Basic Java OOP 10/19/2015 Hsuan-Tien Lin (

(1)

Basic Java OOP

10/19/2015

Hsuan-Tien Lin (林軒田) htlin@csie.ntu.edu.tw

Department of Computer Science

& Information Engineering

National Taiwan University ( 國立台灣大學資訊工程系)

Hsuan-Tien Lin (NTU CSIE) Basic Java OOP 0/45

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What We Have Done

•

designing object (what variables? what methods?)

—[0] extended types, [1] method declaration/implementation

•

creating “first” object and calling its first action (done: java ClassName will call main)

•

creating other objects —[2] new and beyond

•

calling other objects (partially done: method invocation, will talk more in [1])

•

manipulating object status (partially done: instance variable assignments)

•

deleting objects —[TODO 3] object lifecycle

(3)

More on Constructors: Key Point

often better to use self-defined and overloaded constructors to help initialize

Hsuan-Tien Lin (NTU CSIE) 2/45

(4)

[3] Object Lifecycle

(5)

Garbage Collection (1/2)

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

2 p r i v a t e i n t sc or e ;

3 }

4 p u b l i c c l a s s RecordDemo {

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

6 i n t i ; Record r 1 ;

7 f o r( i = 0 ; i < 100; i + + ) {

8 r 1 = new Record ( ) ;

9 }

10 }

11 }

•

100 instances created, only 1 alive after the loop

•

the other 99 memory slots: automatically recycled

(6)

Garbage Collection (1/2)

3 }

7 f o r( i = 0 ; i < 100; i + + ) {

9 }

10 }

11 }

•

100 instances created, only 1 alive after the loop

•

the other 99 memory slots: automatically recycled

(7)

Garbage Collection (2/2)

1 c l a s s Record {

2 p r i v a t e Record prev ;

3 p u b l i c Record ( Record p ) { prev = p ; }

4 }

7 i n t i ; Record r 1 = n u l l;

8 f o r( i = 0 ; i < 100; i + + ) {

9 Record tmp = new Record ( r 1 ) ;

10 r 1 = tmp ;

11 }

12 }

13 }

•

100 instances created, all of them alive

(8)

Garbage Collection (2/2)

2 p r i v a t e Record prev ;

3 p u b l i c Record ( Record p ) { prev = p ; }

4 }

7 i n t i ; Record r 1 = n u l l;

8 f o r( i = 0 ; i < 100; i + + ) {

9 Record tmp = new Record ( r 1 ) ;

10 r 1 = tmp ;

11 }

12 }

13 }

•

100 instances created, all of them alive

(9)

Garbage Collection: Key Point

Garbage Collection: when a memory slot be- comes an orphan (and) system in need of memory

(10)

Finalizer (1/2)

3 p u b l i c Record ( ) { sys . mem_usage += 1 0 ; }

4 p u b l i c v o i d when_truck_comes ( ) { sys . mem_usage −= 1 0 ; }

5 }

9 f o r( i = 0 ; i < 100; i + + ) {

11 }

12 }

13 }

•

finalizer: something you want to do when truck comes

•

calculate memory usage, write something back (say, on BBS), ...

(11)

Finalizer (1/2)

4 p u b l i c v o i d when_truck_comes ( ) { sys . mem_usage −= 1 0 ; }

5 }

9 f o r( i = 0 ; i < 100; i + + ) {

11 }

12 }

13 }

•

finalizer: something you want to do when truck comes

•

calculate memory usage, write something back (say, on BBS), ...

(12)

Finalizer (2/2)

4 p r o t e c t e d v o i d f i n a l i z e ( ) t h r o w s Throwable {

5 sys . mem_usage −= 1 0 ;

6 System . o u t . p r i n t l n ( " Good Bye ! " ) ;

7 }

8 }

•

GC: no guarantee on when the truck comes

•

if JVM halts before truck comes, even no finalizer calls

• use carefully

(13)

Finalizer (2/2)

4 p r o t e c t e d v o i d f i n a l i z e ( ) t h r o w s Throwable {

5 sys . mem_usage −= 1 0 ;

6 System . o u t . p r i n t l n ( " Good Bye ! " ) ;

7 }

8 }

•

GC: no guarantee on when the truck comes

•

if JVM halts before truck comes, even no finalizer calls

• use carefully

(14)

Finalizer: Key Point

finalizer:

a mechanism to let the instance say goodbye

(15)

Object Lifecycle (1/1)

3 p u b l i c Record (i n t i n i t _ s c o r e ) { sc or e = i n i t _ s c o r e ; }

4 p r o t e c t e d v o i d f i n a l i z e ( ) t h r o w s Throwable { }

5 }

8 Record r ; / / v a r i a b l e d e c l a r e d

9 r = new Record ( 6 0 ) ; / / memory a l l o c a t e d (RHS)

10 / / and c o n s t r u c t o r c a l l e d

11 / / v a r i a b l e l i n k e d ( LHS )

12 r . show_score ( ) ; / / i n s t a n c e a c t i o n performed

13 r = n u l l;/ / memory s l o t orphaned

14 / / . . . .

15 / / f i n a l i z e r c a l l e d

16 / / o r JVM t e r m i n a t e d

17 }

18 }

(16)

Object Lifecycle: Key Point

we control birth, life, death, funeral design, but not the exact funeral time

(17)

[4] Back to Class

(18)

[4] Back to Class

Static Variables (1/3)

2 p r i v a t e i n t t o t a l _ r e c ;

3 p u b l i c Record ( ) {

4 t o t a l _ r e c += 1 ;

5 }

6 p u b l i c v o i d s h o w _ t o t a l _ r e c ( ) {

7 System . o u t . p r i n t l n ( t o t a l _ r e c ) ;

8 }

9 }

12 Record r 1 = new Record ( ) ;

13 r 1 . s h o w _ t o t a l _ r e c ( ) ;

15 r 2 . s h o w _ t o t a l _ r e c ( ) ;

16 }

17 }

•

no shared space to store the total records

(19)

[4] Back to Class

Static Variables (1/3)

2 p r i v a t e i n t t o t a l _ r e c ;

3 p u b l i c Record ( ) {

4 t o t a l _ r e c += 1 ;

5 }

8 }

9 }

13 r 1 . s h o w _ t o t a l _ r e c ( ) ;

15 r 2 . s h o w _ t o t a l _ r e c ( ) ;

16 }

17 }

•

no shared space to store the total records

(20)

[4] Back to Class

Static Variables (2/3)

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

2 p r i v a t e i n t cou nt ;

3 p u b l i c v o i d i n c r e a s e _ c o u n t ( ) { c oun t ++; }

4 p u b l i c i n t g e t _ c o u n t ( ) { r e t u r n c oun t ; }

5 }

7 RecordShared shared ;

8 p u b l i c Record ( RecordShared s ) {

9 share = s ; shared . i n c r e a s e _ c o u n t ( ) ;

10 }

12 System . o u t . p r i n t l n ( shared . g e t _ c o u n t ( ) ) ;

13 }

14 }

17 RecordShared shared_space = new RecordShared ( ) ;

18 Record r 1 = new Record ( shared_space ) ;

19 r 1 . s h o w _ t o t a l _ r e c ( ) ;

21 r 2 . s h o w _ t o t a l _ r e c ( ) ;

22 }

23 }

•

do-able, but complicated, and requires many explicit steps

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[4] Back to Class

Static Variables (2/3)

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

2 p r i v a t e i n t cou nt ;

3 p u b l i c v o i d i n c r e a s e _ c o u n t ( ) { c oun t ++; }

4 p u b l i c i n t g e t _ c o u n t ( ) { r e t u r n c oun t ; }

5 }

7 RecordShared shared ;

8 p u b l i c Record ( RecordShared s ) {

9 share = s ; shared . i n c r e a s e _ c o u n t ( ) ;

10 }

12 System . o u t . p r i n t l n ( shared . g e t _ c o u n t ( ) ) ;

13 }

14 }

17 RecordShared shared_space = new RecordShared ( ) ;

19 r 1 . s h o w _ t o t a l _ r e c ( ) ;

21 r 2 . s h o w _ t o t a l _ r e c ( ) ;

22 }

23 }

•

do-able, but complicated, and requires many explicit steps

(22)

[4] Back to Class

Static Variables (3/3)

2 p r i v a t e s t a t i c i n t t o t a l _ r e c = 0 ;

3 p u b l i c Record ( ) { t o t a l _ r e c ++; }

6 }

7 }

11 r 1 . s h o w _ t o t a l _ r e c ( ) ;

13 r 2 . s h o w _ t o t a l _ r e c ( ) ;

14 System . o u t . p r i n t l n ( Record . t o t a l _ r e c ) ;

15 }

16 }

•

static: shared between all X-type instances

•

like a global variable within the scope of the class

• use scarcely

(23)

