Both those lines try to call the displayGreeting() method on an object of class s (by the way, don't use lower case letters for class names - especially single letters! - as it makes it harder to read). But that class doesn't gave that method.
Your displayGreeting() method is accessible from anywhere. But it belongs to the Outer class. Which means you can only call it on an instance of an Outer object. You can't then just call it on any old object.
In object-oriented programming you'll often see interactions between objects described as "sending messages". One object sends a message to another object, which replies. The messages you can send to an object correspond to the methods that object has.
In this case, you're trying to send a message to an object that doesn't understand it. You can tell a dog to bark, you can't tell a daffodil to!
Do you want ants? Because that's how you get ants. And a tiny ads: