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Published byDaniella Lane Modified over 9 years ago
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Constructors & An Introduction to Methods
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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");
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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
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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.
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Methods
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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.
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Method Structure A method must take the following form returnType name (argument list){ // method body }
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Defining Arguments for methods In the method definition: –The argument list contains zero or more formal arguments separated by commas and enclosed in parenthesis
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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.
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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));
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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.
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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
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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 }
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