Functional Interfaces - Java pre-defined functional interfaces

What are some standard Java pre-defined functional interfaces?

Some of the famous pre-defined functional interfaces from previous Java versions are Runnable, Callable, Comparator, and Comparable. While Java 8 introduces functional interfaces like Supplier, Consumer, Predicate, etc. Please refer to the java.util.function doc for other predefined functional interfaces and its description introduced in Java 8.

Runnable: use to execute the instances of a class over another thread with no arguments and no return value.

Callable: use to execute the instances of a class over another thread with no arguments and it either returns a value or throws an exception.

Comparator: use to sort different objects in a user-defined order

Comparable: use to sort objects in the natural sort order

See Comparator interface vs Comparable interface

What are the different functional interfaces present in Java 8 - built in interfaces in Java 8

Look at the interfaces from java.util.function package:

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/function/package-summary.html

The interfaces in this package are general purpose functional interfaces used by the JDK, and are available to be used by developers as well. While they do not identify a complete set of function shapes to which lambda expressions might be adapted, they provide enough to cover common requirements. Other functional interfaces provided for specific purposes, such as FileFilter, are defined in the packages where they are used.

The interfaces in this package are annotated with FunctionalInterface. This annotation is not a requirement for the compiler to recognize an interface as a functional interface, but merely an aid to capture design intent and enlist the help of the compiler in identifying accidental violations of design intent.

Functional interfaces often represent abstract concepts like functions, actions, or predicates. In documenting functional interfaces, or referring to variables typed as functional interfaces, it is common to refer directly to those abstract concepts, for example using “this function” instead of “the function represented by this object”. When an API method is said to accept or return a functional interface in this manner, such as “applies the provided function to…”, this is understood to mean a non-null reference to an object implementing the appropriate functional interface, unless potential nullity is explicitly specified.

Lambda expression is a type of function without a name. It may or may not have results and parameters. It is known as an anonymous function as it does not have type information by itself. It is executed on-demand. It is beneficial in iterating, filtering, and extracting data from a collection.

What are the various categories of pre-defined functional interfaces?

Function: To transform arguments in returnable value.

Predicate: To perform a test and return a Boolean value.

Consumer: Accept arguments but do not return any values.

Supplier: Do not accept any arguments but return a value.

Operator: Perform a reduction type operation that accepts the same input types.