Java 8: Debugging Stream Pipelines

I've found that stream pipelines can be difficult to debug because stack traces involving lambda expressions are quite cryptic. Consider the following contrived example:

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;

public class Test {
  public static void main(final String[] args) {
    final List<String> list = Arrays.asList("foo", null, "bar");
    list.stream()
        .map(Function.identity())
        .filter(x -> true)
        .map(String::length)
        .forEach(System.out::println);
  }
}

You may have already guessed that the code above will throw a NullPointerException when String.length is called on the null element in the list. I've added extra map and filter operations, which do nothing, just to make the example a bit more interesting. In the real world, you will probably have a number of different operations in your stream pipeline.

Running the code, produces the following stack trace:

Exception in thread "main" java.lang.NullPointerException
  at Test$$Lambda$3/455659002.apply(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.Spliterators$ArraySpliterator.forEachRemaining(Unknown Source)
  at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
  at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
  at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
  at java.util.stream.ReferencePipeline.forEach(Unknown Source)
  at Test.main(Test.java:12)

The stack trace shows that a NullPointerException occurred but it doesn't tell you which operation in the pipeline failed. What does Test$$Lambda$3/455659002.apply(Unknown Source) mean and why is there no line number?! Since lambda expressions don't have a name, the compiler makes one up (similar to anonymous classes). In this case, it is Test$$Lambda$3 but that doesn't help us track the bug in our code.

So, what can we do? Let's go old-school and add some logging to our code! We can use peek to print out each element before it is consumed by the next operation in the pipeline.

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;

public class Test {
  public static void main(final String[] args) {
    final List<String> list = Arrays.asList("foo", null, "bar");
    list.stream()
        .peek(x -> System.out.println("Running identity on: " + x))
        .map(Function.identity())
        .peek(x -> System.out.println("Running filter on: " + x))
        .filter(x -> true)
        .peek(x -> System.out.println("Running string length on: " + x))
        .map(String::length)
        .peek(x -> System.out.println("Running print on: " + x))
        .forEach(System.out::println);
  }
}

Running it produces the following output:

Running identity map on: foo
Running filter on: foo
Running string length on: foo
Running print on: 3
3
Running identity map on: null
Running filter on: null
Running string length on: null
Exception in thread "main" java.lang.NullPointerException
  at Test$$Lambda$6/295530567.apply(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$2$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source)
  at java.util.stream.ReferencePipeline$11$1.accept(Unknown Source)
  at java.util.Spliterators$ArraySpliterator.forEachRemaining(Unknown Source)
  at java.util.stream.AbstractPipeline.copyInto(Unknown Source)
  at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(Unknown Source)
  at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(Unknown Source)
  at java.util.stream.AbstractPipeline.evaluate(Unknown Source)
  at java.util.stream.ReferencePipeline.forEach(Unknown Source)
  at Test.main(Test.java:16)

Great! Now we know that the NullPointerException was thrown by the string length lambda!

In general, I think stack traces involving lambdas could be improved in future versions of Java.

Java 8: Converting Anonymous Classes to Lambda Expressions

Refactoring anonymous classes (that implement a single method) to lambda expressions, makes your code more succint and readable. For example, here's an anonymous class for a Runnable and its lambda equivalent:

// using an anonymous class
Runnable r = new Runnable() {
  @Override
  public void run() {
    System.out.println("Hello");
  }
};

// using a lambda expression
Runnable r2 = () -> System.out.println("Hello");

However, it's not always that simple!

Here are a couple of gotchas:

1. Different scoping rules

There are different scoping rules between anonymous classes and lambda expressions. For example, in lambda expressions, this and super are lexically scoped, meaning they are relative to the enclosing class, but in an anonymous class, they are relative to the anonymous class itself. Similarly, local variables declared in lambda expressions will conflict with variables declared in the enclosing class, but in anonymous classes, they are allowed to shadow variables in the enclosing class. Here is an example:

int foo = 1;
Runnable r = new Runnable() {
  @Override
  public void run() {
    // this is ok!
    int foo = 2;
  }
};

Runnable r2 = () -> {
  // compile error: Lambda expression's local variable foo cannot
  // redeclare another local variable defined in an enclosing scope.
  int foo = 2;
};

2. Overloaded methods

If you have an overloaded method, using lambda expressions can result in an ambiguous method call and will require explicit casting. Here is an example:

// Functional interface
interface Task {
  public void execute();
}

// Overloaded methods
public static void go(final Runnable r) {
  r.run();
}
public static void go(final Task t) {
  t.execute();
}

// Calling the overloaded method:

// When using an anonymous class, there is no ambiguity because
// the type of the class is explicit at instantiation
go(new Task() {
  @Override
  public void execute() {
     System.out.println("Hello");
  }
});

// When using a lambda expression, there is a compile error!
// The method go(Runnable) is ambiguous
go(() -> {
  System.out.println("Hello");
});

// This ambiguity can be solved with an explicit cast
go((Task)() -> {
  System.out.println("Hello");
});

Java 8: Sorting a List using Lambdas and Method References

This post shows how you can use Java 8 lambda expressions and method references to sort a list of Person objects by age. In Java 8, the List interface has a sort method, which expects a Comparator to compare two objects.

Traditionally, you would either sort a list by creating a specific class that implements the Comparator interface, like this:

public class AgeComparator implements Comparator<Person> {
  @Override
  public int compare(Person p1, Person p2) {
    return Integer.compare(p1.getAge(), p2.getAge());
  }
}

list.sort(new AgeComparator());

or, you would use an anonymous class, like this:

list.sort(new Comparator<Person>() {
  @Override
  public int compare(Person p1, Person p2) {
    return Integer.compare(p1.getAge(), p2.getAge());
  }
});

As you can see, this is quite verbose!

Java 8 introduces lambda expressions, which allow you to pass code more concisely. Since Comparator is a functional interface, you can use a lambda expression to sort the list:

list.sort((p1, p2) -> Integer.compare(p1.getAge(), p2.getAge()));

You may have noticed that the Comparator class has a static method called comparingInt that takes a ToIntFunction and returns a Comparator object. So, we can rewrite the code above to:

import static java.util.Comparator.comparingInt;
list.sort(comparingInt(p -> p.getAge()));

Finally, we can improve our code even further by using a method reference, which is just "syntactic sugar" for a lambda expression:

list.sort(comparingInt(Person::getAge));

The final solution is not only shorter but is also easier to read :)