References (Section 5.2)

(1)

References (Section 5.2)

Hsuan-Tien Lin

Deptartment of CSIE, NTU

OOP Class, March 15-16, 2010

(2)

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 ;

(3)

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)

(4)

Fun Time (2)

What happens in memory?

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 ;

(5)

Fun Time (3)

What happens in memory?

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

2 Record r 2 ;

3 r 1 = new Record ( ) ;

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 ;

(6)

Fun Time (4)

What happens in memory?

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 ;

(7)

Fun Time (5)

What happens in memory?

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 " ;

(8)

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)

(9)

Reference: Key Point

a instance occupies a space in the memory;

老太太住在屏東一個房子裡面

reference (a.k.a. safe pointer): the “address” to the instance;

用"海角七號"就可以找到老太太

class-type variable: holds the reference;

一個"信封"，上面寫著海角七號

any operation on the instance goes thru the reference;

要請老太太"回憶"時，拿個信封上寫"海角七號"，接著寫"回憶"，阿Ja就會使命必達了

老人信封= new 老人(老太太的身家資料);

信封.回憶();

(10)

null Revisited (1/2)

1 c l a s s Record {

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

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

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

9 Record r 1 = new Record ( ) ;

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”

(11)

null Revisited (2/2)

3 S t r i n g ID ;

4 i n t sc or e ;

5 }

6

9 Record r 1 = n u l l;

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 pointer exception (run time error): accessing the component

of “no reference”

(12)

null Revisited: Key Point

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

(13)

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 ;

11 r 2 . name = " HTLin " ; r 2 . 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”

(14)

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 }

null does not equal non-null ... o_O

null equals null ... O_o

(15)

Reference Equal: Key Point

==: reference equal rather than content equal for ex-

tended types

(16)

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 ;

3 s t a t i c S t r i n g s2 ;

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 " ;

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

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

(17)

String Equal: Key Point

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

content equal

(18)

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

(19)

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

(20)

Reference Argument/Parameter (3/3)

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 m yI nt tmp = f i r s t ;

5 f i r s t = second ;

6 second = 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 }

what happens?

(21)

Reference Argument/Parameter: Key Point

argument ⇒ parameter: by reference copying

same for return value

(22)

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 +10) ; }

5 }

which score? which set_to?

this: my (the object’s)

(23)

References (Section 5.2)