Constructors & An Introduction to Methods

Defining Constructor – Car Example Public class car { String Model; double speed; String colour; { Public Car () }{ Public Car (String make) { model = make; … }} Car car1 = new Car(); Car car 2 = new car ("Audi");

Constructors  A constructor is a method that creates an object. In java constructors are instance methods with the same name as their class. Constructors are invoked using the new keyword  A constructor is a special method which is executed only when an object is created  The purpose of a constructor is to setup (or construct) the environment for the object (variable values, etc) and to initialise an objects data when it is created  A constructor method is easily recognised as it must: –Have the same name as the class

Constructors: Summary  A constructor is generally the first method in a class - it has the exact same name as the class (but has parentheses and optional arguments)  The arguments in a constructor are typically used to initialise the instance variables of the class.  Java has a default constructor if none is provided which does not initialisation - this constructor is called the no args constructor.

Methods

Methods  A method is a group of instructions (also called a function or subroutine in other languages)  A method can only be defined within a class definition  Every java program must contain at least one method  Large programs can get very complex. Methods help to reduce complexity by splitting programs into relatively isolated sections.

Method Structure  A method must take the following form returnType name (argument list){ // method body }

Defining Arguments for methods  In the method definition: –The argument list contains zero or more formal arguments separated by commas and enclosed in parenthesis

Writing a method - Example void printHi() {System.out.print(“hi”);}  This method produces no result after it finishes, it requires no value (parameter) to operate.

Writing a method - Example int addNumbers(int number1, int number2) { int result = number1+number2; return result; }  The method produces an integer when it finishes. It also requires two integers to be given to it.  The method would therefore be used like this: System.out.print(“answer=“+addNumbers(5,4));

Method Declaration  The compiler identifies methods by their name, number and type of arguments.  The compiler also knows (from the return type in the declaration) the return type of the method  There can be zero or more arguments passed to a method, but the parenthesis are mandatory (as is a name and return type)  The return operator is required within every method unless the void keyword is used in its declaration or unless it is a constructor.  If the data type returned by the method does not match the return type, the class will not compile.

class Square { int length; int breadth; public Square (int length, int breadth){ this.length = length; this.breadth = breadth; } int area(){ int answer = length*breadth; return answer; } }//end class Square

Square Example class Square_Example { public static void main(String args[]) { Squares; s = new Square(10,20); System.out.println("result = " + s.area()); }//end main }