COMMON MISTAKES CSC 111 Java Program Structure String

COMMON MISTAKES CSC 111 - Java Program Structure String Methods

Program Structure • Read and understand the problem • In all programs their should be: – Named class – Main method import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); } } By: Arwa Alturki 2

Program Structure • Brackets after both of class and main method import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); } } ✓ import java. util. *; public class Revision; public static void main (String [] args); Scanner input = new Scanner(System. in); } } ✗ By: Arwa Alturki 3

Variables • Preserve naming convention • Constant variable public class Revision{ public static void main (String [] args){ final double DOLLAR_PRICE = 3. 75; } } ✓ public class Revision; public static void main (String [] args); double DOLLAR_PRICE = 3. 75; } } ✗ ? By: Arwa Alturki 4

Variables • Casting variable DO NOT change its type public class Revision{ public static void main (String [] args){ char ch = 'A'; double num = (double) ch + 3. 5; System. out. println("The charachter is " + ch); System. out. println("The number is " + num); } } OUTPUT: The character is A The number is 68. 5 By: Arwa Alturki 5

Variables • Complete division public class Revision{ public static void main (String [] args){ int init_price = 150; int sale = 15; double final_price = 150 - (150 * 15 / 100. 00); System. out. println("The price is " + final_price); } } OUTPUT: The price is 127. 5 SR By: Arwa Alturki 6

Variables • Integer division public class Revision{ public static void main (String [] args){ int init_price = 150; int sale = 15; double final_price = 150 - (150 * 15 / 100); System. out. println("The price is " + final_price); } } OUTPUT: The price is 128 SR By: Arwa Alturki 7

Input • Display message to the user before reading import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); int ID; String name; System. out. print("Enter your name: "); name = input. next(); System. out. print("Enter your ID: "); ID = input. next. Int(); } } By: Arwa Alturki ✓ 8

Input • Display message to the user before reading import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); int ID; String name; name = input. next(); System. out. print("Enter your name: "); ID = input. next. Int(); } ✗ ? } By: Arwa Alturki 9

Input • One Scanner object for all readings import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); int ID; String name; System. out. print("Enter your name: "); name = input. next(); System. out. print("Enter your ID: "); ID = input. next. Int(); } } By: Arwa Alturki ✓ 10

Input • One Scanner object for all readings import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input 1 = new Scanner(System. in); Scanner input 2 = new Scanner(System. in); int ID; String name; System. out. print("Enter your name: "); name = input 1. next(); System. out. print("Enter your ID: "); ID = input 2. next. Int(); } } By: Arwa Alturki ✗ 11

Input • Use next. Line method to read a line of strings – next method read until the first space in the line import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); System. out. print("Enter your full name: "); String str = input. next. Line(); } } By: Arwa Alturki 12

Output • Use comma (, ) NOT plus (+) in printf public class Revision{ public static void main (String [] args){ int year = 2014; double days = 365. 25; System. out. printf("We are on %d where there are %. 2 f days", year, days); } } ✓ public class Revision{ public static void main (String [] args){ int year = 2014; double days = 365. 25; System. out. printf("We are on %d where there are %. 2 f days" + year + days); } } By: Arwa Alturki ✗ 13

String Operations • Make your solution general as possible as you can – Use the given format import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); System. out. print("Enter the time as hh: mm: "); String str = input. next(); int index = str. index. Of(': '); String h = str. substring(0, index); String m = str. substring(index+1); System. out. print("The hour now is “+h+" and “+m); } } ✓ By: Arwa Alturki 14

String Operations • Make your solution general as possible as you can – Use the given format import java. util. *; public class Revision{ public static void main (String [] args){ Scanner input = new Scanner(System. in); System. out. print("Enter the time as hh: mm: "); String str = input. next(); String h = str. substring(0, 2); String m = str. substring(3); System. out. print("The hour now is “+h+" and “+m); } } ✗ By: Arwa Alturki 15

String Operations • Str. Index. Of(char) public class Revision{ public static void main (String [] args){ String str = "1: 15"; int index = str. index. Of(': '); } } public class Revision{ public static void main (String [] args){ String str = "1: 15"; int index = str. index. Of(: ); } } By: Arwa Alturki ✓ ✗ 16

String Operations • Str. Index. Of(char, index to start from) public class Revision{ public static void main (String [] args){ String str = "1: 15: 30"; int index 1 = str. index. Of(': '); int index 2 = str. index. Of(': ', index 1+1); } } ✓ public class Revision{ public static void main (String [] args){ String str = "1: 15: 30"; int len = str. length(); int index 1 = str. index. Of(': '); int index 2 = str. index. Of(': ', len); } By: Arwa Alturki } ✗ 17

String Operation • Parsing (convert string to number) public class Revision{ public static void main (String [] args){ String str = "135"; int num 1 = Integer. parse. Int(str); double num 2 = Double. parse. Double("125. 75"); } } ✓ public class Revision{ public static void main (String [] args){ String str = "135"; int num 1 = Integer. parse. Int("str"); double num 2 = Double. parse. Double(125. 75); } } By: Arwa Alturki ✗ 18

String Operations • Remember: – index. Of method returns int value, which is the index of the given character – char. At method returns char value, which is the character at the given index – Substring method return string NOT numbers By: Arwa Alturki 19

Control Structure (If/Else) • Use compare. To or equals method to compare two strings public class Revision{ public static void main (String [] args){ String str = "Test String Comparison"; if (str. compare. To("Test String") == 0) System. out. println("Similar strings"); } } ✓ public class Revision{ public static void main (String [] args){ String str = "Test String Comparison"; if (str. equals("Test String")) System. out. println("Similar strings"); } } By: Arwa Alturki ✓ 20

Control Structure (If/Else) • Use compare. To or equals method to compare two strings public class Revision{ public static void main (String [] args){ String str = "Test String Comarision"; if (str == "Test String") System. out. println("Similar strings"); } } By: Arwa Alturki ✗ 21

QUESTIONS ? ! By: Arwa Alturki 22