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What is a functional interface in Java 8?
A functional interface is an interface whose relevant abstract methods amount to one function contract. The Java Language Specification describes it as an interface with “just one abstract method (aside from the methods of Object), and thus represents a single function contract.” (Java Language Specification, §9.8.)
The phrase SAM—single abstract method—is a useful shorthand, but it can mislead if taken to mean that every abstract declaration must be counted separately. Inherited declarations with override-equivalent signatures can represent the same logical method. Public instance methods of Object are excluded from the count, and the relevant abstract methods must have compatible signatures and return types. (Java Language Specification, §9.8.)
Default methods have implementations, so they are not abstract methods for this purpose. An interface can therefore have default methods and still be functional, provided it has one logical abstract-method contract. (Java SE 8 FunctionalInterface API.)
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What does @FunctionalInterface do?
@FunctionalInterface is optional. It documents that the interface is intended to have a single function contract, and asks the compiler to check that it qualifies. If an annotated interface does not meet the functional-interface rules, compilation fails. An interface that meets the rules remains functional even without the annotation. (Java SE 8 FunctionalInterface API.)
Use the annotation on custom interfaces when you want the intent to be clear and accidental changes to the contract to be caught at compile time:
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@FunctionalInterface
interface Formatter {
String format(String value);
}How lambdas and method references use a functional interface
A lambda expression is checked against a target functional-interface type. Its parameters and result must be compatible with that interface’s function contract. Java can use that target in an assignment, method invocation, or cast context when the lambda is congruent with the target function type. Method references can likewise create instances of functional-interface types. (Java Language Specification, §15.27.3; Java SE 8 FunctionalInterface API.)
Predicate<String> hasText = text -> !text.isEmpty();
Predicate<String> methodReference = String::isEmpty;In the first assignment, Predicate<String> supplies the target type: Java checks a String input and a boolean result. The second expression is also checked against that target contract; the method reference’s shape must be compatible. A lambda does not stand alone as an independently typed value in these examples—the target type tells the compiler which contract to check.
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Java 8 functional-interface examples
Java 8’s standard library includes functional interfaces for common parameter and return shapes. Choose one whose contract matches the operation.
| Interface | Inputs | Result | Abstract method | Typical role |
|---|---|---|---|---|
Predicate<T> |
One value of type T |
boolean |
test |
Test a condition. (Java SE 8 API.) |
Consumer<T> |
One value of type T |
No result | accept |
Perform an operation on a value; consumers are expected to operate through side effects. (Java SE 8 API.) |
BiFunction<T,U,R> |
Two values, of types T and U |
A value of type R |
apply |
Combine or transform two inputs into a result. (Java SE 8 API.) |
Runnable |
None | No result | run |
Represent code intended to be executed by a thread. (Java SE 8 API.) |
For example, a predicate asks a yes-or-no question; a consumer receives a value and performs an operation without returning one; a bifunction receives two values and produces a result; and a runnable takes no arguments and returns nothing.
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When should you define a custom functional interface?
Use a standard interface when its input and result shape expresses your operation clearly. Define a custom one when the domain needs a more meaningful contract or the standard shapes do not fit. A custom interface is functional if it satisfies the same Java language rules; adding @FunctionalInterface makes that intention explicit and compiler-checked.
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