Methods in Java

Methods
in
Java

Jussi
Pohjolainen

Tampere
University
of
Applied
Sciences

Problem

public class HugeProgram {
public static void main(String []
args) {
1. codeline
2. codeline
3. codeline
...
10 000. codeline
}
}
How
are
you

going
to

manage
this

huge

codeblock?

Problem

•  It's
possible
that
your
whole
program
is

implemented
inside
the
main
–
method

•  Result:
No
one
will
understand
the
code!

•  You
have
to
create
the
code
that
is

– Reusable

– You
and
others
can
manage
it

•  Well
how?

How
to
divide
your
code?

•  Best
pracJce:
Use
Object-‐Orientated

Programming

– Classes,
Methods,
Inheritance,
...

•  Lot
of
possibiliJes
with
OO!

•  But
ﬁrst
let's
concentrate
on
methods!

Methods:
Divide
your
Code?

public static void main(String [] args) {
doSomething1();
doSomething2();
doSomething3();
}
public static void doSomething1() {
// something
}
// something
}
// something
}
}

Methods:
Divide
your
Code?

doSomething1();
doSomething2();
}
System.out.print("Hello ");
}
System.out.println("World!");
}
} java HugeProgram
Hello World!

Method
Call

Method

Method

Examples

•  Flash
demo:
Method
1
(no
parameters,
no

return)

•  MethodDemo1.java



•

IT'S
NOT
THAT
EASY:
PARAMETERS

AND
RETURN

Real
Life
Example

I
really
want
to
buy
christmas
gis

for
my
children.
The
doll
costs
5

euros
and
the
toy
car
costs
6
euros.

I
have
12
euros,
is
that
enough?

Why
didn't
I
listen
to
the
math

teacher
at
school?
Why
didn't
I

learn
sum
–
calculaJons?
Why
oh

why?

Real
Life
Example

Hello
Hello!
My
name
is
Mr.

Sum
and
I
can
sum
everything!

Please
use
my
knowledge!

Real
Life
Example

Well
Mr
Sum.
I
have
a
problem
I

don't
know
what
is
5
plus
6.

Real
Life
Example

Don't
understand
your

problem.
I
only
do
simple

calculaJons,
I
really
need
to

have
two
integer
values
to
be

able
to
do
the
sum.
Please
just

give
my
two
numbers!

Real
Life
Example

Well
thanks,
at
least
I
know
my

numbers
and
11
is
less
than
12,

so
great!
My
children
will
be

very
happy!

Mr.
Sum?

Mr.
Sum

5

6

11

parameter
return

Real
Life
Example
in
the
Nerd
World

I
am
implemenJng
a
applicaJon

and
I
really
need
a
random

ﬂoaJng
point
value
between

[0,1[

Well
Howdy,
my
name
is
Mr.

Random
and
I
live
in
a
town

called
Math.
I
can
give
you
a

random
ﬂoaJng
point
value

between
[0,1[

Real
Life
Example
in
the
Nerd
World

Give
random
number.

0.34523456

Mr.
Random?

Mr.
Random

0.34523456

return

Real
Life
Example
in
the
Nerd
World

Did
you
know
that
in
the
town
Math

there
other
guys
living
too?
For

example:
Mr.
Round,
Mr
Max,
Mr
Min,

Mr
Sin...

The
usage
of
Class
Library

import java.lang.Math;
public class Example {
double myRandomNumber = Math.random();
System.out.println(myRandomNumber);
}
}
Import
the
class

class
method

Mr.
Max
in
Code

import java.lang.Math;
int maxNumber = Math.max(5,10);
System.out.println(maxNumber );
}
}
Import
the
class

class
method
parameters

Your
own
Methods

printSomething();
}
public static void printSomething() {
System.out.println("Hello World!");
}
}

Your
own
Methods

PrintClass.printSomething();
}
}
class PrintClass {
public static void printSomething() {
System.out.println("Hello World!");
}
}

Parameters

MyMath.max(5, 10);
}
}
class MyMath {
public static void max(int a, int b) {
if(a > b)
System.out.println(a);
else
System.out.println(b);
}
}

Return

int maxNumber = MyMath.max(5, 10);
System.out.println(maxNumber);
}
}
class MyMath {
public static int max(int a, int b) {
if(a > b)
return a;
else
return b;
}
}

In
One
class

int maxNumber = max(5, 10);
System.out.println(maxNumber);
}
public static int max(int a, int b) {
if(a > b)
return a;
else
return b;
}
}

Examples

•  Flash-‐demo:
Method
2

Method
3

Method
4


VISIBILITY
OF
VARIABLES

This
does
not
work,
why?

import java.util.Scanner;
class Example {
Scanner input = new Scanner(System.in);
int x = input.nextInt();
if(x == 5) {
int b = 5;
}
System.out.println(b);
}
}

This
does
not
work,
why?

class MethodDemo3 {
askName();
printName();
}
public static void askName() {
String name = input.nextLine();
}
public static void printName() {
System.out.println(name);
}
}

SoluJon

class MethodDemo3 {
String n = askName();
printName(n);
}
public static String askName() {
String name = input.nextLine();
return name;
}
public static void printName(String name) {
System.out.println(name);
}
}

Exercise

•  Implement
a
Java-‐app
that
prints
square.
The

height
of
the
square
is
asked
from
the
user.

java Rectangle
Height? 4
XXXX
X X
X X
XXXX

class Rectangle {
Scanner input = new Scanner(System.in)a;
int height = input.nextInt();
// Go all the lines
for(int line=0; line<height; line++) {
// If the line is the first or last
if(line == 0 || line == height-1) {
// Print 'X' height amount of times
} else {
// Print 'X'
// Print ' ' height-2 amount of times
// Print 'X'
}
// Print enter
System.out.println();
}
}
}

Let's
use
Methods!

class Rectangle {
public static void print(char mark, int amount) {
for(int i=0; i<amount; i++) {
System.out.print(mark);
}
}
...
}

int height = input.nextInt();
// Go all the lines
for(int line=0; line<height; line++) {
// If the line is the first or last
if(line == 0 || line == height-1) {
print('X', height); // METHOD CALL!
} else {
print('X', 1); // METHOD CALL!
print(' ', height-2); // METHOD CALL!
print('X', 1); // METHOD CALL!
}
// Print enter
System.out.println();
}
}

Methods in Java

More Related Content

What's hot

Similar to Methods in Java

More from Jussi Pohjolainen

Recently uploaded

In this document

Methods in Java