Programming in Java

1. Programming in Java • main concepts • classes • objects and values • variables • methods • constructors • parameters • statements • other key concepts • casting • operators • keywords • packages, imports • declaration • etc.

2. Main Concepts: Class and Object • a class represents a category (type) of objects • defines what instance variables (properties) and methods (capabilities) an object has • an object is an instance of a class • has individual values in the instance variables • the methods defined in its class can be called • must be constructed before its methods can be used and the values of its properties can be accessed • an object is constructed or instantiated • using new followed by the name of its class • e.g.: new BallApplication(); • Java program is a collection of classes • execution starts in the main class

3. Main Concepts: Values and Types • Java computes with values • either objects • or primitive values • e.g. integer number 45, floating point number 3.14, character 'A' • each value has a type • either the class of the object • or a primitive type • e.g. int (integer numbers) • float (floating point), • double (double precision floating point numbers) • char (characters) • etc.

4. Main Concepts: Variable • a variable holds a value • variable references the place in memory where the value is stored • variable has a name, type and value • think of a house, the house is the value, the variable holds its address • a variable must be declared • its type, name and possibly initial value, e.g.: • String name; • double pi = 3.14; • a variable declared directly in the class (outside methods) is an instance variable • it holds the values of object's properties

5. Main Concepts: Variable's Values • a variable has initially a default value • e.g. 0 or 0.0 • variable's value is retrievedsimply by using its name • System.out.println(name); writes the value in name • variable can be assigned another value using = • the value of the expression on the right-hand side of = is stored in the variable on the left-hand side • e.g. name = "Doublja"; then name will have value "Doublja" • assignment overwrites the old value

6. Main Concepts: Methods • a method tells an object to do something • methods are declared within a class of the object • they can be called on every object of this class • method call is preceded by object reference and dot . • e.g. System.out.println(); • where System.out is the variable referencing the object • and println(); is its method • method declaration has a return type, name, parameter list and body • if the method doesn't return any value, its return type is void • parameter list is enclosed in parentheses ( and ); it can be empty • body is enclosed in braces { and }; • method's body is a sequence of statements • the most common statements are assignments and method calls • local variables (declared in the body) hold temporary values

7. Main Concepts: Constructors • a constructor is a special case of a method • constructordeclaration is like a method declaration • except constructor has the same name as the class • and constructor doesn't have a return type • constructor is called when an object is created using new • e.g. new Ellipse(); • if a class has no constructor, Java inserts a default one • default constructor has no parameters • constructor is mostly used to initialize the object

8. Main Concepts: Parameters • a method (or constructor) can have parameters • parameters in the method call (actual parameters)and in the method declaration (formal parameters)must correspond, i.e. • in their number • in the type of each parameter • parameters form a comma-separated list enclosed in parentheses ( and ) • a formal parameter in method declaration must be declared i.e. with their type and name • e.g. String substring (int from, int to) {} • when method is called, the formal parameter will be assigned the value of the actual parameter • e.g. when substring (2, 5); is called, the parameter from will be assigned 2 and to will be assigned 5

9. Main Concepts: Program and Execution • an application program is set of classes written as text files • the name of the file must be the class name and extension .java • Eclipse does this for us • a compiler translates the text into bytecode • the name of the bytecode file is the class name and extension .class • bytecode is an intermediate language • bytecode is the same forany hardware • e.g. .class files from PC can be used on Macs • Eclipse does this for us, compiles whenever a java file is saved • we run (or execute) the program • the Virtual Machine (VM) interprets bytecodeon yourhardware • program control decides what to do next • a program (or application) starts in main method • method's statements are (mostly) executed in sequence • when method is called, control branches to the called method • its statements are executed • when the called method returns control goes back to calling method

10. Other Concepts: Assignment and Casting • the expression on the right-hand side of = must be compatible withthevariable on the left-hand side of = • their types must match • either be the same • or the expression's type must be converted to the variable's type • some conversions are automatic, e.g.: • doubleprice = 4000; // converted to 4000.0 • String king = "Henry " + 8; // converted to "Henry 8" • casting is a manual kind of type conversion • to cast an expression proceed it with the type enclosed in(and) • e.g.: int rounded = (int) (number + 0.5); • typically, casting is necessary if information may be lost • watch out: double fraction = 5 / 9; // is 0! • 5/9 is integer division, discards fractional part!

11. Programming Errors • syntax errors • each class must be syntactically correct • i.e. obey the grammatical rules of the Java programming language • are discovered by the compiler • easy to correct • Eclipse highlights syntax errors for us continuously • Eclipse suggests error corrections • programming "by error" saves typing time • we can make a conscious error and let Eclipse do the "correction" • run-time errors (exceptions) • occur when program's execution attempts an illegal operation • e.g. division by 0 • harder to correct • logical errors • program runs correctly, but the results are wrong • hardest to correct

12. Debugging • debugging = correcting run-time and logical errors • to find the place where things went wrong • we must where a variable gets a wrong value • or where a method didn't get called as expected (or did) • either by writing variables' values • to write the value of any variable xto the console, use: • System.out.println ("x=" + x); • this works for any type! • disadvantages: • writing and deleting the statements takes time • program can't be stopped • or with the debugger • included in Eclipse • we can step through the program in smaller or larger steps • defied breakpoints • we can see all variables • disadvantages: • variables are hard to find • starting debugger, stepping through takes time • we can overstep

13. Class String • represents the category of simple strings of characters • a String object is: • is a sequence of letters, digits, other characters • has implicit numbering: 1st character has index 0 • constant String is enclosed in quotes • e.g. "How'zit Brah" • operator + concatenates two strings • e.g. "How'zit" + "Brah" is "How'zitBrah" • if one operand of + is not a String,it will be converted to a String • some usefull methods: • length(): returns the number of characters • "How'zit Brah".length()returns 12 • toUpperCase(): returns a String where all letters are in uppercase • "How'zit".toUpperCase()returns "HOW'ZIT" • substring(m,n): returns a String of the characters numbered m to n-1 • "How'zit Brah". substring(2,5)returns "w'z"

14. System, PrintStream, InputStream, Scanner • class System provides objects out and in for console I/O • out is of type PrintStream, which provides methods: • print(String string) writes the string to the console • println()starts a new line on the console • println(String string) writes the string to the console and then starts a new line • inis of type InputStream, which provides methods for reading from console • too primitive to be useful (reads just one character) • class Scanner provides for reading from a stream • use Scanner scanner = new Scanner (System.in); to read fromSystem . in • nextLine()reads a line, returns it as a String • nextInt()reads an integer number, returns it as an int • nextDouble()reads a decimal number, returns it as a double • hasNextInt()returns true if integer can be read, false otherwise • hasNextDouble()returns true if number can be read, false otherwise