Adjective interface: Way to do callbacks. Include performance of these kinds of services in my contract.

How to use interfaces

• Guideline: Use interfaces to say "What objects can do" or "What can be done to an object."
• interfaces don't say "What objects are."
• interfaces say what objects can do no matter what objects are

Some interface Examples

• Comparable means: an object can compare itself with another object of the same type if I invoke compareTo() on the object

  public interface Comparable {
  
      int compareTo(Object o);
  }
  

• Cloneable means: I can clone an object (meta-semantics)

  public interface Cloneable {
  }
  

• Comparable also means: I can sort an array or collection with Arrays.sort() or Collections.sort() (meta-semantics)

Pulling interfaces out of Thin Air

• So interfaces are great. They separate implementation from class signatures, allowing us more freedom to change our implementation. So should I use an interface to wrap every class whose implementation might change in the future? Depending on your level of paranoia about implementation changes, this could include almost every class of a program.
• Don't look at classes and try to figure out what method subsets should form interfaces

  Sum (k=0 to n) n! / (n - k)! k!, where n = num methods

• Create interfaces to accept as parameters to, or provide as return types of, your class's methods
• interface parameter types: your class/interface more versatile
• interface return types: your class/interface easier to evolve, subclass
• Also, can discover interfaces as concepts independent from any classes, but which don't serve as parameter or return value types: e.g., UI factory interfaces.

Adding a Notification Service to OverdraftAccount

• interfaces arise when you define the semantic contract of a class (define how the class will be used)
• What if another part of your program needs to know when an OverdraftAccount gets overdrawn (or paid back)?

 1 // In file ex3/OverdraftAccount.java
 2
 3 public class OverdraftAccount extends Account {
 4
       //...
74
75     public void addOverdraftListener(OverdraftListener l) {
76         //...
77     }
78
79     public void removeOverdraftListener(
80         OverdraftListener l) {
81         //...
82     }
83
84 }

 1 // In file ex3/OverdraftEvent.java
 2
 3 public class OverdraftEvent
 4     extends java.util.EventObject {
 5
 6     private long overdraft;
 7
 8     // overdraft is the current value of overdraft
 9     // in US pennies. overdraft will be either some
10     // positive number or zero. If zero, it indicates
11     // that the overdraft has just been paid back.
12     // If positive, it means that the account was just
13     // overdrawn. (If $50 has been loaned to the
14     // account holder, overdraft will be 5000)
15     public OverdraftEvent(Object source, long overdraft) {
16
17         super(source);
18         this.overdraft = overdraft;
19     }
20
21     public long getOverdraft() {
22         return overdraft;
23     }
24
25 }

 1 // In file ex3/OverdraftListener.java
 2
 3 public interface OverdraftListener
 4     extends java.util.EventListener {
 5
 6     void overdraftOccurred(OverdraftEvent e);
 7
 8     void overdraftRepaid(OverdraftEvent e);
 9
10 }