More About Methods

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More About Methods

Hsuan-Tien Lin

Deptartment of CSIE, NTU

OOP Class, March 8-9, 2010

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Method (1/2, Callee’s View)

1 p u b l i c c l a s s OOPStudent {

2 p u b l i c i n t s co re ;

3 p u b l i c S t r i n g name ;

4 p u b l i c v o i d s e t _ s c o r e (i n t new_score ) { sc or e = new_score ; }

5 p u b l i c S t r i n g get_name ( ) { r e t u r n name ; }

6 p u b l i c double g e t _ a d j u s t e d ( ) { r e t u r n (60 + sc or e ∗ 0 . 4 ) ; }

7 }

method: what I (the object) do parameter: what I get from the caller return value: what I want to tell the caller

H.-T. Lin (NTU CSIE) More About Methods OOP 03/08-09/2010 1 / 24

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Method (2/2, Caller’s View)

7 }

1 p u b l i c c l a s s OOPStudentDemo {

2 p u b l i c s t a t i c v o i d main ( S t r i n g [ ] arg ) {

3 OOPStudent C h a r l i e L = new OOPStudent ( ) ;

4 C h a r l i e L . s e t _ s c o r e ( 8 0 ) ;

5 System . o u t . p r i n t l n ( C h a r l i e L . g e t _ a d j u s t e d ( ) ) ;

6 System . o u t . p r i n t l n ( C h a r l i e L . get_name ( ) ) ;

7 }

8 }

method: what I (caller) want the object to do argument: what I tell the callee

return value: what I want to hear from the callee

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Method: Key Point

method: an abstraction of action, where informa- tion is passed through argument/parameter and re- turn values

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Return Values (1/1)

7 }

1 p u b l i c c l a s s OOPStudentDemo {

3 OOPStudent C h a r l i e L = new OOPStudent ( ) ;

4 C h a r l i e L . s e t _ s c o r e ( 8 0 ) ;

5 System . o u t . p r i n t l n ( C h a r l i e L . g e t _ a d j u s t e d ( ) ) ;

6 System . o u t . p r i n t l n ( C h a r l i e L . get_name ( ) ) ;

7 }

8 }

void: must-have to mean no return value

primitive/extended return types possible

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Return Values: Key Point

void for no return value

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Primitive Argument/Parameter (1/1)

1 p u b l i c c l a s s Tool {

2 p u b l i c v o i d swap (i n t f i r s t , i n t second ) {

3 i n t tmp = f i r s t ;

4 f i r s t = second ;

5 second = tmp ;

6 System . o u t . p r i n t l n ( f i r s t ) ;

7 System . o u t . p r i n t l n ( second ) ;

8 }

9 }

10 p u b l i c c l a s s Demo{

12 Tool t = new Tool ( ) ;

13 i n t i = 3 ; i n t j = 5 ;

14 t . swap ( i , j ) ;

15 System . o u t . p r i n t l n ( i ) ;

16 System . o u t . p r i n t l n ( j ) ;

17 }

18 }

first, second: swapped

i, j: didn’t

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Primitive Argument/Parameter: Key Point

argument ⇒ parameter: call by value (copying) –change in parameter does not change argument

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Extended Argument/Parameter

to be discussed in Chapter 5

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this (1/1)

4 p u b l i c S t r i n g get_the_name ( OOPStudent WHO)

5 { r e t u r n WHO. name ; }

6 }

CharlieL.get_name() returns CharlieL.name Ptt.get_name() returns Ptt.name

how? a hidden parameter WHO can do

1 p u b l i c S t r i n g get_name ( ) { r e t u r n t h i s. name ; }

2 p u b l i c v o i d s e t _ s c o r e (i n t sc or e ) { t h i s. sc or e = sc or e ; }

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this: Key Point

this: a hidden parameter in the method to keep in

touch with the instance

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Local Variables (1/7)

1 p u b l i c c l a s s FibCompute {

2 p u b l i c i n t f i b (i n t n ) {

3 i n t r e s ;

4 S t r i n g s = " f i b ( " + n + " ) c a l l e d " ; System . o u t . p r i n t l n ( s ) ;

5 i f ( n <= 1 ) r e s = 1 ;

6 e l s e{ r e s = f i b ( n−1) ; r e s = r e s + f i b ( n−2) ; }

7 r e t u r n r e s ;

8 }

9 }

1 p u b l i c c l a s s FibDemo {

3 FibCompute f = new FibCompute ( ) ;

4 System . o u t . p r i n t l n ( " r e s = " + f . f i b ( 5 ) ) ;

5 }

6 }

a so-called recursive method

local primitive n: allocated, and assigned by argument ⇒ parameter

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Local Variables (2/7)

