Download Java Coding 4 - Bilkent University Computer Engineering Department

Java Coding 4
Method madness
David Davenport
Computer Eng. Dept.,
Bilkent University
Ankara - Turkey.
email: [email protected]
IMPORTANT…

Students…
This presentation is designed to be used in class as
part of a guided discovery sequence. It is not selfexplanatory! Please use it only for revision purposes
after having taken the class. Simply flicking through
the slides will teach you nothing. You must be actively
thinking, doing and questioning to learn!

Instructors…
You are free to use this presentation in your classes
and to make any modifications to it that you wish. All
I ask is an email saying where and when it is/was
used. I would also appreciate any suggestions you
may have for improving it.
thank you,
David.
Using Java methods…
output
result type
// in Scanner class
public int nextInt()
int i = scan.nextInt();
answer = f( x, y, z );
// in … class
public void println( String s)
System.out.println( “Hello Java”);
// in Math class
public static double sin( double d)
double d = Math.sin( 0.5);
// in String class…
public String substring( int beginIndex, int endIndex)
String shortString = longString.substring( 1, 3);
Method
name
input
Parameters
& types
Use ClassName.method(…)
for “static” methods
varName.method(…)
for non-static
methods
Methods

Methods are


Don’t need to
know HOW they
work in order to
use them
“Black” boxes that do something
In Java


methods have any no of named inputs
“only” 1 output.
abc
f
z
y = sin(x)
functions 
methods in Java
z=
f
( a, b, c)
Methods

Meaningfully named blocks of commands

facilitate: reuse, structure & top-down design.
Methods
can be
reused
Easier to
build,
test &
debug
Each method is short
(7 ± 2 lines) and so
easy to understand
Reusing code…

Identical code…
aMethod()
main
main
aMethod()
aMethod()
aMethod()

Almost identical…
sum = 0;
for ( int i = 5; i < 25; i++)
sum = sum + i;
Sys… ( sum);
sum = 0;
for ( int i = 19; i < 27; i++)
sum = sum + i;
Sys… ( sum);
sumBtw( low, high)
sum = 0;
for ( int i = low; i < high; i++)
sum = sum + i;
Sys… ( sum);
Methods & Algorithms

public static type f( type a, type b, type c)
Output type
or void
Method
name
List of
inputs &
types
Ask for and get x
(iii)
Compute y using x
(iv)
Print y
(ii)
f
(i)
(ii)
a
b
c
(iii)
(iv)
a
b
c
f
f
z
f
z
Declaring Methods

Syntax
modifiers resultType methodName( parameters)
statement;

where
Formal parameters
matched to actual
 statement is any Java statement
parameter values
 methodName is identifier
at run-time
(convention as for variables)
 resultType is any Java type or “void”
 parameters is comma separated list of 0 or more
“type name” pairs, eg. int age, String s, …
Modifiers

Access


coming next…
Others


final – cannot be changed!
static – no need for object, only one copy,
can be accessed via class name.
y = Math.sin( x);
str = Integer.toString( 5, 2);
b = Character.isDigit( ‘3’);
public static double sin( double value) {…}
public static String toString( int i, int radix) {…}
public static boolean isDigit( char ch) {…}
public static void main( String[] args) {…}
Modifiers

Access




public – accessible/usable by all
protected – package & inherited
default – package only
private – only in current class
Methods are collections of statements
Classes are collections of methods (+)
Packages are collections of classes
Packages can contain other packages
Need full “path” to class to use its methods
e.g. java.util.Scanner & java.lang.Math
mypackage.MyMethods
Packages, Classes & Methods
Default package
java package
lang package
main(..)
MyProg
class
cs1
package
Math
class
random()
swing
package
sin()
String
class
abs()
toUpperCase()
util
package
charAt()
Scanner
class
Character
class
Examples (1)

Declare method to print fixed message
public static void showErrorMsg() {
System.out.println( “Error, try again!”);
}

Use… (from inside same class)
…
showErrorMsg();
…

…
if ( x > 10)
showErrorMsg();
else
…
…
…
while ( !done) {
showErrorMsg();
…
}
…
Use… (from outside class)
mypackage.mysubpackage.MyClassName.showErrorMsg();
mypackage.MyClass.showErrorMsg();
MyClass.showErrorMsg();
Examples (2)