[4] Back to Class

Static Variables (3/3)

6 }

7 }

11 r 1 . s h o w _ t o t a l _ r e c ( ) ;

13 r 2 . s h o w _ t o t a l _ r e c ( ) ;

15 }

16 }

•

static: shared between all X-type instances

•

like a global variable within the scope of the class

• use scarcely

(24)

[4] Back to Class

Static Variables: Key Point

staticvariable:

of the

class (shared), not of an instance

(25)

[4] Back to Class

Static Variables Revisited (1/1)

3 p r i v a t e i n t i d ;

4 p u b l i c Record ( ) { i d = t o t a l _ r e c + + ; }

5 }

9 Record r 2 = n u l l;

11 System . o u t . p r i n t l n ( r 1 . t o t a l _ r e c ) ;

14 System . o u t . p r i n t l n ( r 1 . i d ) ;

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

17 }

18 }

•

r2.total_rec⇒ Record.total_rec in compile time

(26)

[4] Back to Class

Static Variables Revisited (1/1)

3 p r i v a t e i n t i d ;

4 p u b l i c Record ( ) { i d = t o t a l _ r e c + + ; }

5 }

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

17 }

18 }

•

r2.total_rec⇒ Record.total_rec in compile time

(27)

[4] Back to Class

Static Variables Revisited: Key Point

staticvariable:

of the

class (shared), not of an instance;

compile-time binding (i.e. static binding)

(28)

[4] Back to Class

Static Methods (1/2)

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

2 p u b l i c double mean (double a , double b ) {

3 r e t u r n ( a + b ) ∗ 0 . 5 ;

4 }

5 }

6 p u b l i c c l a s s MathDemo {

8 double i = 3 . 5 ;

9 double j = 2 . 4 ;

10 myMath m = new MyMath ( ) ;

11 System . o u t . p r i n t l n (m. mean ( i , j ) ) ;

12 }

13 }

•

new a myMath instance just for computing mean –unnecessary

(29)

[4] Back to Class

Static Methods (1/2)

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

2 p u b l i c double mean (double a , double b ) {

3 r e t u r n ( a + b ) ∗ 0 . 5 ;

4 }

5 }

8 double i = 3 . 5 ;

9 double j = 2 . 4 ;

10 myMath m = new MyMath ( ) ;

11 System . o u t . p r i n t l n (m. mean ( i , j ) ) ;

12 }

13 }

•

new a myMath instance just for computing mean –unnecessary

(30)

[4] Back to Class

Static Methods (2/2)

1 c l a s s myMath {

2 s t a t i c double mean (double a , double b ) {

3 r e t u r n ( a + b ) ∗ 0 . 5 ;

4 }

5 }

8 double i = 3 . 5 ;

9 double j = 2 . 4 ;

10 System . o u t . p r i n t l n ( myMath . mean ( i , j ) ) ;

11 System . o u t . p r i n t l n ( ( new myMath ( ) ) . mean ( i , j ) ) ;

12 }

13 }

•

make the method a static (class) one

—no need to new an instance

•

similar to static variable usage

(31)

[4] Back to Class

Static Methods (2/2)

1 c l a s s myMath {

2 s t a t i c double mean (double a , double b ) {

3 r e t u r n ( a + b ) ∗ 0 . 5 ;

4 }

5 }

8 double i = 3 . 5 ;

9 double j = 2 . 4 ;

10 System . o u t . p r i n t l n ( myMath . mean ( i , j ) ) ;

11 System . o u t . p r i n t l n ( ( new myMath ( ) ) . mean ( i , j ) ) ;

12 }

13 }

•

make the method a static (class) one

—no need to new an instance

•

similar to static variable usage

(32)

[4] Back to Class

Static Methods: Key Point

staticmethod:

associated with the

class,

no need to create an instance

(33)

[4] Back to Class

Use of Static Methods (1/2)

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

3 System . o u t . p r i n t l n ( Math . PI ) ;

4 System . o u t . p r i n t l n ( Math . s q r t ( 2 . 0 ) ) ;

5 System . o u t . p r i n t l n ( Math . max ( 3 . 0 , 5 . 0 ) ) ;

6 System . o u t . p r i n t l n ( I n t e g e r . t o B i n a r y S t r i n g ( 1 5 ) ) ;

7 }

8 }

•

commonly used as utility functions (so don’t need to create instance)

