This document provides a survival guide for lambdas and generics in Java. It discusses key concepts like covariance, capture, inference, erasure and synthetic methods. It provides examples and explanations of how these concepts work. It also includes tips for avoiding warnings and exceptions when using generics and lambdas. Clean code techniques for refactoring lambda expressions are presented at the end. Useful external references on generics and lambdas are provided.

1. © Henri Tremblay 2015
Henri Tremblay
Senior Software Engineer
Terracotta, a Software AG company
@henri_tremblay
Lambdas and generics survival guide

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Henri Tremblay

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Henri Tremblay

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Henri Tremblay
• More or less made possible class mocking and proxying
• Coined the term “partial mocking”

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Henri Tremblay
• More or less made possible class mocking and proxying
• Coined the term “partial mocking”

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March 18 2014

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Java 7 End of Life

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September 2004

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Lambda
return Tweet.TWEETS.stream()
.collect(Collectors
.partitioningBy(
t->t.containsHashTag("#lambda")));

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Lambda = Fun with generics
Stream<Tweet> stream = Tweet.TWEETS.stream();
Predicate<Tweet> lambda = t -> t.containsHashTag("#lambda");
Collector<Tweet, ?, Map<Boolean, List<Tweet>>> collector =
Collectors.partitioningBy(lambda);
return stream.collect(collector);

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What do I need to know?
Why

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What do I need to know?
Why
Covariance

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What do I need to know?
Why
Covariance
Capture

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What do I need to know?
Why
Covariance
Capture
Inference

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What do I need to know?
Why
Covariance
Capture
Inference
Erasure

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What do I need to know?
Why Covariance
Capture Inference
Erasure Synthetic

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Compile

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Dreaded warnings
Type safety: The expression of type List needs
unchecked conversion to conform to List<String>
Type safety: Unchecked cast from
List<capture#1-of ?> to List<String>

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Ostrich defense
@SuppressWarnings("unchecked")

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Why

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Rule #1
A code compiling
without warning should
never ever cause a
ClassCastException

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Covariance

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Arrays
Arrays are covariant:
Number n = Integer.MAX_VALUE;
Number[] list = new Integer[0];
Generics are not:
List<Number> l =
new ArrayList<Integer>(); // Illegal

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Why not?
List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415));
int i = li.get(0); // ClassCastException
Would work if covariant And allow to break rule #1

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Why for array?
Integer[] list = // ...
foo(list);
public void foo(Object[] o) {
// ...
}

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Arrays and generics don’t mix well
Can’t have an array of generics
List<String>[] lsa = new List<String>[10];// illegal

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Because
If it was allowed
List<String>[] lsa = new List<String>[10]; // illegal
Object[] oa = lsa; // OK (covariant)
oa[0] = new ArrayList<Integer>(); // OK
oa[0].add(42);
String s = lsa[0].get(0); // bad

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Exception
List<?>[] l = new ArrayList<?>[3];

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@SafeVarargs // Not actually safe!
static void m(List<String>... stringLists) {
Object[] array = stringLists;
List<Integer> tmpList = Arrays.asList(42);
array[0] = tmpList; // Semantically invalid, but compiles without warnings
String s = stringLists[0].get(0); // ClassCastException
}
@SafeVarargs (Java 7)

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Capture

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Capture usually is
Type
List<?> bar();
<T> IExpectationSetters<T> expect(T value);
void andReturn(T value); // Method of IExpectationSetters
expect(bar()).andReturn(new ArrayList<String>());
And you get
The method andReturn(List<capture#6-of ?>) in the type
IExpectationSetters<List<capture#6-of ?>> is not applicable for the arguments
(ArrayList<String>)

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Detail
List<?> bar();
<T> IExpectationSetters<T> expect(T value);
void andReturn(T value);
expect(bar()).andReturn(new ArrayList<String>());
List<Capture#6> bar = bar();
IExpectationSetters<List<Capture#6>> es =
expect(bar());
es.andReturn(List<Capture#6> value);

