
To be able to write larger programs
◦ By breaking them down into smaller parts and
passing data between the parts.

To understand the concepts of Methods
◦ To be able to write Methods for use in your
programs.

Consider the following code:
public class MethodDemo {
public static void main (String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
‘main’ program:
the entry point of
our code
public static void sayHello() {
System.out.println("Hello! ");
}
public static int square(int x) {
return x * x;
}
}
methods that the
‘main’ method
will call
public class MethodDemo {
Call to sayHello
public static void main (String args[]) {
sayHello();
Enters the
int result = square(2);
System.out.println(result);
method, executes
}
public static void sayHello() {
System.out.println("Hello! ");
}
all the code, and
then returns to the
main program.
public static int square(int x) {
return x * x;
}
}
The code which is actually run so far . . .
public class MethodDemo {
public static void main (String args[]) {
System.out.println("Hello! ");
}
}
public class MethodDemo {
public static void main (String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
public static void sayHello() {
System.out.println("Hello! ");
}
public static int square(int x) {
return x * x;
}
}
The code which is actually run so far . . .
public class MethodDemo {
public static void main (String args[]) {
System.out.println("Hello! ");
int result = square(2);
System.out.println(result);
}
}
public class MethodDemo {
public static void main (String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
public static void sayHello() {
System.out.println("Hello! ");
}
public static int square(int x) {
return x * x;
}
}
The code which is actually run so far . . .
public class MethodDemo {
public static void main (String args[]) {
System.out.println("Hello! ");
int result = 2 * 2;
System.out.println(result);
}
}
Call to square
Enters the method
with a parameter
value 2, executes
all the code, and
then returns to the
main program
with the answer 4.
public class MethodDemo {
public static void main (String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
4
2
public static void sayHello() {
System.out.println("Hello! ");
}
public static int square(int x) {
return x * x;
}
}
The code which is actually run so far . . .
public class MethodDemo {
public static void main (String args[]) {
System.out.println("Hello! ");
int result = 2 * 2;
System.out.println(result);
}
}
public class MethodDemo {
public static void main (String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
public static void sayHello() {
System.out.println("Hello! ");
}
public static int square(int x) {
return x * x;
}
}
The total code which is actually run.
public class MethodDemo {
public static void main (String args[]) {
System.out.println("Hello! ");
int result = 2 * 2;
System.out.println(result);
}
}

The sayHello() method in more detail:
nothing is
returned
name of
method
empty
parameter
list ‘()’
public static void sayHello() {
System.out.println("Hello! ");
}
our method does not
require any
parameters

The square() method in more detail:
an integer
is
returned...
name of
method
public static int square(int x) {
return x * x;
}
… and the value of that integer is
given by this expression
this
method
requires
an integer

In our main method, we have the following
code:
public static void main(String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
}
◦ The program will:
 output the word Hello
 calculate the result of 2 * 2
 output the result


In our main method, we now have the following code:
The program will:
◦
◦
◦
◦
◦
◦
◦
◦
◦
public static void main(String args[]) {
sayHello();
int result = square(2);
System.out.println(result);
result = square(5);
System.out.println(result);
result = square(34);
System.out.println(result);
result = square(2000);
System.out.println(result);
}
output the word Hello
calculate the result of 2 * 2
output the result
calculate the result of 5 * 5
output the result
calculate the result of 34 * 34
output the result
calculate the result of 2000 * 2000
output the result
public static void
main(String args[]) {
4
2
sayHello();
int result = square(2);
public static int square(int x) {
System.out.println(result);
return x * x;
5
25
result = square(5);
}
System.out.println(result);
34
2,000
1,156
result = square(34);
System.out.println(result);
result = square(2000);
System.out.println(result);
}
4,000,000
Parameters
Used for passing
data to a method
int calculateArea(int w, int h) {
int area = w * h;
return area;
Return type
Defines the type
of data to be
passed from the
method.
Keyword void is
used here if
method returns
no data.
}
Return statement
Used for passing
data from the
method.
Omitted for void
methods.
Parameter values
Numbers 2 and 5
are transferred to the
method from main.
int calculateArea(int w, int h) {
int area = w * h;
return area;
}
2&5
10
Return value
Result 10 is
passed from the
method into
variable area.
int area;
area = calculateArea(2, 5);
System.out.println( "Area = " + area);

Consider these two methods:
public void square1 (int y)
{
System.out.println(y * y);
}


public int square2 (int y)
{
return (y * y);
}
Both calculate the square of the number that is
supplied by the input parameter.
The first simply prints out the computed value.
◦ i.e. the value is lost

The second returns the computed value
◦ i.e. the value can be used by the program that called
the method
Four Types of Method exist:
1.
Those which ‘do something’ but don’t require any
data from the main program and which don’t
return any data to the main program (no data in
or out).
2.
Those which ‘do something’ where the
‘something’ depends on data supplied by the
main program but don’t return any data to the
main program (data in).
3.
Those which ‘do something’ and then do return
resulting data to the main program (data out).
4.
A combination of the last 2 – Require data from
the main program and return data back to the
main program (data in and out).

A Method may require more than one Input
parameter
◦ or none at all

A Method can only return a single Output
return value
◦ or none at all

Some Methods may have Optional input
parameters
◦ these will take Default values if not supplied

Draft timetable has the same times as this
semester: ◦ Monday 9:00 am to 12:00 noon, 1:00 pm to 3:00 pm.
◦ Tuesday 9:00 am to 12:00 noon, 1:00 pm to 3:00 pm.
◦ Thursday 9:00 am to 12:00 noon, 1:00 pm to 3:00 pm.

Not necessarily lectures in the afternoons.
◦ Some modules have lectures during the morning and
workshops/tutorials in the afternoons.


Module classes are mostly on the same day (but
not always).
It depends on which modules that you are taking.