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 characters String 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:stmt1 stmt2 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 b done!! 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:stmt1 stmt3 condition is false:stmt2 stmt3

Control structures if ( a > b) System.out.writeln(“a is larger than b”); else System.out.writeln(“a is less than b”); System.out.println(“done!!”); a=8; b=3; a > b is true output: a is larger than b done!! a=5; b=7; a > b is false output: a is less than b done!!

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:

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 it 2: 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 2 2: 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:

Write a method: void triangle2 (int n) that for a given parameter n displays:

Write a method: void triangle3 (int n) that for a given parameter n displays:

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’