[NETBEANS-3802] Hint suggests invalid replacement with lambda for implementations of interfaces with only default methods

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 11.2, 12.5
• Component/s: java - Hints
• Labels: hints

Environment

JDK 13

• Assignee: Unassigned
• Reporter: dennis lucero
• Created: 06/Feb/20 16:29
• Updated: 28/Nov/21 22:27

Description

    interface Dog {
        default void run() {
        }
        default void bark() {
        }
    }

    public static void foo() {
        final Dog dog = new Dog() {
            @Override
            public void bark() {
                System.out.println("Woof");
            }
        };
    }

For this code NetBeans has a hint stating

This anonymous inner class creation can be turned into a lambda expression.

in the line with new Dog(). Replacing the code using the hint results in

final Dog dog = () -> {
    System.out.println("Woof");
};

which is invalid code

incompatible types: Dog is not a functional interface
no abstract method found in interface Dog

(Real world example: http://jdbi.org/apidocs/org/jdbi/v3/core/statement/SqlLogger.html This style of interfaces would also be a nice alternative to Swing’s *Listener/*Adapter pairs.)