Mittwoch, 2. Januar 2013

JDK8 lambdas and anonymous classes

Preview releases of the upcoming JDK8 including the long-awaited lambdas are available for several months meanwhile. Time to have a look at lambdas to see what they are and what you can expect from them.

So today, I downloaded the latest preview release of the JDK8 from jdk8.java.net/lambda to have a look at the upcoming lambdas in JDK8. To my despair, this code snippet did not compile: 

        List<Integer> ints = new ArrayList<>();
        ints.add(1);
        ints.add(2);
        ints.add(3);

        int sum = 0;
        ints.forEach(i -> { sum += i; });
 

The compiler error was: "value used in lambda expression should be effectively final". The compiler complains here that the variable sum had not been declared final. Also see this blog post, that is part of the JDK8 lambda FAQ, which explains the matter (I perpetually insist on having found the issue independently from this post ;-)). So lambdas in JDK8 carry exactly the same restriction as anonymous classes and you have to resort to the same kind of workaround:


int sumArray[] = new int[] { 0 };
ints.forEach(i -> {sumArray[0] += i;}); 

println(sumArray[0]);


This works and prints 6 as expected. Note, that the compiler did not complain here about sumArray not being declared final as it is effectively final: "A variable is effectively final if it is never assigned to after its initialization" (see link). This is a new feature in JDK8 as the code below does not compile with a pre-JDK8 if value is not declared final:


final long[] value = new long[] { 0 };
Runnable runnable = new Runnable() {          
    public void run() {
        value[0] = System.currentTimeMillis();
        System.out.println(value[0]);
    }
};


However, this means that JDK8 lambdas are not true closures since they cannot refer to free variables, which is a requirement for an expression to be a closure:

"When a function refers to a variable defined outside it, it's called a free variable. A function that refers to a free lexical variable is called a closure.". Paul Graham, ANSI Common Lisp, Prentice Hall, 1996, p.107.

The free variable gives the closure expression access to its environment:


"A closure is a combination of a function and an environment.". Paul Graham, ANSI Common Lisp, Prentice Hall, 1996, p.108.


In the end we can conclude that JDK8 lambdas are less verbose than anonymous classes (and there is no instantiation overhead as with anonymous classes as lambdas compile to method handles), but they carry the same restrictions as they do. The lambda specification (JSR 335) also says so explicitly: "For both lambda bodies and inner classes, local variables in the enclosing context can only be referenced if they are final or effectively final. A variable is effectively final if it is never assigned to after its initialization.". Here is also a link to an article where Neal Gafter himself (who was a member of the BGGA team) tried to explain why inner classes are no closures (read the comments section). However, all this is only a little tear drop as the usefulness of closures is preserved to a large extend. An imense amount of pre-JDK8 boilerplate code can be replaced with much more concise expressions now. And in the end, you can anyway still do this:
       
        int sum = ints.stream().reduce(0, (x, y) -> x + y);
       

Nevertheless, the difference between JDK8 lambdas and closures is worth a note as it is good to know. There is a nice write-up about many the things you can do with JDK8 lambdas in this blog post. Here is some sample code from it: 


List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Dave");
names

   .mapped(e -> { return e.length(); })
   .asIterable()
   .filter(e -> e.getValue() >= 4)
   .sorted((a, b) -> a.getValue() - b.getValue())
   .forEach(e -> { System.out.println(e.getKey() + '\t' + e.getValue()); });


We can also reference a static method:


executorService.submit(MethodReference::sayHello);
private static void sayHello() {
        System.out.println("hello");
}


The lambda FAQ says about the restriction on local variable capture explained in this article: "The restriction on local variables helps to direct developers using lambdas aways from idioms involving mutation; it does not prevent them. Mutable fields are always a potential source of concurrency problems if sharing is not properly managed; disallowing field capture by lambda expressions would reduce their usefulness without doing anything to solve this general problem.". 

The author is making the point here that immutable variables, like those declared final, cannot be changed inadvertently by some other thread. A free variable referenced from within a closure expression (but declared outside the closure) is allocated somewhere on the heap, which means that it is not local to some specific stack (hence it is free). Being allocated on the heap a free variable can be seen by all other threads as well. This way a free variable can effectively become a variable shared between threads for which access needs to be synchronized to prevent stale data from happening. So the finalness restriction for JDK8 lambdas helps in avoiding that kind of trouble. Note, however, that this is only true for simple types or objects that are not nested as can be seen in the sample code in the beginning of this text with the single-element-array sumArray: the final variable sumArray cannot be re-assigned, but the single element it holds can be changed any time.

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