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Only solution
We need to cast
expect((List<String>) bar())
.andReturn(new ArrayList<String>());
But still a warning
Type safety: Unchecked cast from List<capture#6-of
?> to List<String>
Framework coder tip:
Try to never return a wildcard unless necessary
Tell to expect we want a
List<String>

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Inference

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Diamonds are a programmer best friend
List<String> l = new ArrayList<>();

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How the compiler tells the type
<T> T anyObject(T t)
<T> T anyObject(Class<T> clazz)
The parameterDetermine the
return value
type

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How the compiler tells the type
MyType var = <T> T anyObject()
Determine the return
type
The assigned
variable

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But watch out with overloading
public void foo(String s)
public void foo(Object o)
foo(anyObject());
Can’t guess
the type

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Trick #1
<T> T anyObject(Class<T> clazz)
Artificially give the type with a
dedicated parameter
Determine the return
value type

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But how to pass a generic?
public void foo(String s)
public void foo(Object o)
foo(anyObject(List<String>.class));
Illegal

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Some solutions
foo((String) anyObject());
foo((List<String>) anyObject()); // Warning
This would work
But still doesn’t work for
generics

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Trick #2
So the only solution is called a “type witness”
foo(EasyMock.<List<String>> anyObject());
… which sadly doesn’t support static imports
foo(.<List<String>> anyObject()); // Illegal

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Trick #2
So the only solution is called a “type witness”
foo(EasyMock.<List<String>> anyObject());
… which sadly doesn’t support static imports
foo(.<List<String>> anyObject()); // Illegal

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Trick #2 applied to Lambda
return Tweet.TWEETS.stream()
.collect( Collectors.<Tweet, Boolean,
List<Tweet>, Map<Boolean, List<Tweet>>>
groupingBy(t->t.containsHashTag("#lambda"),
HashMap::new, ArrayList::new));
Return type: Map<Boolean, List<Tweet>>

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Lambda = Inference
return Tweet.TWEETS.stream()
.collect(Collectors
.partitioningBy(
t->t.containsHashTag("#lambda"));

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How did it do it?
Tweet.TWEETS.stream()
List<Tweet> list = Tweet.TWEETS;
Stream<Tweet> stream = list.stream();

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How did it do it?
Stream<Tweet> stream = list.stream();
R result = stream.collect(Collector<? super T, ?, R> collector);
R result = stream.collect(Collector<? super Tweet, ?, R>
collector);

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How did it do it?
stream.collect(Collector<? super Tweet, ?, R> collector);
Collector<T, ?, Map<Boolean, List<T>>> collector =
Collectors.partitioningBy(Predicate<? super T> predicate);
Collector<Tweet, ?, Map<Boolean, List<Tweet>>> collector =
Collectors.partitioningBy(Predicate<? super Tweet> predicate);

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We now know that
So the best t can be is a Tweet
How did it do it?
Predicate<? super Tweet> lambda = t -> t.containsHashTag("#lambda");
Predicate<? super Tweet> lambda = (Tweet t) -> t.containsHashTag("#lambda");

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Trick #3: Lambda inference
Object o = (Runnable) () -> {
System.out.println("hi");
};
Collections.sort(strings,
(String a, String b) -> a.compareTo(b));

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Erasure

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Erasure…
public void foo() {
List<String> l = new ArrayList<String>();
for (String s : l) {
System.out.println(s);
}
}
No type
public void foo() {
List l = new ArrayList();
for (String s : l) {
System.out.println(s);
}
}
Compilation

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… or not erasure
public class A extends ArrayList<String> {}
public static void main(final String[] args) {
ParameterizedType type = (ParameterizedType)
A.class.getGenericSuperclass();
System.out.println(
type.getActualTypeArguments()[0]);
}
 prints class java.lang.String

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Type class
java.lang.reflect.Type
• GenericArrayType
• ParameterizedType
• TypeVariable
• WildcardType
• Implemented by Class
java.lang.reflect.GenericDeclaration
Implemented by Class, Method, Constructor
New powers
unleashed!

