Lecture 2 (Chapter 2) Introduction to Java Programming PDF

Programming 1 Lecture 2 Chapter 2: Introduction to Java Programming Dr/ Mahmoud Gamal 1 A Java Program: Output: 2 A Java Program: Package: Group of classes. Class: 1. Data items 2. Methods (...

Programming 1 Lecture 2 Chapter 2: Introduction to Java Programming Dr/ Mahmoud Gamal 1 A Java Program: Output: 2 A Java Program: Package: Group of classes. Class: 1. Data items 2. Methods ( or functions or operations) The main method is the entry point to the program. Java is case sensitive language. Console window: Text box into which the program's output is printed. 3 Structure of a Java program class: a program public class name { public static void main(String[] args) { Body of statement; statement; main method: a named group method... statement; of statements } } statement: a command to be executed Every executable Java program consists of a class, that contains a method named main, that contains the statements (commands) to be executed. System.out.println Statement: A statement that prints a line of output on the console. Two ways to use System.out.println : System.out.println("text"); Prints the given message as output. System.out.println(); Prints a blank line of output. 5 Example: public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); System.out.println(); System.out.println("This program produces"); System.out.println("four lines of output"); } } Output: Hello, world! This program produces four lines of output 6 Compile/run a program 1. Write it. code or source code: The set of instructions in a program. 2. Compile it. javac: translates the program from Java to bytecode bytecode: runs on many computer types (any computer with JVM) 3. Run (execute) it. output: whatever the programmer instructs the program to do source code byte code output compile run Hello, World! javac Hello.java java Hello Identifier: A name given to an item in your program. Must start with a letter or _ or $ Do not use space or any special character. Do not use any reserved name like (void, class, int) Do not start with a number. legal: _myName TheCure ANSWER_IS_42 $bling$ illegal: me+u 49ers side-swipe Ph.D’s Example: public class MyClass { 8 Syntax: Syntax: The set of legal structures and commands that can be used in a particular language. Every basic Java statement ends with a semicolon ; The contents of a class or method occur between { and } 9 Programming Errors: 1- Syntax Errors: Detected by the compiler. 2- Runtime Errors: Causes the program to abort. 10 Programming Errors: 3- Logic Errors: Produces incorrect results. 11 Strings: string: A sequence of characters to be printed. Starts and ends with a " quote " character. The quotes do not appear in the output. Examples: "hello" "This is a string. It's very long!" Restrictions: May not span multiple lines. "This is not a legal String." May not contain a " character. "This is not a "legal" String either."12 Escape sequence: Escape Sequence: A special sequence of characters used to represent certain special characters in a string. \t tab character \n new line character \" quotation mark character \\ backslash character Example: System.out.println("\\hello\nhow\tare \"you\"?\\\\"); Output: \hello how are "you"?\\ 13 Questions What is the output of the following println statements? System.out.println("\ta\tb\tc"); System.out.println("\\\\"); System.out.println("'"); System.out.println("\"\"\""); System.out.println("C:\nin\the downward spiral"); Answer: a b c \\ ' """ C: in he downward spiral Questions Write a println statement to produce this output: / \ // \\ /// \\\ Answer: System.out.println("/ \\ // \\\\ /// \\\\\\"); Comments: Comment: A note written in source code by the programmer to describe or clarify the code. Comments are not executed when your program runs. Syntax: // comment text, on one line or, Examples: // This is a one-line comment. Using comments Where to place comments: at the top of each file (a "comment header") at the start of every method (seen later) to explain complex pieces of code Comments are useful for: Understanding larger, more complex programs. Multiple programmers working together, who must understand each other's code. Comments Example public class BaWitDaBa { public static void main(String[] args) { // first verse System.out.println("Bawitdaba"); System.out.println("da bang a dang diggy diggy"); System.out.println(); // second verse System.out.println("diggy said the boogy"); System.out.println("said up jump the boogy"); } } print vs println: println print