•

main is static (called by classname during ’java className’)

•

tools for other static methods

(34)

[4] Back to Class

Use of Static Methods (1/2)

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

3 System . o u t . p r i n t l n ( Math . PI ) ;

4 System . o u t . p r i n t l n ( Math . s q r t ( 2 . 0 ) ) ;

5 System . o u t . p r i n t l n ( Math . max ( 3 . 0 , 5 . 0 ) ) ;

6 System . o u t . p r i n t l n ( I n t e g e r . t o B i n a r y S t r i n g ( 1 5 ) ) ;

7 }

8 }

•

commonly used as utility functions (so don’t need to create instance)

•

main is static (called by classname during ’java className’)

•

tools for other static methods

(35)

[4] Back to Class

Use of Static Methods (2/2)

4 p u b l i c s t a t i c v o i d s h o w _ t o t a l _ r e c ( ) {

6 }

7 }

11 Record . s h o w _ t o t a l _ r e c ( ) ;

12 }

13 }

•

class related actions rather than instance related actions

(36)

[4] Back to Class

Use of Static Methods (2/2)

4 p u b l i c s t a t i c v o i d s h o w _ t o t a l _ r e c ( ) {

6 }

7 }

11 Record . s h o w _ t o t a l _ r e c ( ) ;

12 }

13 }

•

class related actions rather than instance related actions

(37)

[4] Back to Class

Use of Static Methods: Key Point

staticmethod:

•

compile time determined (bound)

•

per class

•

sometimes useful

(38)

[4] Back to Class

Fun Time (1)

What happens in memory?

1 i n t i ;

2 s h o r t j ;

3 double k ;

4 char c = ’ a ’ ;

5 i = 3 ; j = 2 ;

6 k = i ∗ j ;

(39)

[4] Back to Class

Life Cycle of a Primitive Variable (C/Java)

•

declared and created

1 i n t c oun t ;

•

used and modified

1 co unt += 1 ;

•

destroyed

–automatically (when out of scope)

(40)

[4] Back to Class

Life Cycle of a Primitive Variable (C/Java)

•

1 i n t c oun t ;

•

used and modified

1 co unt += 1 ;

•

destroyed

(41)

[4] Back to Class

Life Cycle of a Primitive Variable (C/Java)

•

1 i n t c oun t ;

•

used and modified

1 co unt += 1 ;

•

destroyed

(42)

[4] Back to Class

Fun Time (2)

1 S t r i n g s = " l a l a l a " ;

2 S t r i n g t = " abc " ;

3 S t r i n g a = s + t ;

(43)

[4] Back to Class

Fun Time (3)

1 Record r 1 ; / / r 1 . name , r 1 . sc or e

2 Record r 2 ;

4 r 2 = r 1 ; / / how many r e c o r d s are t h e r e ?

5 r 1 . name = " HTLin " ;

6 r 2 . sc or e = 9 8 ;

(44)

[4] Back to Class

Fun Time (4)

1 c l a s s Person { S t r i n g name ; Person spouse ; }

2

3 Person George ;

4 Person Marry ;

5 George = new Person ( ) ;

6 George . name = " George " ;

7 Marry = new Person ( ) ;

8 Marry . name = " Marry " ;

9 Mary . spouse = George ;

10 George . spouse = Marry ;

(45)

[4] Back to Class

Fun Time (5)

1 c l a s s Person { S t r i n g name ; Person spouse ; }

2

3 Person George ;

4 George = new Person ( ) ;

5 George . name = " George " ;

6 George . spouse = new Person ( ) ;

7 George . spouse . name = " Marry " ;

8 George . spouse = new Person ( ) ;

9 George . spouse . name = " L i s a " ;

(46)

[4] Back to Class

Life Cycle of an Object Instance (Java)

•

reference declared

1 Record r ;

•

instance created

1 r = new Record ( ) ;

•

used and modified

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

•

destroyed

–automatically (when out of

use)

(47)

[4] Back to Class

Life Cycle of an Object Instance (Java)

•

reference declared

1 Record r ;

•

instance created

•

used and modified

•

destroyed

use)

(48)

[4] Back to Class

Life Cycle of an Object Instance (Java)

•

reference declared

1 Record r ;

•

instance created

•

used and modified

•

destroyed

use)

(49)

[4] Back to Class

Life Cycle of an Object Instance (Java)

