Slide pack for my JavaZone talk Iteration 2.0: Stream. Official page: http://2014.javazone.no/presentation.html?id=640bf7b0

1. Iteration 2.0
Stream
Martin Skarsaune
Java Developer and Co-Owner

2. Iteration Problem
Root
x
y z
public class Node {
private Object data;
private Map<String, Node> children;
}
u
v

3. Recursion
• Need to traverse model
void recursiveX() {
x();
for (final Node child : children.values())
child.recursiveX();
}
• recursiveY()
• recursiveZ()
• ...

4. External Iteration : Iterator
• Any way to implement recursion once and for
all ?
• Iterator pattern (Java >= 1.2 )
• Wikipedia:
” the iterator pattern is a design pattern in
which an iterator is used to traverse a
container and access the container's
elements”

5. Client In Control
Iterator Behaviour
Root
x
y z
u
v
Client
Iterator()
next()
Iterator

6. Iterator Issues
• The iterator instance has state
– Strong references to provider and internal state
• implementation often difficult
• Thread unsafe by design

7. Internal Iteration - Visitor
• The visitor pattern
• Wikipedia:
” the visitor design pattern is a way of
separating an algorithm from an object
structure on which it operates”

8. Visitor Behaviour
Provider in Control
Root
x
y z
u
v
Client
visitor()
Visitor
accept(Visitor)
visit(Node)
visit(Node)
visit(Node)

9. Visitor Implementation
interface Visitor {
void visit(Node node);
}
void recursiveX() {
x();
for (final Node child : children.values())
child.recursiveX();
}

10. Visitor Implementation
interface Visitor {
void visit(Node node);
}
void accept(Visitor visitor) {
visitor.visit(this);
for (final Node child : children.values())
child.accept(visitor);
}

11. Visitor Usage
node.accept(n -> n.x());
node.accept(n -> n.y());
node.accept(n -> n.z());

12. Visitor – Limitations (Lambda)
• Limited access to local scope
– Final and effectively final variables
• Early exit not possible
– break
– return

13. Internal iteration in Java 8 -
Stream
• java.util.Stream
• Not limited to collections
• Many powerful methods
• Potential gains
– Lazy evaluation
– Parallell execution
• The default implementation (StreamSupport) is
based on (spl)iterators

14. LIMITATIONS
callback()
Stream In Control
Stream
Support
(Spl)Iterator
Default Stream
Implementation
Root
x
y z
u
v
stream() Client
next()

15. Visitor Stream Implementation
interface Visitor {
void visit(Node node);
}

16. Visitor Stream Implementation
- GEneralizaion
interface Visitor<T> {
void visit(T node);
}
interface Visitable<T> {
void accept(Visitor<T> visitor);
}

17. Visitor Stream Implementation
- Concrete
class Node {
void accept(Visitor visitor){
...
visitor.accept(this);
...
}
}

18. Visitor Stream Implementation
- Concrete
class Node {
void accept(Visitor<Node> visitor){
...
visitor.accept(this);
...
}
}

19. Visitor Stream Implementation
- Concrete
class Node implements Visitable<Node> {
void accept(Visitor<Node> visitor){
...
visitor.accept(this);
...
}
}

20. Visitor Stream Implementation
- Concrete
class VisitorStream<T> implements Stream<T> {
private Visitable<T> source;
public VisitorStream(Visitable<T> source) {
this.source = source;
}
...
}

21. interface Visitor<T> {
void visit(T node);
Stream Method Categories
default boolean isDone(){
return false;
}
Provider VisitorStream Filter Map
public void forEach(Consumer<? super T> action) {
this.source.accept(item -> action.accept(item));
}
}
✔ ✔ ✔
Reduce
• max
• min
• reduce
• count
Iterator
• iterator
• spliterator
Concurrency
• sequential
• paralell
Output
• toArray
Pipeline
• filter
• map
• flatMap
• skip(long)
Stateful
• sorted
• distinct
• ordered
Short circuit
• *Match(Predicate)
• limit(int)
• find*
Iteration
• forEach ✔
✔ ✔
✔
interface SplitVisitable<T> {
Collection<Visitable<T>> split();
}
implements SortedVisitable ...
implements DistinctVisitable ...
implements OrderedVisitable ...

22. Summary
• Many advantages to internal iteration
– Control
– Simplified implementation
– Nice abstraction
• Streams are powerful
– Create your own – it is not that hard 

23. Thank You for
Your Time!
Martin Skarsaune
Java Developer and Co-Owner