OOPClass,May31-June1,2010 Hsuan-TienLin Generics

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Generics

Hsuan-Tien Lin

Department of CSIE, NTU

OOP Class, May 31-June 1, 2010

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Motivation of Generics

1 c l a s s ANYArray {

2 p r i v a t e ANY [ ] myarr ;

3 p u b l i c ANYArray (i n t l e n ) { myarr = new ANY [ l e n ] ; }

4 p r o t e c t e d ANY g e t (i n t n ) { r e t u r n myarr [ n ] ; }

5 p r o t e c t e d v o i d s e t (i n t n , ANY o ) { myarr [ n ] = o ; }

6 p u b l i c v o i d s h o w A l l ( ) {

7 f o r(i n t i = 0 ; i <myarr . l e n g t h ; i ++)

8 System . o u t . p r i n t l n ( myarr [ i ] ) ;

9 }

10 }

Yes, by identifying the common parts, and then replacing manual way: sed ’s/ANY/String/’ ANYArray.java >

StringArray.java

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C++ Solution (roughly)

1 t e m p l a t e <c l a s s ANY>

2 c l a s s A r r a y {

4 p u b l i c A r r a y (i n t l e n ) { myarr = new ANY [ l e n ] ; }

10 }

11 }

12

13 {

14 Array < S t r i n g > s a r r ( 5 ) ;

15 s a r r . s e t ( 3 , " l a l a l a " ) ;

16 }

basically, the step sed ’s/ANY/String/’ ANYArray.cpp >

StringArray.cpp done by compiler

code automatically duplicates during complication as you use

Array<String>, Array<Integer>, Array<Double>, ...

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Java Solution (roughly)

1 c l a s s Array <ANY> {

3 p u b l i c A r r a y (i n t l e n ) { myarr = (ANY [ ] ) (new O b j e c t [ l e n ] ) ; }

9 }

10 }

11

12 {

13 Array < S t r i n g > s a r r ( 5 ) ;

14 s a r r . s e t ( 3 , " l a l a l a " ) ;

15 }

the ANY → Object step is automatically done by compiler: a true

one-class solution

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How does duplicating solution compare

with one-class solution?

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How can we write one class for arbitrary

sets of arbitrary size while keeping type

information?

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Should StringSet extend ObjectSet?

3FNIGX%VVE] EVV ! RI[ 7XVMRK%VVE]

EVV EHH RI[ -RXIKIV

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Java Solution: Generics (since 1.4)

no manual duplicating (as opposed to old languages): save coding efforts

no automatic duplicating (as opposed to C++): save code size and re-compiling efforts

check type information very strictly by compiler (as opposed to single-object polymorphism): ensure type safety in JVM

Note: type information erased after compilation

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Type Erasure: Mystery 1

1 c l a s s Set <T> {

2 Set ( ) {

3 T [ ] a r r = new T [ 1 0 ] ;

4 a r r [ 0 ] = new T ( ) ;

5 }

6 }

cannot new with an “undetermined type” T (no T in runtime)

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Type Erasure: Mystery 2

1 c l a s s Set <T> {

2 }

3 p u b l i c c l a s s Fun {

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

5 Set < S t r i n g > [ ] a r r = new Set < S t r i n g > [ 2 0 ] ;

6 a r r [ 0 ] . addElement (new I n t e g e r ( 3 ) ) ;

7 }

8 }

cannot create generic array (after type erasure, no type

guarantee)

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Use of Generics: Java Collection Framework

interfaces: Collection (Set, List) and Map

abstract classes: AbstractCollection (AbstractSet, AbstractList) and AbstractMap

concrete classes: HashSet, ArrayList, HashMap

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SimpleArray I

1 c l a s s SimpleArray <E> {

2 E [ ] a r r ; i n t co unt ;

3

4 v o i d i n i t 1 (i n t i n i t _ s i z e ) {

5 a r r = new E [ i n i t _ s i z e ] ;

6 }

8 a r r = new O b j e c t [ i n i t _ s i z e ] ;

