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