Generics
Hsuan-Tien Lin
Department of CSIE, NTU
OOP Class, May 24-25, 2010
How can we write a class for an Integer set of arbitrary size?
H.-T. Lin (NTU CSIE) Generics OOP 05/24-25/2010 1 / 20
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How can we write a class a String set of arbitrary size?
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How can we write classes for Inte- ger/String/Double/Professor sets of arbi- trary size?
H.-T. Lin (NTU CSIE) Generics OOP 05/24-25/2010 3 / 20
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How can we write one class for arbitrary
sets of arbitrary size?
Motivation of Generics (1/3)
1 c l a s s S t r i n g A r r a y {
2 p r i v a t e S t r i n g [ ] myarr ;
3 p u b l i c S t r i n g A r r a y (i n t l e n ) { myarr = new S t r i n g [ l e n ] ; }
4 p u b l i c S t r i n g g e t (i n t n ) { r e t u r n myarr [ n ] ; }
5 p u b l i c v o i d s e t (i n t n , S t r i n g s ) { myarr [ n ] = s ; }
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 }
11 c l a s s P r o f e s s o r A r r a y {
12 p r i v a t e P r o f e s s o r [ ] myarr ;
13 p u b l i c P r o f e s s o r A r r a y (i n t l e n ) { myarr = new P r o f e s s o r [ l e n ] ; }
14 p u b l i c P r o f e s s o r g e t (i n t n ) { r e t u r n myarr [ n ] ; }
15 p u b l i c v o i d s e t (i n t n , P r o f e s s o r p ) { myarr [ n ] = p ; }
16 p u b l i c v o i d s h o w A l l ( ) {
17 f o r(i n t i = 0 ; i <myarr . l e n g t h ; i ++)
18 System . o u t . p r i n t l n ( myarr [ i ] ) ;
19 }
20 }
Can we avoid writing the same boring things again and again?
H.-T. Lin (NTU CSIE) Generics OOP 05/24-25/2010 5 / 20
Motivation of Generics (2/3)
1 c l a s s O b j e c t A r r a y {
2 p r i v a t e O b j e c t [ ] myarr ;
3 p u b l i c O b j e c t A r r a y (i n t l e n ) { myarr = new O b j e c t [ l e n ] ; }
4 p r o t e c t e d O b j e c t 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 , O b j e c t 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 }
11
12 c l a s s S t r i n g A r r a y extends O b j e c t A r r a y {
13 p u b l i c S t r i n g A r r a y (i n t l e n ) { super( l e n ) ; }
14 p u b l i c S t r i n g g e t (i n t n ) { r e t u r n ( S t r i n g )super. g e t ( n ) ; }
15 p u b l i c v o i d s e t (i n t n , S t r i n g s ) { super. s e t ( n , s ) ; }
16 }
Yes, by inheritance and polynormphism—everything is an Object
Motivation of Generics (3/3)
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 sed ’s/ANY/String/’ ANYArray.java >
StringArray.java
H.-T. Lin (NTU CSIE) Generics OOP 05/24-25/2010 7 / 20
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 {
3 p r i v a t e ANY [ ] myarr ;
4 p u b l i c A r r a y (i n t l e n ) { myarr = new ANY [ l e n ] ; }
5 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 ] ; }
6 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 ; }
7 p u b l i c v o i d s h o w A l l ( ) {
8 f o r(i n t i = 0 ; i <myarr . l e n g t h ; i ++)
9 System . o u t . p r i n t l n ( myarr [ i ] ) ;
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
Java Solution (roughly)
1 c l a s s Array <ANY> {
2 p r i v a t e ANY [ ] myarr ;
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 ] ) ; }
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 }
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
H.-T. Lin (NTU CSIE) Generics OOP 05/24-25/2010 9 / 20
