Lambda functions in java 8

What’s a lambda function?
 A.K.A.:
 Anonymous function
 Anonymous method
 Function literal
 Function constant
 Closure
 Wikipedia: “a function (or a subroutine)
defined, and possibly called, without being
bound to an identifier”
 Typically can’t be recursive directly
 Simple example: (int x, int y) -> x + y
 Supported in a bunch of languages…

Already supported in…
 ActionScript, C#, C++, Clojure, Curl, D,
Dart, Delphi, Dylan, Erlang, F#, Frink,
Go, Groovy, Haskell, JavaScript, Lisp,
Logtalk, Lua, Mathematica, Maple,
Matlab, Maxima, Ocaml, Octave,
Objective-C, Perl, PHP, Python, R,
Racket, Ruby, Scala, Scheme,
Smalltalk, Vala, Visual Basic, Visual
Prolog…

More Examples
 x -> x + 1
 (x) -> x + 1
 (int x) -> x + 1
 (int x, int y) -> x + y
 (x, y) -> x + y
 (x, y) -> { System.out.printf("%d +
%d = %d%n", x, y, x+y); }
 () -> { System.out.println("I am a
Runnable"); }

Meanwhile, in Java 7…
 Functional Interfaces
 SAM – “Single Abstract Method”
 Interfaces that have just 1 method

 java.lang.Runnable
 java.util.concurrent.Callable
 java.security.PrivilegedAction
 java.util.Comparator
 java.io.FileFilter
 java.nio.file.PathMatcher
 java.lang.reflect.InvocationHandler
 java.beans.PropertyChangeListener
 java.awt.event.ActionListener
 javax.swing.event.ChangeListener

Meanwhile, in Java 7…
 Typically:
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
panel.doStuff(e.getModifiers());
}
});
 Five lines to do one statement

 This would become:
button.addActionListener(e -> { panel.doStuff(e.getModifiers()); });

More Anonymous Class Issues(?)

 Lexical scoping
 What does this mean?
final Runnable r = () -> {
// This reference to 'r' is legal:
if (!allDone) { workQueue.add(r); }
else { displayResults(); }
};

 Accessing non-final variables
 Lambdas: same variable access as you’d
have just outside the expression

java.util.functions
 Predicate
List<String> names = Arrays.asList("Alice", "Bob",
"Charlie", "Dave");
List<String> filteredNames = names
.filter(e -> e.length() >= 4)
.into(new ArrayList<String>());
 filter: only retain elements where the
predicate holds true
 into: fills up an ArrayList with the filtered
elements, then gets set as filteredNames

java.util.functions
 Block
 No more for loops…?
"Charlie", "Dave");
names
.filter(e -> e.length() >= 4)
.forEach(e -> { System.out.println(e); });

 forEach: Can only see one element at a
time
 Lazy element computation

java.util.functions
 Method chaining
"Charlie", "Dave");
names
.mapped(e -> { return e.length(); })
.asIterable() // returns an Iterable of BiValue
// elements; an element's key is the
// person's name, its value is the
// string length
.filter(e -> e.getValue() >= 4)
.sorted((a, b) -> a.getValue() - b.getValue())
.forEach(e -> { System.out.println(e.getKey() +
't' + e.getValue()); });
 Is method chaining good…?

Method References
 Can pass around references to methods
like you can with anonymous functions
by using ::
 Static and instance methods
Arrays.asList("Alice", "Bob", "Charlie",
"Dave").forEach(System.out::println);

 Make factories easily:
Factory<Biscuit> biscuitFactory = Biscuit::new;
Biscuit biscuit = biscuitFactory.make();

Default Methods
 Currently, interfaces are basically “set in stone”
 Altering an interface will break existing implementations
 Blurred line between abstract/concrete methods…?
interface Foo {
void bar() default {
System.out.println(“baz”);
}
}

interface AnotherFoo extends Foo {
void bar() default {
System.out.println(“not baz”);
}
}

interface YetAnotherFoo extends Foo {
void bar();
}

More Java 8 Stuff…
 http://openjdk.java.net/projects/jigsaw/

 Sources:
 http://java.dzone.com/news/java-8-lambda-
syntax-closures
 http://datumedge.blogspot.ca/2012/06/java-
8-lambdas.html
 http://cr.openjdk.java.net/~briangoetz/lambd
a/lambda-state-4.html

Lambda functions in java 8

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