Do lambda expressions have any use other than saving lines of code?

Do lambda expressions have any use other than saving lines of code?

Are there any special features provided by lambdas which solved problems which weren't easy to solve? The typical usage I've seen is that instead of writing this:

Comparator<Developer> byName = new Comparator<Developer>() {
  @Override
  public int compare(Developer o1, Developer o2) {
	return o1.getName().compareTo(o2.getName());
  }
};

We can use a lambda expression to shorten the code:

Comparator<Developer> byName =
(Developer o1, Developer o2) -> o1.getName().compareTo(o2.getName());

Lambda expressions do not change the set of problems you can solve with Java in general, but definitely make solving certain problems easier, just for the same reason we’re not programming in assembly language anymore. Removing redundant tasks from the programmer’s work makes life easier and allows to do things you wouldn’t even touch otherwise, just for the amount of code you would have to produce (manually).

But lambda expressions are not just saving lines of code. Lambda expressions allow you to define functions, something for which you could use anonymous inner classes as a workaround before, that’s why you can replace anonymous inner classes in these cases, but not in general.

Most notably, lambda expressions are defined independently to the functional interface they will be converted to, so there are no inherited members they could access, further, they can not access the instance of the type implementing the functional interface. Within a lambda expression, this and super have the same meaning as in the surrounding context, see also this answer. Also, you can not create new local variables shadowing local variables of the surrounding context. For the intended task of defining a function, this removes a lot of error sources, but it also implies that for other use cases, there might be anonymous inner classes which can not be converted to a lambda expression, even if implementing a functional interface.

Further, the construct new Type() { … } guarantees to produce a new distinct instance (as new always does). Anonymous inner class instances always keep a reference to their outer instance if created in a non-static context¹. In contrast, lambda expressions only capture a reference to this when needed, i.e. if they access this or a non-static member. And they produce instances of an intentionally unspecified identity, which allows the implementation to decide at runtime whether to reuse existing instances (see also “Does a lambda expression create an object on the heap every time it's executed?”).

These differences apply to your example. Your anonymous inner class construct will always produce a new instance, also it may capture a reference to the outer instance, whereas your (Developer o1, Developer o2) -> o1.getName().compareTo(o2.getName()) is a non-capturing lambda expression that will evaluate to a singleton in typical implementations. Further, it doesn’t produce a .class file on your hard drive.

Given the differences regarding both, semantic and performance, lambda expressions may change the way programmers will solve certain problems in the future, of course, also due to the new APIs embracing ideas of functional programming utilizing the new language features. See also Java 8 lambda expression and first-class values.

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¹ From JDK 1.1 to JDK 17. Starting with JDK 18, inner classes may not retain a reference to the outer instance if it is not used. For compatibility reasons, this requires the inner class not be serializable. This only applies if you (re)compile the inner class under JDK 18 or newer with target JDK 18 or newer. See also JDK-8271717

Programming languages are not for machines to execute.

They are for programmers to think in.

Languages are a conversation with a compiler to turn our thoughts into something a machine can execute. One of the chief complaints about Java from people who come to it from other languages (or leave it for other languages) used to be that it forces a certain mental model on the programmer (i.e. everything is a class).

I'm not going to weigh in on whether that's good or bad: everything is trade-offs. But Java 8 lambdas allow programmers to think in terms of functions, which is something you previously could not do in Java.

It's the same thing as a procedural programmer learning to think in terms of classes when they come to Java: you see them gradually move from classes that are glorified structs and have 'helper' classes with a bunch of static methods and move on to something that more closely resembles a rational OO design (mea culpa).

If you just think of them as a shorter way to express anonymous inner classes then you are probably not going to find them very impressive in the same way that the procedural programmer above probably didn't think classes were any great improvement.

Saving lines of code can be viewed as a new feature, if it enables you to write a substantial chunk of logic in a shorter and clearer manner, which takes less time for others to read and understand.

Without lambda expressions (and/or method references) Stream pipelines would have been much less readable.

Think, for example, how the following Stream pipeline would have looked like if you replaced each lambda expression with an anonymous class instance.

List<String> names =
    people.stream()
          .filter(p -> p.getAge() > 21)
          .map(p -> p.getName())
          .sorted((n1,n2) -> n1.compareToIgnoreCase(n2))
          .collect(Collectors.toList());

It would be:

List<String> names =
    people.stream()
          .filter(new Predicate<Person>() {
              @Override
              public boolean test(Person p) {
                  return p.getAge() > 21;
              }
          })
          .map(new Function<Person,String>() {
              @Override
              public String apply(Person p) {
                  return p.getName();
              }
          })
          .sorted(new Comparator<String>() {
              @Override
              public int compare(String n1, String n2) {
                  return n1.compareToIgnoreCase(n2);
              }
          })
          .collect(Collectors.toList());

This is much harder to write than the version with lambda expressions, and it's much more error prone. It's also harder to understand.

And this is a relatively short pipeline.

To make this readable without lambda expressions and method references, you would have had to define variables that hold the various functional interface instances being used here, which would have split the logic of the pipeline, making it harder to understand.

