Functional interfaces, lambda syntax, method references, Optional, and the Stream API with map, filter, reduce and collectors.
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- What is a functional interface in Java?
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An interface with exactly one abstract method, used as the target type for lambda expressions and method references.- What does the @FunctionalInterface annotation do?
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It marks an interface as functional and causes a compile error if the interface violates the single abstract method rule.- What is the Single Abstract Method (SAM) rule?
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A functional interface must have exactly one abstract method; default and static methods do not count.- Can a functional interface have default or static methods?
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Yes, it can have any number of default and static methods; only the count of abstract methods matters.- What does the Function<T,R> functional interface represent?
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A function that takes a parameter of type T and returns a result of type R.- What is the basic syntax of a lambda expression?
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(parameters) -> body, where parameters are the inputs and body is the expression or statement block.- How do you write a lambda with zero parameters?
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() -> expression or () -> { statements; }, using empty parentheses.- How do you write a lambda with a single parameter without explicit type?
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x -> expression, omitting the parentheses when there is exactly one parameter and no type is declared.- How do you write a lambda with multiple parameters?
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(x, y) -> expression or (x, y) -> { statements; }, with parentheses around multiple parameters.- What is type inference in lambda expressions?
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The compiler deduces parameter types from the context, so you can omit explicit types like (String x) -> x.length().- How do you write a multi-statement lambda?
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(params) -> { statement1; statement2; return result; }, using curly braces and explicit return statement.- What is variable capture in lambdas?
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A lambda can access and use variables from its enclosing scope (captured variables).- Why must captured local variables be final or effectively final?
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To ensure thread safety and prevent unexpected behavior; if a variable could change, the lambda's behavior would be unpredictable.- What are method references?
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A shorthand for lambdas that call a single existing method, using the syntax ClassName::methodName.- What is the syntax for a static method reference?
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ClassName::staticMethodName, for example String::valueOf or Math::abs.
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