•

reference declared

1 Record r ;

•

instance created

•

used and modified

•

destroyed

use)

(50)

[4] Back to Class

null Revisited (1/2)

2 S t r i n g name ;

3 S t r i n g ID ;

4 i n t sc or e ;

5 }

6

10 System . o u t . p r i n t l n ( r 1 . sc or e ) ;

11 System . o u t . p r i n t l n ( r 1 . name ) ;

12 }

13 }

•

null: Java’s reserved word of saying “no reference”

•

default initial value for extended types (if initialized automatically)

•

0, NULL, anything equivalent to integer 0: C’s way of saying “no reference”

(51)

[4] Back to Class

null Revisited (1/2)

3 S t r i n g ID ;

4 i n t sc or e ;

5 }

6

12 }

13 }

•

null: Java’s reserved word of saying “no reference”

•

default initial value for extended types (if initialized automatically)

•

0, NULL, anything equivalent to integer 0: C’s way of saying “no reference”

(52)

[4] Back to Class

null Revisited (2/2)

3 S t r i n g ID ;

4 i n t sc or e ;

5 }

6

12 }

13 }

•

null pointer exception (run time error): accessing the component of “no reference”

(53)

[4] Back to Class

null Revisited (2/2)

3 S t r i n g ID ;

4 i n t sc or e ;

5 }

6

12 }

13 }

•

null pointer exception (run time error): accessing the component of “no reference”

(54)

[4] Back to Class

null Revisited: Key Point

null: Java’s special way of saying “no reference”

(55)

[4] Back to Class

Reference Equal (1/2)

3 i n t sc or e ;

4 }

5

8 Record r1 , r 2 ;

9 r 1 = new Record ( ) ; r 2 = new Record ( ) ;

10 r 1 . name = " HTLin " ; r 1 . s co re = 9 5 ;

12 System . o u t . p r i n t l n ( r 1 == r 2 ) ;

13 r 2 = r 1 ;

15 }

16 }

•

reference equal: comparison by “reference value”

(56)

[4] Back to Class

Reference Equal (1/2)

3 i n t sc or e ;

4 }

5

8 Record r1 , r 2 ;

9 r 1 = new Record ( ) ; r 2 = new Record ( ) ;

13 r 2 = r 1 ;

15 }

16 }

•

reference equal: comparison by “reference value”

(57)

[4] Back to Class

Reference Equal (2/2)

3 i n t sc or e ;

4 }

5

8 Record r1 , r 2 ;

9 r 1 = n u l l; r 2 = new Record ( ) ;

11 r 2 = r 1 ;

13 }

14 }

•

nulldoes not equal non-null ... o_O

•

nullequals null ... O_o

(58)

[4] Back to Class

Reference Equal (2/2)

3 i n t sc or e ;

4 }

5

8 Record r1 , r 2 ;

9 r 1 = n u l l; r 2 = new Record ( ) ;

11 r 2 = r 1 ;

13 }

14 }

•

nulldoes not equal non-null ... o_O

•

nullequals null ... O_o

(59)

[4] Back to Class

Reference Equal: Key Point

==: reference equal rather than content equal for extended types

(60)

[4] Back to Class

String Equal (1/1)

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

2 s t a t i c S t r i n g s1 ;

5 s1 = " HTLin " ;

6 s2 = " HTLin " ;

7 System . o u t . p r i n t l n ( s1 == s2 ) ;

8 s1 = s1 + " l a l a l a " ;

9 s2 = s2 + " l a l a l a " ;

11 System . o u t . p r i n t l n ( s1 . e q u a l s ( s2 ) ) ;

12 }

13 }

•

first true: compiler allocates one constant string only

•

second false: two different string references

•

third true: an action (method) for content comparison

(61)

[4] Back to Class

String Equal (1/1)

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

5 s1 = " HTLin " ;

6 s2 = " HTLin " ;

8 s1 = s1 + " l a l a l a " ;

9 s2 = s2 + " l a l a l a " ;

11 System . o u t . p r i n t l n ( s1 . e q u a l s ( s2 ) ) ;

12 }

13 }

•

first true: compiler allocates one constant string only

•

second false: two different string references

•

third true: an action (method) for content comparison

(62)

[4] Back to Class

String Equal: Key Point

String ==: still reference equal, use .equals if want content equal

(63)

[4] Back to Class

Reference Argument/Parameter (1/3)

1 c l a s s Tool {

2 b o o l t r i c k y ( S t r i n g s1 , S t r i n g s2 ) {

3 s2 = s2 + " " ;

