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1. Overview

Now that Java 8 has reached wide usage, patterns and best practices have begun to emerge for some of its headlining features. In this tutorial, we'll take a closer look at functional interfaces and lambda expressions.

Further reading:

Why Do Local Variables Used in Lambdas Have to Be Final or Effectively Final?

Learn why Java requires local variables to be effectively final when used in a lambda.

Java 8 – Powerful Comparison with Lambdas

Elegant Sort in Java 8 - Lambda Expressions go right past syntactic sugar and bring powerful functional semantics into Java.

2. Prefer Standard Functional Interfaces

Functional interfaces, which are gathered in the java.util.function package, satisfy most developers' needs in providing target types for lambda expressions and method references. Each of these interfaces is general and abstract, making them easy to adapt to almost any lambda expression. Developers should explore this package before creating new functional interfaces.

Let's consider an interface Foo:

@FunctionalInterface
public interface Foo {
    String method(String string);
}

In addition, we have a method add() in some class UseFoo, which takes this interface as a parameter:

public String add(String string, Foo foo) {
    return foo.method(string);
}

To execute it, we would write:

Foo foo = parameter -> parameter + " from lambda";
String result = useFoo.add("Message ", foo);

If we look closer, we'll see that Foo is nothing more than a function that accepts one argument and produces a result. Java 8 already provides such an interface in Function<T,R> from the java.util.function package.

Now we can remove interface Foo completely and change our code to:

public String add(String string, Function<String, String> fn) {
    return fn.apply(string);
}

To execute this, we can write:

Function<String, String> fn = 
  parameter -> parameter + " from lambda";
String result = useFoo.add("Message ", fn);

3. Use the @FunctionalInterface Annotation

Now let's annotate our functional interfaces with @FunctionalInterface. At first, this annotation seems to be useless. Even without it, our interface will be treated as functional as long as it has just one abstract method.

However, let's imagine a big project with several interfaces; it's hard to control everything manually. An interface, which was designed to be functional, could accidentally be changed by adding another abstract method/methods, rendering it unusable as a functional interface.

By using the @FunctionalInterface annotation, the compiler will trigger an error in response to any attempt to break the predefined structure of a functional interface. It is also a very handy tool to make our application architecture easier to understand for other developers.

So we can use this:

@FunctionalInterface
public interface Foo {
    String method();
}

Instead of just:

public interface Foo {
    String method();
}

4. Don't Overuse Default Methods in Functional Interfaces

We can easily add default methods to the functional interface. This is acceptable to the functional interface contract as long as there is only one abstract method declaration:

@FunctionalInterface
public interface Foo {
    String method(String string);
    default void defaultMethod() {}
}

Functional interfaces can be extended by other functional interfaces if their abstract methods have the same signature:

@FunctionalInterface
public interface FooExtended extends Baz, Bar {}
	
@FunctionalInterface
public interface Baz {	
    String method(String string);	
    default String defaultBaz() {}		
}
	
@FunctionalInterface
public interface Bar {	
    String method(String string);	
    default String defaultBar() {}	
}

Just as with regular interfaces, extending different functional interfaces with the same default method can be problematic.

For example, let's add the defaultCommon() method to the Bar and Baz interfaces:

@FunctionalInterface
public interface Baz {
    String method(String string);
    default String defaultBaz() {}
    default String defaultCommon(){}
}

@FunctionalInterface
public interface Bar {
    String method(String string);
    default String defaultBar() {}
    default String defaultCommon() {}
}

In this case, we'll get a compile-time error:

interface FooExtended inherits unrelated defaults for defaultCommon() from types Baz and Bar...

To fix this, the defaultCommon() method should be overridden in the FooExtended interface. We can provide a custom implementation of this method; however, we can also reuse the implementation from the parent interface:

@FunctionalInterface
public interface FooExtended extends Baz, Bar {
    @Override
    default String defaultCommon() {
        return Bar.super.defaultCommon();
    }
}

It's important to note that we have to be careful. Adding too many default methods to the interface is not a very good architectural decision. This should be considered a compromise, only to be used when required for upgrading existing interfaces without breaking backward compatibility.

5. Instantiate Functional Interfaces With Lambda Expressions

The compiler will allow us to use an inner class to instantiate a functional interface; however, this can lead to very verbose code. We should prefer to use lambda expressions:

Foo foo = parameter -> parameter + " from Foo";

Over an inner class:

Foo fooByIC = new Foo() {
    @Override
    public String method(String string) {
        return string + " from Foo";
    }
};

The lambda expression approach can be used for any suitable interface from old libraries. It is usable for interfaces like Runnable, Comparator, and so on; however, this doesn't mean that we should review our whole older code base and change everything.

6. Avoid Overloading Methods With Functional Interfaces as Parameters

We should use methods with different names to avoid collisions:

public interface Processor {
    String process(Callable<String> c) throws Exception;
    String process(Supplier<String> s);
}

public class ProcessorImpl implements Processor {
    @Override
    public String process(Callable<String> c) throws Exception {
        // implementation details
    }

    @Override
    public String process(Supplier<String> s) {
        // implementation details
    }
}

At first glance, this seems reasonable, but any attempt to execute either of the ProcessorImpl‘s methods:

String result = processor.process(() -> "abc");

Ends with an error with the following message:

reference to process is ambiguous
both method process(java.util.concurrent.Callable<java.lang.String>) 
in com.baeldung.java8.lambda.tips.ProcessorImpl 
and method process(java.util.function.Supplier<java.lang.String>) 
in com.baeldung.java8.lambda.tips.ProcessorImpl match

To solve this problem, we have two options. The first option is to use methods with different names:

String processWithCallable(Callable<String> c) throws Exception;

String processWithSupplier(Supplier<String> s);

The second option is to perform casting manually, which is not preferred:

String result = processor.process((Supplier<String>) () -> "abc");

7. Don’t Treat Lambda Expressions as Inner Classes

Despite our previous example, where we essentially substituted inner class by a lambda expression, the two concepts are different in an important way: scope.

