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Java Basics 1 Compiling A “compiler” is a program that translates from one language to another Typically from easy-to-read to fast-to-run e.g. from C or Assembly to machine code Java must be (explicitly) compiled before it is run The Java compiler turns Java source code (.java) into Java bytecode (.class) 2 The Java Platform The Java Virtual Machine (JVM) is responsible for running bytecode The idea: bytecode can be interpreted quickly The same bytecode can be interpreted on any architecture: write once, run anywhere Code (C,C++) compiled to machine code must be compiled to a specific system 3 The Java Language Created by Sun Microsystems Introduced in 1995, initial popularity grew due to Internet applications Excitement surrounding Java applets Confusion with Javascript Steady rise in popularity has continued for “better” programming reasons 4 A Historical Interlude: The Java Team Java originally intended to be used on “smart” consumer electronics Bill Joy James Gosling (“the father of Java”) Founded Sun, 1982 Intelligent robots will replace humanity in the near future… University of Calgary grad First JVM, compiler, interpreter also developed Emacs Patrick Naughton Arrested in late 90s on child predator charges Not mentioned so much as a founding father anymore 5 The Java Language (cont’d) … is a high-level programming language … is very object oriented … is similar to C++ and C … typically compiled to Java bytecode … is often confused with the Java Platform, but these are two different aspects of “Java” 6 Syntax and Semantics The syntax rules of a language define how we can combine reserved words, symbols, and identifiers The semantics of a program statement define what the statement means Problem with program syntax = “error” Problem with program semantics = “bug” 7 Java Program Structure A Java program consists of: One or more classes A class contains one or more methods A method contains program statements We will explore these terms in detail 8 Java Program Structure // comments about the class public class MyProgram { class header class body Comments can be placed almost anywhere } 9 Java Program Structure // comments about the class public class MyProgram { // comments about the method public static void main (String[] args) { method body method header } } 10 Hello World // HelloWorld.java public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } } 11 Hello World // HelloWorld.java public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } } Creates a “class” called HelloWorld Compiled to HelloWorld.class Classes used to define objects… later 12 Hello World // HelloWorld.java public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } } The “main” method is where it starts to run Ignore “public static void” and “String[] args” for now 13 Hello World // HelloWorld.java public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } } Contains one “statement” The System.out.println function comes from the Java “class library” Ends with a semicolon (all statements do) 14 Compiling and Running Create the file HelloWorld.java in a text editor Compile: Run: javac HelloWorld.java java HelloWorld Output: Hello World! 15 Comments Three kinds of comments: // a one-line comment /* a multi-line comment */ /** a javadoc comment */ To simplify: comments are good 16 Reserved Words and Identifiers Reserved words are specified by the language All Java reserved words are in the text Identifiers are specified by a programmer Maybe you: e.g. HelloWorld Maybe someone else: e.g. println 17 Restrictions and Conventions Restriction Identifiers can not start with a digit Conventions Title case for class names: HelloWorld Uppercase for constants: MAX 18 White Space Conventions Idea: make programs easy to read Use consistent indentation Use blank lines and comments to visually separate methods The fact that it compiles doesn’t make it right… 19 Strong Typing Java is a “strongly typed” language All variables and values have a specific type Type is known when the program is compiled…. before it is run So all variables and values must be declared with a type before being used 20 Declaring Variables Syntax: <variable declaration> ::= <type> <declarator>, …. ; <declarator> ::= <identifier> <declarator> ::= <identifier> = <expression> Examples: int count1, int count 2; int count = 0; String course1 = “CMPT 126”; 21 Assignment We use the = operator for variable assignment Initialization is a special case When a value is assigned, the old value is overwritten In Java, we use the final modifier to declare a variable constant final int MAX_HEIGHT = 6; 22 Primitive Data Types in Java Four integer types: Two floating point types float, double One of them is for characters byte, short, int, long char One of them is for boolean values boolean 23 Expressions and Assignment An expression is a combination of one or more operators and operands Arithmetic operators: +, -, *, /, % Use the normal order of operations e.g. int exp = 2 * 5 +7; count = count + 1; count++; Boolean operators: &&, || 24 More Assignment Operators x += y is equivalent to x = x + y Also: -= *= /= %= 25 Data Conversion Non-matching types can be converted A widening conversion is automatic A narrowing conversion may lose information e.g. from short to int e.g. from float to int Three kinds of conversion: Assignment Promotion Casting 26 Assignment Conversion final int dollars = 6; double money; money = dollars; Only works for widening conversion 27 Promotion int count = 2; float mass = 18.342; mass = mass / count; Passing count to an operator that expects floating point values 28 Casting float mass = 18.342; int roundedmass = (int) mass; Casting works for widening and narrowing In this example, decimal part is just lost Note: this does not actually round 29 Object Types The primitive types aren’t really enough Java also allows object types, or classes Object variables hold references to objects Typically capitalized The declaration only creates a reference This is different from primitive types Variables of primitive type hold a value 30 Example: String Objects We have already seen one object type in Java: String A String object is a list of characters e.g. “Hello world!” or “My name is Aaron” Can be passed to print or println Can be concatenated using the (+) operator e.g. “Hello world! ” + “My name is Aaron” “I can also append numbers, like “ + 2 31 Object Instances We must create a new “instance” of an object to store something Each object type has a constructor (more later) Create instances using the reserved world new e.g. course = new String(“CMPT 126”); This creates a new String in memory It stores the characters “CMPT 126” The assignment sets course to refer to this instance 32 References and Instances String course; course: new String(“CMPT 126”) CMPT 126 course = new String(“CMPT 126”); course: CMPT 126 33