Lambda Expressions versus Anonymous Classes
Implementation
A lambda expression can only be used to provide implementation of exactly one functional interface. It represents an anonymous function. Unlike an object, it has only behavior and no state.
An anonymous class is restricted to either implementing one interface or extending one class, but it is not restricted to implementing only one abstract method from its supertype.
No separate class file with Java bytecode is created for a lambda expression, in contrast to a separate class file for each anonymous class declared in a source file.
Scope
Lambda expressions do not introduce a new naming scope, and follow the lexical scope rules for nested blocks. Names in a lambda expression are resolved lexically in its enclosing block and enclosing class.
An anonymous class introduces a new naming scope, where names are resolved according to the inheritance hierarchy of the anonymous class, its local enclosing block, and its enclosing class.
Accessing Inherited Members from the Inheritance Hierarchy
Members in the functional interface implemented by a lambda expression are not accessible in the lambda body.
Since an anonymous class can declare and inherit members from its supertype, instances of anonymous classes can have state. The this and super references can be used to access members in the current instance of the anonymous class and its superclass object, respectively.
Accessing Local Declarations in the Enclosing Block
An anonymous class and a lambda expression can only access effectively final local variables in their enclosing local context.
A local variable in a lambda expression cannot shadow a local variable with the same name in the local context because local variables cannot be redeclared. A field name in the anonymous class can shadow a local variable with the same name in the local context.
Accessing Members in the Enclosing Class
A local variable in a lambda expression and a member (either inherited or declared) in an anonymous class can hide a member with the same name in the enclosing class.
A lambda expression and an anonymous class can access any non-hidden members in the enclosing class by their simple names.
In a non-static context, a lambda expression and an anonymous class can access any hidden members in the enclosing class by the this reference and the qualified this reference, respectively.
In a static context, a lambda expression and an anonymous class can access any hidden members in the enclosing class by their qualified names.
Best Practices
Defining an anonymous class can be verbose. Even an implementation of a single method requires a lot of boilerplate code to encapsulate the method in a class definition, with the added risk of making the code hard to read and understand.
The obvious choice for implementing functional interfaces is lambda expressions. Anything beyond that, and there is little choice but to bring in the anonymous classes.