✅ We can assign greeting::length to Supplier<Integer> because their method signatures match.

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🔗 Let’s Compare the Signatures

Supplier Functional Interface

@FunctionalInterface
public interface Supplier<T> {
    T get();
}

• Method signature: Integer get()

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Method Reference — greeting::length

public int length()

• This method belongs to String, and greeting is a specific String object.
• It takes no arguments and returns int (which gets autoboxed to Integer).

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✅ Matching the Signature

Expected (Supplier<Integer>)	Actual (greeting::length)	Matching?
Integer get()	int length()	✔️ Yes (autoboxed to Integer)

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📣 Key Rule for Method References

Method Reference = Allowed only if it fits the target functional interface’s method signature.

This is why:

Supplier<Integer> bound = greeting::length;  // Works

But this would fail:

Function<String, Integer> func = greeting::length;  // ❌ Doesn't match Function<T, R>

🔥 Pro Tip for Interviews

✅ When asked “Why does this work?”, just say:

“The method reference fits the functional interface’s method signature.”