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Useful!
class A {}
abstract class BaseDao<T> {
public T load(final long id) {
// …
}
}
class ADao extends BaseDao<A> {}

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Useful!
@SuppressWarnings("unchecked")
public T load(final long id) {
ParameterizedType type =
(ParameterizedType) getClass()
.getGenericSuperclass();
Type actualType = type.getActualTypeArguments()[0];
return em.find((Class<T>) actualType, (Long) id);
}
ADao
A
BaseDao<A>
Unsafe cast

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Synthetic methods

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Everything seems normal…
class A<T> {
abstract void set(T value);
}
class B extends A<String> {
String value;
@Override
void set(final String value) {
this.value = value;
}
}

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But is not
class B extends A<String> {
void set(String value) {
this.value = value;
}
volatile void set(Object o){
set((String)o);
}
}

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Example
A a = new B();
a.set(new Object());
But at runtime:
java.lang.ClassCastException
Raw type warning Perfectly compiling

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The actual problem being
B.class.getDeclaredMethods()
volatile void set(java.lang.Object)
void B.set(java.lang.String)
This
Returns that
And gives you no way to find out
which method is bridged

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What about lambdas?
public class A {
public static void main(String[] args) {
Method[] methods = A.class.getDeclaredMethods();
Arrays.stream(methods).forEach(m ->
System.out.println(m + " "
+ m.isBridge() + " " + m.isSynthetic()));
}
}

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What about lambdas?
public class A {
public static void main(String[] args) {
Method[] methods = A.class.getDeclaredMethods();
Arrays.stream(methods).forEach(m ->
System.out.println(m + " "
+ m.isBridge() + " " + m.isSynthetic()));
}
}
Prints this
public static void A.main(java.lang.String[]) false false
private static void A.lambda$0(java.lang.reflect.Method) false true

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Clean Code Bonus!

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What’s wrong?
public static Set<Tweet> containing(String hashtag) {
return Tweet.TWEETS.stream()
.filter(t -> t.hasHashtag(hashtag))
.collect(Collectors.toSet());
}

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What’s wrong?
public static Stream<Tweet> containing(String hashtag)
{
return Tweet.TWEETS.stream()
.filter(t -> t.hasHashtag(hashtag));
}
Return a Stream whenever you can

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What’s wrong?
Tweet.TWEETS
.stream()
.filter(t -> t.hasHashtag("#lambda"))
.sorted(Comparator.comparing(Tweet::getSender)
.thenComparing(Tweet::getTimestamp))
.forEachOrdered(System.out::println);
}

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What’s wrong?
public static void main(String[] args) {
getForHashtag("#lambda”).forEachOrdered(System.out::println);
}
private static Stream<Tweet> getForHashtag(String hashtag) {
return Tweet.TWEETS.stream()
.filter(hasHashtagPredicate(hashtag))
.sorted(bySenderThenTimestamp());
}
private static Predicate<Tweet> hasHashtagPredicate(String hashtag) {
return t -> t.hasHashtag(hashtag);
}
private static Comparator<Tweet> bySenderThenTimestamp() {
return Comparator.comparing(Tweet::getSender).thenComparing(Tweet::getTimestamp);
}
Refactor your one-liner

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Who has learned
something today?
?

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Useful links
Ninja Squad – Nice lambda dojo (in French)
• http://lambda.ninjackaton.ninja-squad.com/
Brian Goetz - Description of inference types on lambda
• http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html
• http://www.slideshare.net/jaxlondon2012/lambda-a-peek-under-the-hood-brian-goetz
Angelika Langer - Everything on generics
• http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html
Maurice Naftalin – Facts and books
• http://www.lambdafaq.org/
• Java Generics and Collections: Speed Up the Java Development Process
• Mastering Lambdas: Java Programming in a Multicore World

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Questions? http://montreal-jug.org
?
http://easymock.org
http://objenesis.org
http:/ehcache.org
Henri Tremblay
http://blog.tremblay.pro
@henri_tremblay