3 i n t r e s ;

5 i f ( n <= 1 ) r e s = 1 ;

8 }

9 }

5 }

6 }

local extended this: allocated, and assigned by argument (f) ⇒

parameter (this)

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Local Variables (3/7)

3 i n t r e s ;

5 i f ( n <= 1 ) r e s = 1 ;

8 }

9 }

5 }

6 }

local primitive res: allocated, not initialized assigned by ourselves

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Local Variables (4/7)

3 i n t r e s ;

5 i f ( n <= 1 ) r e s = 1 ;

8 }

9 }

5 }

6 }

local extended s: allocated, not initialized, links to a valid instance

by ourselves

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Local Variables (5/7)

3 i n t r e s ;

5 i f ( n <= 1 ) r e s = 1 ;

8 }

9 }

5 }

6 }

some other local variables generated by compiler: allocated, not initialized, used internally

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Local Variables (6/7)

3 i n t r e s ;

5 i f ( n <= 1 ) r e s = 1 ;

8 }

9 }

5 }

6 }

when call returns: the result is “copied” to the caller’s space-of-interest somewhere

local variables: discarded

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Local Variables (7/7, courtesy of Prof. Chuen-Liang Chen )

local variable

instance variable

class(static) variable

b l t method

i t l

belong to method

invocation instance class declaration within method within class within class

difi i O O S

modifier static NO NO YES

allocation (when) method invocation

instance creation

class loadingg allocation (where) stack memory heap memory heap memory

initial to 0 NO YES YES

de-allocation method return

automatic garbage collection

NO

scope

usage range direct access range from

declaration to whole class whole class

Chuen-Liang Chen, NTU CS&IE / 5 OOP

declaration to end of block

whole class whole class

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Local Variables: Key Point

local variables: the “status” of the current method frame

—by spec not necessarily initialized

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Method Overloading (1/2)

1 c l a s s P r i n t e r {

2 v o i d p r i n t I n t e g e r (i n t a ) ;

3 v o i d p r i n t D o u b l e (double a ) ;

4 v o i d p r i n t S t r i n g ( S t r i n g a ) ;

5 }

“Integer” and “int” are basically saying the same thing lazy people don’t want to type so many words

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Method Overloading (2/2)

2 v o i d p r i n t (i n t a ) ;

3 v o i d p r i n t (double a ) ;

4 i n t p r i n t ( S t r i n g a ) ;

5 v o i d p r i n t (i n t a , i n t b ) ;

6 }

7 c l a s s P r i n t e r T h a t C o m p i l e r S e e s {

8 v o i d p r i n t _ i n t (i n t a ) ;

9 v o i d p r i n t _ d o u b l e (double a ) ;

10 i n t p r i n t _ S t r i n g ( S t r i n g a ) ;

11 v o i d p r i n t _ i n t _ i n t (i n t a , i n t b ) ;

12 }

Java’s (and many modern language’s) solution: one method name, many possible argument types

called method overloading

Java “signature” of a method: include name and parameters (but

NO return types)

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Method Overloading (2/2)

2 s t a t i c v o i d p r i n t (i n t a ) ;

3 s t a t i c i n t p r i n t (i n t a ) ;

4 s t a t i c double p r i n t (i n t a ) ;

5

6 s t a t i c v o i d main ( S t r i n g [ ] argv ) {

7 P r i n t e r . p r i n t ( P r i n t e r . p r i n t ( 3 ) + 5 ) ;

8 / / which one do you want t o c a l l ?

9 }

10 }

determine programmer’s intention from arguments: easy

determine programmer’s intention from return value: hard —can cast, can discard, etc.

Java “signature” of a method: include name and parameters only compiler’s job: from arguments (type), determine which method (name+parameters) to call

cannot have two methods with the same signature

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Method Overloading: Key Point

method overloading: a compiler’s help by looking at

“signature” rather than “name” in calling

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Operator Overloading

1 c l a s s Demo{

2 s t a t i c v o i d main ( S t r i n g [ ] argv ) {

3 i n t a = 5 ;

4 System . o u t . p r i n t l n ( 3 + a ) ; / / p l u s ( 3 , a )

5 System . o u t . p r i n t l n ( " " + a ) ; / / p l u s ( " " , a )

6 }

7 }

Java: a limited special case for String (actually, StringBuffer); the usual cases for primitive types; but not for other extended types C++: can overload almost “any” operator for any class

More About Methods