如何在Java中使用接口作为类型？

Question

I have created an interface named Leen and created a class X that implements this interface, but when i go to main class to create a variable named x like this

Leen x;
x.printname();

The compiler gives me error:

Connect.java:10: error: variable x might not have been initialized

Answer 1

You can't use an interface without implementation. Implementation can be a concrete class or abstract class which implements your interface. Based on your code, clearly there is no implementation for you interface. I'm also not sure where did you use the variable p . Kindly provide your whole class so that we can help you.

Answer 2

Suppose if u want to access in the variable with out instantiating it u can make the variable static in the class. say App is ur class name and xyz is ur interface.

class App implements xyz{
    public static int x;
    public static void printme(){
        //Your code goes here...
    }

}

Now u can access that from ur mainclass like

App app= null;
app.x= 6;
app.printme();

Answer 3

As mentioned earlier, it's because you declared the variable but never instantiated it, meaning it does not exsist yet, it's just a placeholder for the type defined at the declaration, what you need to do is

Leen x; // declare the variable
x = new implementationClass(); // instantiate the variable

x.PrintName(); // then you functions

Now since the interface is used, all functions on the interface is available, but the code will attempt to use the implementation form whatever class implemented the interface and was intantiated

Why is this smart ? well say you lave a List<Leen> then all elements in the list can use the functions on Leen regardless of what implementation of Leen they were created with