Method to print any message in box
public static void showMsg( String msg) {
System.out.println( “************” );
System.out.println( “ “ + msg);
System.out.println( “************” );
}

Use…
…
showMsg( “Hello”);
…
…
if ( x > 10)
showMsg( “too big!”);
else
showMsg( “ok!”);
…
Examples (3)

Method to simulate a die throw
public static int randomDieThrow() {
return (int) (Math.random() * 6) + 1;
}

Use…
…
int faceValue;
faceValue = randomDieThrow();
if ( faceValue == 6)
System.out.println( “free throw”);
…
Return statement

Used to indicate result of method/function
return value;

where value is






Literal
Variable or constant
Expression
⋮
if (… )
return 0;
⋮
return -1;
Type of value must match method return type!
Execution of method stops immediately
Can have multiple return statements
Examples (4)

Method to find hypotenuse of triangle
public static double hypotenuse( double side1, double side2) {
return Math.sqrt( side1 * side1 + side2 * side2);
}

Use…
double z = hypotenuse( 3, 4);
Actual & formal
parameters are
matched one-toone in sequence,
e.g side1 = 3,
side2 = 4
Definition & Use

Static methods
Methods declared in class
(but outside main)
// header
// Author, date
public class ClassName {
public static void myMethod1( String s) {
System.out.println( s);
}
private static int myMethod2( int i) {
return i * 2 + 1;
}
Return statement
indicates what value will
be considered the result of
the method (function)
public static void main( String[] args) {
myMethod1( “Hello”);
Use in main
or other method
int j = myMethod2( 5);
System.out.println( j);
}
}
Examples (5)

Method to find absolute difference
public static int absDiff( int x, int y) {
int z;
z = x – y;
Any variables, like z, that are not
if ( z < 0)
parameters, should be defined locally.
z = -z;
return z;
}

Use…
int a = absDiff( 5, 3);
Actual & formal
parameters are
matched one-toone in sequence,
e.g x = 5,
y=3
Methods calling methods
public static boolean isLessThan( int a, int b) {
boolean z;
⋮
a:-4, b:0
}
z:true
public static int absDiff( int x, int y) {
int z;
z = x – y;
.. isLessThan( z, 0);
⋮
}
public static void main( String[] args) {
int x, y;
x = 5; y = 9;
... absDiff( x, y);
x:5, y:9
… absDiff( y, x);
... isLessThan( x, y);
}

Methods should be
independent of each
other

Parameters & variables
are different, even if
same name

Only connected when
called… actual & formal
params matched oneto-one in order, then
values passed.
x:5, y:9
z:-4
Methods calling methods
public static boolean isLessThan( int a, int b) {
boolean z;
⋮
a:4, b:0
}
z:false
public static int absDiff( int x, int y) {
int z;
z = x – y;
.. isLessThan( z, 0);
⋮
}
public static void main( String[] args) {
int x, y;
x = 5; y = 9;
... absDiff( x, y);
x:5, y:9
… absDiff( y, x);
... isLessThan( x, y);
}

Methods should be
independent of each
other

Parameters & variables
are different, even if
same name

Only connected when
called… actual & formal
params matched oneto-one in order, then
values passed.
x:9, y:5
z:4
Methods calling methods
public static boolean isLessThan( int a, int b) {
boolean z;
⋮
a:5, b:9
}
z: true
public static int absDiff( int x, int y) {
int z;
z = x – y;
.. isLessThan( z, 0);
⋮
}
public static void main( String[] args) {
int x, y;
x = 5; y = 9;
... absDiff( x, y);
… absDiff( y, x);
x:5, y:9
... isLessThan( x, y);
}

