Inheritance: Interfaces

Interfaces

Dr Andy Evans

Interfaces

Interfaces are the ultimate abstract class.
They are lists of methods that must be defined in classes that implement them. All variables in them are final.
Why the devil would you want to do that...?
You can make classes that rely on finding certain methods wherever they are used. By forcing people to implement these methods you can guarantee they exist.

	
BlueBankRequest  b1 = 
		new BlueBankRequest();
b1.setPin (getAtmPin());
boolean pinOk = 
    redBankConnection.isPinOk(b1);

	
boolean isPinOk (GenericRequest qr) {
    if (qr.getPin() == customerPin) {
		return true;
    } else { 
		return false;
    }
}

Here then, is our banking example. Let's imagine, now, that GenericRequest is an interface, not a class. What would it look like?

Interfaces

	
public interface GenericRequest {

    public int getPin ();

}

	
public class BlueBankRequest 
				    implements GenericRequest  {
    private int pin = 0;

    public void setPin (int p) { 
		pin = p; 
    }

    public int getPin () {
		return pin;
    }
}

Interfaces are *so* important, that they have their own keywords to define and inherit them.

	
BlueBankRequest  b1 = 
		new BlueBankRequest();
b1.setPin (getAtmPin());
boolean pinOk = 
    redBankConnection.isPinOk(b1);

	
boolean isPinOk (GenericRequest qr) {
    if (qr.getPin() == customerPin) {
		return true;
    } else { 
		return false;
    }
}

In short, the process is identical.

Inheritance

We can extend a Class to give us all its functionality.
```
	
class A extends B {
```
We can implement an Interface, which means we promise to supply its methods.
```
class A implements C {
```
We can only extend one Class, but we can implement many Interfaces using commas.
```
	
class A extends B implements C, D, E {
```

interface variables

Interfaces classically also contain final static variables of use with the interface:

	
interface GenericRequest {

    final static String STANDARDS_AGENCY = 
				"Bank Standards Inc.";
    int getPin ();

}

	
BlueBankRequest  b1 = new BlueBankRequest();
b1.setPin (1234);
boolean ok = redBankConnection.isPinOk(b1);
System.out.printToATM
    ("Your connection is brought to you by " +
	GenericRequest.STANDARDS_AGENCY);