Java Generics

1
Java Generics

• COMP204 Bernhard Pfahringer
• (with input from Robi Malik)

2
Generic types for collections

• Old Java (1.4 and older)
• List strings new ArrayList()
• strings.add(hello)
• String word (String) strings.get(0)
• New (since 1.5)
• ListltStringgt strings new ArrayListltStringgt()
• strings.add(hello)
• String word strings.get(0)

3
Advantages

• Better readability
• Better type-safety no casts (runtime checks),
  compiler can already catch problems

4
Writing your own generic code

• public class StackltEgt
• public void push(E element)
• contents.add(element)
• public E pop()
• int top contents.size()-1
• E result contents.get(top)
• contents.remove(top)
• return result
• private ListltEgt contents new ArrayListltEgt()

5
Formal type parameter

• public class StackltEgt
• convention Short (single-char) uppercase
• can be used wherever a Type is needed
• will be replaced with actual Type

6
Problems with sub-types

• class Student extends Person ..
• ListltStudentgt students new ArrayListltStudentgt()
  
• ListltPersongt people students
• // should this be possible? -gt no

7
No, because

• Person sam new Person()
• people.add(sam)
• // if this was legal, we would have just sneaked
  a non-student onto the students list
• on the other hand, how do you write generic code
  then accepting lists of sub-types of Persons ???

8
Wildcards

• void printCollection(Collectionlt?gt c)
• for(Object o c)
• System.out.println(o)
• // only read access (everything is an Object),
  but cannot add, because do not know correct type
  (except null, which is any type)

9
Bounded wildcards

• public void drawAll(Collectionlt? extends Shapegt
  c)
• for(Shape shape c)
• shape.draw()
• // again, only read access, allows collections of
  Shape or any sub type of Shape