When we use an inner class, it creates a new scope. We can hide local variables from the enclosing scope by instantiating new local variables with the same names. We can also use the keyword this inside our inner class as a reference to its instance.

Lambda expressions, however, work with enclosing scope. We can’t hide variables from the enclosing scope inside the lambda’s body. In this case, the keyword this is a reference to an enclosing instance.

For example, in the class UseFoo, we have an instance variable value:

private String value = "Enclosing scope value";

Then in some method of this class, place the following code and execute this method:

public String scopeExperiment() {
    Foo fooIC = new Foo() {
        String value = "Inner class value";

        @Override
        public String method(String string) {
            return this.value;
        }
    };
    String resultIC = fooIC.method("");

    Foo fooLambda = parameter -> {
        String value = "Lambda value";
        return this.value;
    };
    String resultLambda = fooLambda.method("");

    return "Results: resultIC = " + resultIC + 
      ", resultLambda = " + resultLambda;
}

If we execute the scopeExperiment() method, we'll get the following result: Results: resultIC = Inner class value, resultLambda = Enclosing scope value

As we can see, by calling this.value in IC, we can access a local variable from its instance. In the case of the lambda, this.value call gives us access to the variable value, which is defined in the UseFoo class, but not to the variable value defined inside the lambda's body.

8. Keep Lambda Expressions Short and Self-explanatory

If possible, we should use one line constructions instead of a large block of code. Remember, lambdas should be an expression, not a narrative. Despite its concise syntax, lambdas should specifically express the functionality they provide.

This is mainly stylistic advice, as performance will not change drastically. In general, however, it is much easier to understand and to work with such code.

This can be achieved in many ways; let's have a closer look.

8.1. Avoid Blocks of Code in Lambda's Body

In an ideal situation, lambdas should be written in one line of code. With this approach, the lambda is a self-explanatory construction, which declares what action should be executed with what data (in the case of lambdas with parameters).

If we have a large block of code, the lambda's functionality is not immediately clear.

With this in mind, do the following:

Foo foo = parameter -> buildString(parameter);
private String buildString(String parameter) {
    String result = "Something " + parameter;
    //many lines of code
    return result;
}

Instead of:

Foo foo = parameter -> { String result = "Something " + parameter; 
    //many lines of code 
    return result; 
};

It is important to note, we shouldn't use this “one-line lambda” rule as dogma. If we have two or three lines in lambda's definition, it may not be valuable to extract that code into another method.

8.2. Avoid Specifying Parameter Types

A compiler, in most cases, is able to resolve the type of lambda parameters with the help of type inference. Consequently, adding a type to the parameters is optional and can be omitted.

We can do this:

(a, b) -> a.toLowerCase() + b.toLowerCase();

Instead of this:

(String a, String b) -> a.toLowerCase() + b.toLowerCase();

8.3. Avoid Parentheses Around a Single Parameter

Lambda syntax only requires parentheses around more than one parameter, or when there is no parameter at all. That's why it's safe to make our code a little bit shorter, and to exclude parentheses when there is only one parameter.

So we can do this:

a -> a.toLowerCase();

Instead of this:

(a) -> a.toLowerCase();

8.4. Avoid Return Statement and Braces

Braces and return statements are optional in one-line lambda bodies. This means that they can be omitted for clarity and conciseness.

We can do this:

a -> a.toLowerCase();

Instead of this:

a -> {return a.toLowerCase()};

8.5. Use Method References

Very often, even in our previous examples, lambda expressions just call methods which are already implemented elsewhere. In this situation, it is very useful to use another Java 8 feature, method references.

The lambda expression would be:

a -> a.toLowerCase();

We could substitute it with:

String::toLowerCase;

This is not always shorter, but it makes the code more readable.

9. Use “Effectively Final” Variables

Accessing a non-final variable inside lambda expressions will cause a compile-time error, but that doesn’t mean that we should mark every target variable as final.

According to the “effectively final” concept, a compiler treats every variable as final as long as it is assigned only once.

It's safe to use such variables inside lambdas because the compiler will control their state and trigger a compile-time error immediately after any attempt to change them.

For example, the following code will not compile:

public void method() {
    String localVariable = "Local";
    Foo foo = parameter -> {
        String localVariable = parameter;
        return localVariable;
    };
}

The compiler will inform us that:

Variable 'localVariable' is already defined in the scope.

This approach should simplify the process of making lambda execution thread-safe.

10. Protect Object Variables From Mutation

One of the main purposes of lambdas is use in parallel computing, which means that they're really helpful when it comes to thread-safety.

The “effectively final” paradigm helps a lot here, but not in every case. Lambdas can't change a value of an object from enclosing scope. But in the case of mutable object variables, a state could be changed inside lambda expressions.

Consider the following code:

int[] total = new int[1];
Runnable r = () -> total[0]++;
r.run();

This code is legal, as total variable remains “effectively final,” but will the object it references have the same state after execution of the lambda? No!

Keep this example as a reminder to avoid code that can cause unexpected mutations.

11. Conclusion

In this article, we explored some of the best practices and pitfalls in Java 8's lambda expressions and functional interfaces. Despite the utility and power of these new features, they are just tools. Every developer should pay attention while using them.

The complete source code for the example is available in this GitHub project. This is a Maven and Eclipse project, so it can be imported and used as is.

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