Day 1 — Let’s learn about lambdas

From today, I am kicking off 7 Days with Java 8 blog series with first blog on Lambdas. One of the most important features in Java 8 is the introduction of Lambda expressions. They make your code concise and allows you to pass behaviour around. For some time now, Java is criticised for being verbose and for lacking functional programming capabilities. With functional programming becoming more popular and relevant, Java is forced to embrace the functional style of programming. Else, Java would become irrelevant.

Java 8 is a big step forward in making the world’s most popular language adopt the functional style of programming. To support functional programming style, a language needs to support functions as first class citizen. Prior to Java 8, writing a clean functional style code was not possible without the use of an anonymous inner class boilerplate. With the introduction of Lambda expressions, functions have become first class citizen and they can be passed around just like any other variable.

Lambda expressions allow you to define an anonymous function that is not bound to an identifier. You can use them like any other construct in your programming language like variable declaration. Lambda expressions are required if a programming language needs to support higher order functions. Higher order functions are functions that either accept other functions as arguments or returns a function as a result.

Now, with the introduction of Lambda expressions in Java 8, Java supports higher order functions. Let us look at the canonical example of Lambda expression — a sort function in Java’s Collections class. The sort function has two variants — one that takes a List and another that takes a List and a Comparator. The second sort function is an example of a Higher order function that accepts a lambda expression as shown below in the code snippet.

List names = Arrays.asList("shekhar", "rahul", "sameer");
Collections.sort(names, (first, second) -> first.length() - second.length());

The code shown above sorts the names by their length. The output of the program will be as shown below.

[rahul, sameer, shekhar]

The expression (first, second) -> first.length() – second.length() shown above in the code snippet is a lambda expression of type Comparator.

• The (first, second) are parameters of the compare method of Comparator.
• first.length() - second.length() is the function body that compares the length of two names.
• -> is the lambda operator that separates parameters from the body of the lambda.

Before we dig deeper into Java 8 Lambdas support, let’s look into their history to understand why they exist.

History of Lambdas

Lambda expressions have their roots in the Lambda Calculus. Lambda calculus originated from the work of Alonzo Church on formalizing the concept of expressing computation with functions. Lambda calculus is turing complete formal mathematical way to express computations. Turing complete means you can express any mathematical computation via lambdas.

Lambda calculus became the basis of strong theoretical foundation of functional programming languages. Many popular functional programming languages like Haskell, Lisp are based on Lambda calculus. The idea of higher order functions i.e. a function accepting other functions came from Lambda calculus.

The main concept in Lambda calculus is expressions. An expression can be expressed as:

<expression> := <variable> | <function>| <application>

• variable — A variable is a placeholder like x, y, z for values like 1, 2, etc or lambda functions.

• function — It is an anonymous function definition that takes one variable and produces another lambda expression. For example, λx.x*x is a function to compute square of a number.

• application — This is the act of applying a function to an argument. Suppose you want a square of 10, so in lambda calculus you will write a square function λx.x*x and apply it to 10. This function application would result in (λx.x*x) 10 = 10*10 = 100.You can not only apply simple values like 10 but, you can apply a function to another function to produce another function. For example, (λx.x*x) (λz.z+10) will produce a function λz.(z+10)*(z+10). Now, you can use this function to produce number plus 10 squares. This is an example of higher order function.

Now, you understand Lambda calculus and its impact on functional programming languages. Let’s learn how they are implemented in Java 8.

Passing behavior before Java 8

Before Java 8, the only way to pass behavior was to use anonymous classes. Suppose you want to send an email in another thread after user registration. Before Java 8, you would write code like one shown below.

sendEmail(new Runnable() {
@Override
public void run() {
System.out.println("Sending email...");
}
});

The sendEmail method has following method signature.

public static void sendEmail(Runnable runnable)

The problem with the above mentioned code is not only that we have to encapsulate our action i.e. run method in an object but, the bigger problem is that it misses the programmer intent i.e. to pass behavior to sendEmail function. If you have used libraries like Guava then, you would have certainly felt the pain of writing anonymous classes. A simple example of filtering all the tasks with lambda in their title is shown below.

Iterable lambdaTasks = Iterables.filter(tasks, new Predicate() {
@Override
public boolean apply(Task task) {
return input.getTitle().contains("lambda");
}
});

With Java 8 Stream API, you can write the above mentioned code without the use of a third party library like Guava. We will cover streams on Day 2. So, stay tuned!!

Java 8 Lambda expressions

In Java 8, we would write the code using a lambda expression as show below. We have mentioned the same example in the code snippet above.

sendEmail(() -> System.out.println("Sending email..."));

The code shown above is concise and does not pollute the programmer’s intent to pass behavior. () is used to represent no function parameters i.e. Runnable interface run method does not have any parameters. -> is the lambda operator that separates the parameters from the function body which prints Sending email... to the standard output.

Let’s look at the Collections.sort example again so that we can understand how lambda expressions work with the parameters. To sort a List of names by their length, we used Collections.sort function as shown below.

Comparator comparator = (first, second) -> first.length() - second.length();

The lambda expression that we wrote was corresponding to compare method in the Comparator interface. The signature of compare function is shown below.

int compare(T o1, T o2);

T is the type parameter passed to Comparator interface. In this case it will be a String as we are working over a List of String i.e. names.

In the lambda expression, we didn’t have to explicitly provide the type — String. The javac compiler inferred the type information from its context. The Java compiler inferred that both parameters should be String as we are sorting a List of String and compare method use only one T type. The act of inferring type from the context is called Type Inference. Java 8 improves the already existing type inference system in Java and makes it more robust and powerful to support lambda expressions. javac under the hoods look for the information close to your lambda expression and uses that information to find the correct type for the parameters.

