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");
});