Java 8 introduced a new functional feature which is the Lambda expression. Read on to learn more about what it is and how it works.

JAVA LAMBDA EXPRESSIONS (Java 8)

• Is a function which can be created without belonging to any class
• Can be passed around as if it was an object and executed on demand

Matching Lambdas to Interfaces

• A lambda expression can be used where the type they are matched against is a single method interface (also known as functional interface)
• Conditions for matching a Java lambda expression:
  • Single method interface
  • Parameter match
  • Return type match

Lambda Type Inference

• The type of the lambda expressions can be inferred from the surrounding code.
  • E.g. From the method declaration right before the lambda expression
  • E.g. From the parameter types
• Sometimes, we’ll need to specify the parameter types for a lambda expression if the compiler cannot infer the parameter types from the functional interface method.
  • E.g. (Car car) -> {};

Lambdas as Objects

• A Java lambda expression is essentially an object.
• It can be assigned to a variable and passed around, like with any other object.

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Another similar feature that Java offers that can get confusing with the Java Lambda expressions is the Java anonymous inner class. Below is a quick recap of what it is.

JAVA ANONYMOUS INNER CLASS (Java 7)

• Anonymous inner class: is a class that has no name.
  • Used if you have to override method of class or interface.
  • Used if you need to use a local class only once

Accessing local variables of the enclosing scope:

• Anonymous classes have the same access to local variables of the enclosing scope:

  • They have access to members of its enclosing class
  • Cannot access local variables in its enclosing scope that are not declared as final or effectively final
  • Declaration of a type in an anonymous class shadows any other declarations in the enclosing scope

Same restrictions as local classes:

• Cannot declare static initializers or member interfaces in an anonymous class
• Can have static members, provided they are constant variables

Can declare the following fields in an anonymous class:

• Fields
• Extra methods
• Instance initilizers
• Local classes

Cannot declare constructors in an anonymous classs

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GENERAL RULES:

• Lambda functions can only be used in cases of single method interfaces. If you need to have classes/methods with more than one method, you need to use anonymous functions.
• The body of a lambda expression does not define a new scope -> its scope is the same as the enclosing scope
• If you find yourself using lambda functions in multiple places, consider creating a class