csc 201-lecture 9

8/14/2019 CSC 201-Lecture 9

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CSC 201

Lecture - 9

8/14/2019 CSC 201-Lecture 9


Basic Java Program Structure

Class example

{

public static void main (String[] args){

System.out.println(“Hello world!”);

}}

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Functions/Static Methods

• In a simple Java program we have seen a class

which contained a single method ‘main’. Every

Java program must have a main() method.

• Method is a group of instructions that is given aname and can be called up at any point in a

program simply by quoting that name.

• The power of Object-orientation lies in breaking

tasks down into simpler tasks.

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How to define your method?

class sample

{

public static void method_name(parameters if any)

{

//Write your method definition here.

}

public static void main(String[] args)

{

method_name(); // calling your method to execute

}

}

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Defining Methods

• The only required elements of a method declaration are themethod’s return type, method name, a pair of parenthesis ( )and a body between the braces { }.

• Usually methods have 5 components:

1) Modifiers – such as public, private and others ( later chapter).

2) The return type – The data type of the value returned by themethod or ‘void’ if the method does not return a value.

3) Method name

4) The parameter list in parenthesis ( ) - A comma delimitedlist of input parameters preceded by their data type.

5) Method body – enclosed between { and }.

8/14/2019 CSC 201-Lecture 9


Hello world method

public static void hello_world()

{

System.out.println(“Hello world:”);}

Modifier Return type Method name

Parameters if any

Inside the parenthesis

Body of the method

What is meant by static?

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First program

class method_sample

{

public static void hello_world_method()

{

System.out.println(“Hello world”);

}

public static void main(String args[])

{

hello_world_method();

}

}

Method defined

Method called

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Multiple methods in a programclass multiple_methods

{public static void calculate_area()

{

int length = 10, breadth = 20, area=0;

area = length * breadth;

System.out.println(area);}

public static void hello_world()

{

System.out.println(“Hello world”);

}

Public static void main(String[] args)

{

hello_world();

calculate_area();

}

}

Defining

calculate_area

method

Defining

hello_world

method

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Notes

• We can define as many static methods as

we want in a .java file.

• Each method has a body which consists of

a sequence of statements enclosed in { }.

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Parameter passing

What are Parameters?

Parameters refer to the list of variables in

a method declaration. They are used in

the method body and will take on the

values of the arguments that are passed

in.

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How to pass parameters in Java?

A simple program to demonstrate the method ‘multiply’.

Class sample

{

public static void multiply (int num)

{

num = num * 2;System.out.println(“Your input number multiplied by 2 is:”+num);

}


{

Scanner s = new Scanner(System.in);

System.out.println(“Enter a number and I multiply with 2:”);

int input_num = s.nextInt();

multiply(input_num);

}

}

How does the execution of this program take place? What are arguments?

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Same Program but with Return

typesclass sample

{

public static int multiply (int num)

{

num = num * 2;

return num;

}public static void main(String[] args)

{

Scanner s = new Scanner(System.in);

System.out.println(“Enter a number and I multiply with 2:”);

int input_num = s.nextInt();

int result = multiply(input_num);

System.out.println(“The result after multiplying with 2 is:”+result);

}

}

Can we similarly pass ‘float’ values? What about other datatypes?

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Programs

1) Write a swap function to swap two numbers.

2) Write a program to calculate an area of a

triangle and rectangle.

3) Is there any limit on the numbers of parameters a method can have?

4) What would happen if you write some code

after a return statement in a method?5) What would happen if you do not include a

return statement?

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6) Write a static method odd() that takes 3

boolean inputs and returns true if odd

number of inputs are true, and false

otherwise.

7) What is function calling?

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Recursion Example

class factorial

{

public static int fact(int num)

{

int result;

if(num == 1)return 1;

result = num * fact(num – 1);

return result;

}


{int factorial = fact(5);

System.out.println(“Factorial is”+factorial);

}

}

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What is Recursion?

• Recursion is the process of defining

something in terms of itself. With relation

to Java programming, recursion is the

attribute that allows a method to call itself.A method that calls itself is said to be

recursive.

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Advantages of Functions

• You can customize your functions according toprogrammer’s needs.

• You can minimize the coding time, because youdon’t have to repeat a piece of code every time

you need it.• Functions can take information and process as

you need.• They can return data of a certain data type.

• They are easy to understand and best way toprogram.

• Bug fixing is easy.

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Terminology

Concept Java Construct description

Function

Domain

Range

Formula

Static method

Argument type

Return type

Method body

Mapping

Set of values

where function is

defined

Set of values a

function can

return

Function

definition

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Announcements

• Homework – 2 due date extended to 10/15/09since 10/13/09 is non-working day.

• Special office hours on 10/15/09 from 1:00pm

to 3:00pm before midterm.• Midterm examination CSC201 : 10/20/09

Tuesday 11:00am – 12:50pm.

• CSC 185 lab in-class quiz on 10/27/09

between 10:00am – 11:00am.

csc 201-lecture 9

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