { System.out.println(x + " is a prime number"); } else { System.out.println(x + " is not a prime number"); }

• Write an integer method that given an integer, m, returns the smallest prime number that is larger than n

Test:Scanner inp = new Scanner(System.in);int n = inp.nextInt();

System.out.println(nextPrime(n));

• Write a method:

int countVowels (String s)

that for a given string s, returns number of vowels in s

• Write a method:

void multiplication (int n)

that for a given parameter n displays a nxn multiplication table

• Write a method:

void triangle1 (int n)

that for a given parameter n displays:

11 21 2 31 2 3 41 2 3 4 51 2 3 4 5 6

• Write a method:

void triangle2 (int n)

that for a given parameter n displays:

12 13 2 14 3 2 15 4 3 2 16 5 4 3 2 1

• Write a method:

void triangle3 (int n)

that for a given parameter n displays:

12 43 6 94 8 12 165 10 15 20 256 12 18 24 30 36

• Write a method:

char mostOftenIn(String s)

that for a given string s, returns the character that occurs most often in s.

For example for s = “abcaabbcb”mostOftenIn(s) must return ‘b’