9 }

11 a r r = ( E [ ] ) (new O b j e c t [ i n i t _ s i z e ] ) ;

12 }

13 @SuppressWarnings ( " unchecked " )

15 a r r = ( E [ ] ) (new O b j e c t [ i n i t _ s i z e ] ) ;

16 }

17 O b j e c t [ ] a r r O b j ;

19 a r r O b j = new O b j e c t [ i n i t _ s i z e ] ;

20 }

21

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SimpleArray II

22

23 v o i d add1 ( E element ) {

24 a r r [ co unt ++] = element ;

25 }

26

27 v o i d add2 ( E element ) {

28 a r r O b j [ co unt ++] = element ;

29 }

30 31 32 33 34 35 36 37 38 39 40 41 42 43

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SimpleArray III

44 E pop1 ( ) {

45 r e t u r n a r r [ count −−];

46 }

47

48 E pop2 ( ) {

49 r e t u r n a r r O b j [ count −−];

50 r e t u r n n u l l;

51 }

52

53 E pop3 ( ) {

54 r e t u r n ( E ) a r r O b j [ count −−];

55 }

56

58 E pop4 ( ) {

59 r e t u r n ( E ) a r r O b j [ count −−];

60 }

61 62 63 64 65

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SimpleArray IV

66 boolean equals1 ( SimpleArray <E> a n o t h e r ) {

67 i f ( a n o t h e r . co unt ! = co unt ) r e t u r n f a l s e;

68 f o r(i n t i = 0 ; i < a n o t h e r . c oun t ; i ++)

69 i f ( a r r [ i ] ! = a n o t h e r . a r r [ i ] )

70 r e t u r n f a l s e;

71 r e t u r n t r u e;

72 }

73 boolean equals2 ( SimpleArray <?> a n o t h e r ) {

74 i f ( a n o t h e r . co unt ! = co unt ) r e t u r n f a l s e;

76 i f ( a r r [ i ] ! = a n o t h e r . a r r [ i ] )

77 r e t u r n f a l s e;

78 r e t u r n t r u e;

79 }

80 81 82 83 84 85 86 87

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SimpleArray V

88 v o i d j o i n 1 ( SimpleArray <E> a n o t h e r ) {

90 a r r [ co unt ++] = a n o t h e r . a r r [ i ] ;

91 }

92 v o i d j o i n 2 ( SimpleArray <? extends E> a n o t h e r ) {

95 }

96 v o i d j o i n 3 ( SimpleArray <?> a n o t h e r ) {

99 }

100 101 102 103 104 105 106 107 108 109

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SimpleArray VI

110 E [ ] t o A r r a y 1 ( ) {

111 r e t u r n a r r ;

112 }

113

115 E [ ] t o A r r a y 2 ( ) {

116 r e t u r n ( E [ ] ) a r r O b j ;

117 }

118

119 O b j e c t [ ] t o O b j e c t A r r a y 1 ( ) {

120 r e t u r n a r r ;

121 }

122 O b j e c t [ ] t o O b j e c t A r r a y 2 ( ) {

123 r e t u r n a r r ;

124 }

125

127 <T> T [ ] t o T A r r a y 1 ( T [ ] t y p e ) {

128 r e t u r n ( T [ ] ) a r r ;

129 }

130 131

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SimpleArray VII

132 v o i d j o i n 2 ( SimpleArray <? extends E> a n o t h e r ) {

135 }

136

137 <T extends E> v o i d j o i n 2 w i t h T ( SimpleArray <T> a n o t h e r ) {

140 }

141 142 }

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More on Type Erasure

1 A r r a y L i s t < S t r i n g > l 1 = new A r r a y L i s t < S t r i n g > ( ) ;

2 A r r a y L i s t < I n t e g e r > l 2 = new A r r a y L i s t < I n t e g e r > ( ) ;

3 System . o u t . p r i n t l n ( l 1 . g e t C l a s s ( ) == l 2 . g e t C l a s s ( ) ) ;

4 System . o u t . p r i n t l n ( l 1 i n s t a n c e o f C o l l e c t i o n < S t r i n g > ) ;