Internal iteration

When iterating Java Collections, most developers tend to get an element and then process it. This is, take that item out and then use it, or reinsert it, etc. With pre-8 versions of Java, you can implement an inner class and do something like:

numbers.forEach(new Consumer<Integer>() {
    public void accept(Integer value) {
        System.out.println(value);
    }
});

Now with Java 8 you can do better and less verbose with:

numbers.forEach((Integer value) -> System.out.println(value));

or better

numbers.forEach(System.out::println);

Behaviors as arguments

Guess the following case:

public int sumAllEven(List<Integer> numbers) {
    int total = 0;

    for (int number : numbers) {
        if (number % 2 == 0) {
            total += number;
        }
    } 
    return total;
}

With Java 8 Predicate interface you can do better like so:

public int sumAll(List<Integer> numbers, Predicate<Integer> p) {
    int total = 0;

    for (int number : numbers) {
        if (p.test(number)) {
            total += number;
        }
    }
    return total;
}

Calling it like:

sumAll(numbers, n -> n % 2 == 0);

Source: DZone - Why We Need Lambda Expressions in Java

There are many benefits of using lambdas instead of inner class following as below:

• Make the code more compactly and expressive without introducing more language syntax semantics. you already gave an example in your question.

• By using lambdas you are happy to programming with functional-style operations on streams of elements, such as map-reduce transformations on collections. see java.util.function & java.util.stream packages documentation.

• There is no physical classes file generated for lambdas by compiler. Thus, it makes your delivered applications smaller. How Memory assigns to lambda?

• The compiler will optimize lambda creation if the lambda doesn't access variables out of its scope, which means the lambda instance only create once by the JVM. for more details you can see @Holger's answer of the question Is method reference caching a good idea in Java 8? .

• Lambdas can implements multi marker interfaces besides the functional interface, but the anonymous inner classes can't implements more interfaces, for example:

    //                 v--- create the lambda locally.
    Consumer<Integer> action = (Consumer<Integer> & Serializable) it -> {/*TODO*/};
    
  

Lambdas are just syntactic sugar for anonymous classes.

Before lambdas, anonymous classes can be used to achieve the same thing. Every lambda expression can be converted to an anonymous class.

If you are using IntelliJ IDEA, it can do the conversion for you:

• Put the cursor in the lambda
• Press alt/option + enter

To answer your question, the matter of fact is lambdas don’t let you do anything that you couldn’t do prior to java-8, rather it enables you to write more concise code. The benefits of this, is that your code will be clearer and more flexible.

One thing I don't see mentioned yet is that a lambda lets you define functionality where it's used.

So if you have some simple selection function you don't need to put it in a separate place with a bunch of boilerplate, you just write a lambda that's concise and locally relevant.

Yes many advantages are there.

• No need to define whole class we can pass implementation of function it self as reference.
• Internally creation of class will create .class file while if you use lambda then class creation is avoided by compiler because in lambda you are passing function implementation instead of class.
• Code re-usability is higher then before
• And as you said code is shorter then normal implementation.

Function composition and higher order functions.

Lambda functions can be used as building blocks towards building "higher order functions" or performing "function composition". Lambda functions can be seen as reusable building blocks in this sense.

Example of Higher Order Function via lambda:

Function<IntUnaryOperator, IntUnaryOperator> twice = f -> f.andThen(f);
IntUnaryOperator plusThree = i -> i + 3;
var g = twice.apply(plusThree);
System.out.println(g.applyAsInt(7))

Example Function Composition

Predicate<String> startsWithA = (text) -> text.startsWith("A");
Predicate<String> endsWithX   = (text) -> text.endsWith("x");

Predicate<String> startsWithAAndEndsWithX =
        (text) -> startsWithA.test(text) && endsWithX.test(text);

String  input  = "A hardworking person must relax";
boolean result = startsWithAAndEndsWithX.test(input);
System.out.println(result);

One benefit not yet mentioned is my favorite: lambdas make deferred execution really easy to write.

Log4j2 uses this for example, where instead of passing a value to conditionally log (a value that may have been expensive to calculate), you can now pass a lambda to calculate that expensive value. The difference being that before, that value was being calculated every time whether it got used or not, whereas now with lambdas if your log level decides not to log that statement, then the lambda never gets called, and that expensive calculation never takes place -- a performance boost!

Could that be done without lambdas? Yes, by surrounding each log statement with if() checks, or using verbose anonymous class syntax, but at the cost of horrible code noise.

Similar examples abound. Lambdas are like having your cake and eating it too: all the efficiency of gnarly multi-line optimized code squeezed down into the visual elegance of one-liners.

Edit: As requested by commenter, an example:

Old way, where expensiveCalculation() always gets called regardless of whether this log statement will actually use it:

logger.trace("expensive value was {}", expensiveCalculation());

New lambda efficient way, where expensiveCalculation() call won't happen unless trace log level is enabled:

logger.trace("expensive value was {}", () -> expensiveCalculation());