FunctionalInterface
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AI-generated Key Takeaways
@FunctionalInterface annotation indicates that an interface is designed to be a functional interface, meaning it has a single abstract method.
Default methods and methods overriding java.lang.Object's public methods don't count towards the abstract method count for a functional interface.
Compilers enforce the functional interface requirements when @FunctionalInterface is present, generating an error if the annotated type doesn't meet the criteria.
Functional interfaces can be instantiated using lambda expressions, method references, or constructor references.
Even without the @FunctionalInterface annotation, the compiler recognizes and treats any interface meeting the functional interface definition as such.
public abstract @interface
FunctionalInterface
implements
Annotation
An informative annotation type used to indicate that an interface
type declaration is intended to be a functional interface as
defined by the Java Language Specification.
Conceptually, a functional interface has exactly one abstract
method. Since {@linkplain java.lang.reflect.Method#isDefault()
default methods} have an implementation, they are not abstract. If
an interface declares an abstract method overriding one of the
public methods of java.lang.Object, that also does
not count toward the interface's abstract method count
since any implementation of the interface will have an
implementation from java.lang.Object or elsewhere.
Note that instances of functional interfaces can be created with
lambda expressions, method references, or constructor references.
If a type is annotated with this annotation type, compilers are
required to generate an error message unless:
The type is an interface type and not an annotation type, enum, or class.
The annotated type satisfies the requirements of a functional interface.
However, the compiler will treat any interface meeting the
definition of a functional interface as a functional interface
regardless of whether or not a FunctionalInterface
annotation is present on the interface declaration.
Returns the hash code of this annotation, as defined below:
The hash code of an annotation is the sum of the hash codes
of its members (including those with default values), as defined
below:
The hash code of an annotation member is (127 times the hash code
of the member-name as computed by String.hashCode()) XOR
the hash code of the member-value, as defined below:
The hash code of a member-value depends on its type:
The hash code of a primitive value v is equal to
WrapperType.valueOf(v).hashCode(), where
WrapperType is the wrapper type corresponding
to the primitive type of v (Byte,
Character, Double, Float, Integer,
Long, Short, or Boolean).
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2024-07-10 UTC."],[],["The `@FunctionalInterface` annotation indicates an interface intended as a functional interface, having exactly one abstract method. Default methods and overridden `java.lang.Object` methods are not counted. Instances can be created via lambdas, method references, or constructor references. Compilers must flag errors if an annotated type is not an interface or doesn't meet the functional interface definition. Any interface meeting these criteria is a functional interface, regardless of the annotation's presence. The annotation extends the Annotation Interface and inherits method like annotationType(), equals(), hashCode(), and toString().\n"]]