4 r e t u r n ( s1 == s2 ) ;

5 }

6 }

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

9 Tool t = new Tool ( ) ;

10 S t r i n g sa = " HTLin " ;

11 S t r i n g sb = sa ;

12 System . o u t . p r i n t l n ( t . t r i c k y ( sa , sb ) ) ;

13 System . o u t . p r i n t l n ( sa == sb ) ;

14 System . o u t . p r i n t l n ( t . t r i c k y ( sa + " " , sb ) ) ;

15 }

16 }

•

reference parameter passing: again, value copying

•

sa, sb copied to s1, s2

•

s2 (reference) changed, sb didn’t

(64)

[4] Back to Class

Reference Argument/Parameter (1/3)

1 c l a s s Tool {

2 b o o l t r i c k y ( S t r i n g s1 , S t r i n g s2 ) {

3 s2 = s2 + " " ;

4 r e t u r n ( s1 == s2 ) ;

5 }

6 }

10 S t r i n g sa = " HTLin " ;

11 S t r i n g sb = sa ;

12 System . o u t . p r i n t l n ( t . t r i c k y ( sa , sb ) ) ;

13 System . o u t . p r i n t l n ( sa == sb ) ;

14 System . o u t . p r i n t l n ( t . t r i c k y ( sa + " " , sb ) ) ;

15 }

16 }

•

reference parameter passing: again, value copying

•

sa, sb copied to s1, s2

•

s2 (reference) changed, sb didn’t

(65)

[4] Back to Class

Reference Argument/Parameter (2/3)

1 c l a s s m yI nt {i n t v a l ; m yI nt (i n t v ) { v a l = v ; } }

2 c l a s s Tool {

3 v o i d swap ( m yI nt f i r s t , m yI nt second ) {

4 i n t tmp = f i r s t . v a l ;

5 f i r s t . v a l = second . v a l ;

6 second . v a l = tmp ;

7 System . o u t . p r i n t l n ( f i r s t . v a l ) ;

8 System . o u t . p r i n t l n ( second . v a l ) ;

9 }

10 }

14 m yI nt i = new m yI nt ( 3 ) ;

15 m yI nt j = new m yI nt ( 5 ) ;

16 t . swap ( i , j ) ;

17 System . o u t . p r i n t l n ( i . v a l ) ;

18 System . o u t . p r i n t l n ( j . v a l ) ;

19 }

20 }

•

swapped as requested

(66)

[4] Back to Class

Reference Argument/Parameter (2/3)

2 c l a s s Tool {

4 i n t tmp = f i r s t . v a l ;

5 f i r s t . v a l = second . v a l ;

6 second . v a l = tmp ;

9 }

10 }

16 t . swap ( i , j ) ;

19 }

20 }

•

swapped as requested

(67)

[4] Back to Class

Reference Argument/Parameter (3/3)

2 c l a s s Tool {

4 m yI nt tmp = f i r s t ;

5 f i r s t = second ;

6 second = tmp ;

9 }

10 }

16 t . swap ( i , j ) ;

19 }

20 }

•

what happens?

(68)

[4] Back to Class

Reference Argument/Parameter (3/3)

2 c l a s s Tool {

4 m yI nt tmp = f i r s t ;

5 f i r s t = second ;

6 second = tmp ;

9 }

10 }

16 t . swap ( i , j ) ;

19 }

20 }

•

what happens?

(69)

[4] Back to Class

Reference Argument/Parameter: Key Point

argument ⇒ parameter: by reference copying

same for return value

(70)

[4] Back to Class

this (1/1)

2 i n t sc or e ;

3 v o i d s e t _ t o (i n t s co re ) { t h i s. sc or e = sc or e ; }

4 v o i d a d j u s t _ s c o r e { t h i s. s e t _ t o ( sc or e + 1 0 ) ; }

5 }

•

which score? which set_to?

•

this: my (the object’s)

(71)

[4] Back to Class

this (1/1)

2 i n t sc or e ;

3 v o i d s e t _ t o (i n t s co re ) { t h i s. sc or e = sc or e ; }

4 v o i d a d j u s t _ s c o r e { t h i s. s e t _ t o ( sc or e + 1 0 ) ; }

5 }

•

which score? which set_to?

•

this: my (the object’s)

(72)

[4] Back to Class

this: Key Point

this: the reference variable pointing to the object itself