Given: ``java interface P { public void method1(); } interface Q extends P { public void method1(); } interface R extends P { public void method2(); } interface S { public default void method1() { }…

Question

interface P { public void method1(); }
interface Q extends P { public void method1(); }
interface R extends P { public void method2(); }
interface S { public default void method1() { } }
interface T { public void method1(); public void method2(); }
interface U { public void method1(); public abstract void method2(); }

Options

• AT
• BR
• CP
• DS
• EQ
• FU

• A
  71% (32)
• B
  4% (2)
• C
  2% (1)
• D
  7% (3)
• E
  16% (7)

Explanation

There appears to be an error in the provided answer key. Based on Java's definition of a functional interface, the actual correct answers are C (P) and E (Q), not A and F.

Why C (P) and E (Q) are correct: A lambda expression requires a functional interface - one with exactly one abstract method (SAM: Single Abstract Method). P declares a single abstract method (method1), making it a valid SAM type. Q extends P and redeclares method1, which doesn't add a second abstract method - it simply overrides the same one, so Q also has exactly one abstract method and qualifies.

Why the others are wrong:

• R inherits method1 from P and adds method2, giving it two abstract methods - not a functional interface.
• S has zero abstract methods (method1 is default), so there's nothing for the lambda to implement.
• T explicitly declares method1 and method2 - two abstract methods, disqualifying it.
• U is identical in effect to T: public void and public abstract void mean the same thing in interfaces - still two abstract methods.

Memory tip: Think "SAM = lambda." Count abstract methods only - default and static methods don't count, and re-declaring an inherited method doesn't add a new one. If the count isn't exactly 1, the interface can't be a lambda target.

Note: If this is from an official source, double-check the answer key - T and U both have two abstract methods and are definitively not functional interfaces under the Java Language Specification.

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