Methods should be
independent of each
other

Parameters & variables
are different, even if
same name

Only connected when
called… actual & formal
params matched oneto-one in order, then
values passed.
Parameter passing (1)
main
a
myMethod( x, y)
5
b
3
c
13
a = 5; b = 3;
c = myMethod( a, b);
System.out.println( c);
5
x
3
y
10
z
13
z = 2 * x;
return z + y;
Parameter passing (2)
main
a
myMethod( x, y)
5
b
3
c
11
a = 5; b = 3;
c = myMethod( b, a);
System.out.println( c);
3
x
5
y
6
z
11
z = 2 * x;
return z + y;
Parameter passing (3)
main
Doesn’t
change!
a
myMethod( x, y)
5
b
3
c
19
a = 5; b = 3;
c = myMethod( b, a);
System.out.println( c);
7
3
x
5
y
14
z
19
x=7
z = 2 * x;
return z + y;
Overloaded Methods

Method signature…
public int anyMethod(
anyMethod(parameters)
parameters)

Methods in same class
must have different signatures!
println( int i)
println( double d)
println( String s)
Same name,
different parameter list
Note: result type
must be same!
Be aware

Want methods to be reusable

Methods that read inputs from
keyboard or write output to console are
difficult to reuse (e.g. in GUIs)

So, pass inputs as parameters
& return the output as the result

User interaction is then done
elsewhere, usually main method.
Beware
a = 5;
b = 3;
d = 7;
c = myMethod( a, b);
Sys… ( a + “, ” + b + “, ” + d);
What is output?
Naturally assume method
parameters are inputs only
& method doesn’t have
side effects!
Global variables!
Global, Local, Params…
public class MethodPlay {
global
static int d;

Scope: if local use it, else
look to outer (global)
context.

Rule: any variable used in
a method, that is not a
parameter, must be
defined locally…

otherwise need to trace
everything to see whether
any side-effects change its
value!
public static int myMethod(…) {
}
d = 999;
assigns 999 to
global variable
public static void main( String[] args) {
assigns 7 to
global variable
d = 7;
c = myMethod(…);
Sys..( d);
}
prints 999 from
}
global variable
Global, Local, Params…
public class MethodPlay {
global
static int d;

Scope: if local use it, else
look to outer (global)
context.

Rule: any variable used in
a method, that is not a
parameter, must be
defined locally…

otherwise need to trace
everything to see whether
any side-effects change its
value!
public static int myMethod(…) {
}
d = 999;
assigns 999 to
global variable
public static void main( String[] args) {
assigns 7 to
int d;
local variable
d = 7;
c = myMethod(…);
Sys..( d);
}
prints 7 from
local
}
local variable
Global, Local, Params…
public class MethodPlay {
global
static int d;

Scope: if local use it, else
look to outer (global)
context.

Rule: any variable used in
a method, that is not a
parameter, must be
defined locally…

otherwise need to trace
everything to see whether
any side-effects change its
value!
public static int myMethod(…) {
local
int d:
}
d = 999;
assigns 999 to
local variable
public static void main( String[] args) {
assigns 7 to
global variable
d = 7;
c = myMethod(…);
Sys..( d);
}
prints 7 from
}
global variable
…unless something
else changed it!
Global, Local, Params…
public class MethodPlay {
global
static int d;

Scope: if local use it, else
look to outer (global)
context.

Rule: any variable used in
a method, that is not a
parameter, must be
defined locally…

otherwise need to trace
everything to see whether
any side-effects change its
value!
public static int myMethod( int d) {
param
}
d = 999;
assigned to param
(input only!)
public static void main( String[] args) {
}
}
int d;
d = 7;
c = myMethod(..);
Sys..( d);
local
Global, Local, Params…
public class MethodPlay {
static int d;
global

Scope: if local use it, else
look to outer (global)
context.

Rule: any variable used in
a method, that is not a
parameter, must be
defined locally…

otherwise need to trace
everything to see whether
any side-effects change its
value!
public static int myMethod( …) {
int d;
}
local
d = 999;
public static void main( String[] args) {
}
}
int d;
d = 7;
c = myMethod(..);
Sys..( d);
local
Example…
static Scanner scan = new Scanner( System.in );
global
public static int askForAndGetIntValue()Scanner
{
scan ) {
Scanner scan = new Scanner( System.in );
}
param
local
System.out.print( “Enter value: “);
return scan.nextInt();
oops…
undefined!
 Local:
ok, but may be problem here
due to shared resource?
 Param: ok, create in main & pass
(but may have lots of params!)
 Global: um… for the really lazy!