error while trying to compile an interface

[yotamoo]

hello all
i'm trying to learn about interfaces. my understanding is that an interface forces the class implementing it to call all the method variables defined in it. am i right about the basic idea?

this is a little code i've written just to see if i got it right and i can't compile it for a few reason. i have two .java files: myInterface and myClass:

PHP Code:

// The interface
public interface myInterface {
    public void printWord (String word);
} 


PHP Code:

//Class implementing it 
class myClass implements myInterface {

    public static void main (String[] args) {
    String word="Hello World";
    printWord(word);
    }

    public void printWord(String word) {
    System.out.println(word);
    }
} 


The error i keep getting:

PHP Code:

myClass.java:5: non-static method printWord(java.lang.String) cannot be referenced from a static context
    printWord(word);
    ^
1 error 


so first - how do i make it right.
second - can the interface, class implementing it and the method all be in the same file?

thank you!

[Fou-Lu]

Your implementation is correct, but your usage is wrong.
printWord is not static. You must call it from an instance of myClass:

PHP Code:

public static void main(String[] argv)
{
    String word = "hello world";
    myClass mc = new myClass();
    mc.printWord(word);
} 


An interface is a contract. It guarantees that an Object of type myInterface will have a method called printWord implemented. Its up to the class how to do this, it could very well be nothing more than an exception toss. This is done to emulate multiple inheritance through polymorphism since Java does not natively allow extensions from multiple parents. Interfaces are at least 100x more useful than an extends anyway, so that really doesn't matter.