Private interface methods
Since Java 8 an interface can contain default methods (with a body that implementers inherit) and static helper methods; since Java 9 it can also have private methods shared by those bodies. Default methods let old interfaces grow without breaking existing classes.
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Practice questions
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Create interface Greeter { String name(); default String greet() { ... } } that returns "Hello, <name>!". Implement it in English and in Hindi, where Hindi overrides greet() to return "Namaste, <name>!".
Create two interfaces Walker and Swimmer, each with a default move(). Write Frog implements Walker, Swimmer whose move() returns both results joined by " and ".
Write an interface Temperature with abstract double celsius(), a default fahrenheit(), and a static factory Temperature of(double c) that returns an object of a small implementing class in the same file.
Explain it without notes
Why were default methods added in Java 8?
State the three rules for resolving a method inherited from several places.
How do static interface methods differ from static class methods?
What problem do private interface methods solve?
Why can't a default method override toString()?
Expected output
[SALES Q3]
[# Sales Q3]
Sales Q3 (12 pages)
Office closed Friday (1 page)