In most cases, javac will infer the type from the context. In case it can’t resolve type because of missing or incomplete context information then the code will not compile. For example, if we remove String type information from Comparator then code will fail to compile as shown below.

Comparator comparator = (first, second) -> first.length() - second.length(); // compilation error - Cannot resolve method 'length()'

How does Lambda expressions work in Java 8?

You may have noticed that the type of a lambda expression is some interface like Comparator in the above example. You can’t use any interface with lambda expression. Only those interfaces which have only one non-object method can be used with lambda expressions. These kinds of interfaces are called Functional interfaces and they are annotated with @FunctionInterface annotation. Runnable interface is an example of functional interface as shown below. @FunctionInterface annotation is not mandatory but, it can help tools know that an interface is a functional interface and perform meaningful actions. For example, if you try to compile an interface with multiple methods and FunctionalInterface annotation then compilation will fail with an error Multiple non-overriding abstract methods found. Similarly, if you add @FunctionInterface annotation to an interface without any method i.e. a marker interface then you will get error message No target method found.

@FunctionalInterface
public interface Runnable {
public abstract void run();
}

Let’s answer one of the most important questions that might be coming to your mind. Are Java 8 lambda expressions just the syntactic sugar over anonymous inner classes or how does functional interface gets translated to bytecode?

The short answer is NO. Java 8 does not use anonymous inner classes mainly for two reasons:

1. Performance impact: If lambda expressions were implemented using anonymous classes then each lambda expression would result in a class file on disk. When these classes are loaded by JVM at startup, then startup time of JVM will increase as all the classes needs to be first loaded and verified before they can be used.
2. Possibility to change in future: If Java 8 designers would have used anonymous classes from the start then it would have limited the scope of future lambda implementation changes.

Using invokedynamic

Java 8 designers decided to use invokedynamic instruction added in Java 7 to defer the translation strategy at runtime. Whenjavac compiles the code, it captures the lambda expression and generates an invokedynamic call site (called lambda factory). The invokedynamic call site when invoked returns an instance of the Functional Interface to which the lambda is being converted. For example, if we look at the byte code of our Collections.sort example, it will look like as shown below.

public static void main(java.lang.String[]);
Code:
0: iconst_3
1: anewarray #2 // class java/lang/String
4: dup
5: iconst_0
6: ldc #3 // String shekhar
8: aastore
9: dup
10: iconst_1
11: ldc #4 // String rahul
13: aastore
14: dup
15: iconst_2
16: ldc #5 // String sameer
18: aastore
19: invokestatic #6 // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
22: astore_1
23: invokedynamic #7, 0 // InvokeDynamic #0:compare:()Ljava/util/Comparator;
28: astore_2
29: aload_1
30: aload_2
31: invokestatic #8 // Method java/util/Collections.sort:(Ljava/util/List;Ljava/util/Comparator;)V
34: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
37: aload_1
38: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
41: return
}

The interesting part of the byte code shown above is the line 23 23: invokedynamic #7, 0 // InvokeDynamic #0:compare:()Ljava/util/Comparator; where a call to invokedynamic is made.

The second step is to convert the body of the lambda expression into a method that will be invoked through the invokedynamic instruction. This is the step where JVM implementers have the liberty to choose their own strategy.

I have only glossed over this topic. You can read about internals at http://cr.openjdk.java.net/~briangoetz/lambda/lambda-translation.html.

Do I need to write my own functional interfaces?

By default, Java 8 comes with many functional interfaces which you can use in your code. They exist inside java.util.function package. Let’s have a look at few of them.

java.util.function.Predicate

This functional interface is used to define check for some condition i.e. a predicate. Predicate interface has one method called test which takes a value of type T and return boolean. For example, from a list of names if we want to filter out all the names which starts with s then we will use a predicate as shown below.

Predicate namesStartingWithS = name -> name.startsWith("s");

java.util.function.Consumer

This functional interface is used for performing actions which does not produce any output. Predicate interface has one method called accept which takes a value of type T and return nothing i.e. it is void. For example, sending an email with given message.

Consumer messageConsumer = message -> System.out.println(message);

java.util.function.Function

This functional interface takes one value and produces a result. For example, if we want to uppercase all the names in our names list, we can write a Function as shown below.

Function<String, String> toUpperCase = name -> name.toUpperCase();

We will cover more functional interfaces as we move along in the series.

Method references

There would be times when you will be creating lambda expressions that only calls a specific method like Function strToLength = str -> str.length();. The lambda only calls length() method on the String object. This could be simplified using method references like Function strToLength = String::length;. They can be seen as shorthand notation for lambda expression that only calls a single method. In the expression String::length, String is the target reference, :: is the delimiter, and length is the function that will be called on the target reference. You can use method references on both the static and instance methods.

Static method references

Suppose we have to find a maximum number from a list of numbers then we can write a method reference Function maxFn = Collections::max. max is a static method in the Collections class that takes one argument of type List. You can then call this like maxFn.apply(Arrays.asList(1, 10, 3, 5)). The above lambda expression is equivalent to Function maxFn = (numbers) -> Collections.max(numbers); lambda expression.

Instance method references

This is used for method reference to an instance method for example String::toUpperCase calls toUpperCase method on a String reference. You can also use method reference with parameters for example BiFunction concatFn = String::concat. The concatFn can be called as concatFn.apply("shekhar", "gulati"). The String concat method is called on a String object and passed a parameter like "shekhar".concat("gulati").

Conclusion

In first blog of this series, you learnt the history of lambda expressions, how Java implements them, and finally we looked at few functional interfaces available in Java 8. In the next blog, we will cover how you can process data in collections using the new Stream API.