Object-Orientation - Interfaces (in Java)

# Interfaces

> As far as the customer is concerned, the interface is the product.
>
> –[Jef Raskin](https://en.wikipedia.org/wiki/Jef_Raskin), initial creator of the Apple Macintosh

---

# Agenda

1. [Overview](#concept)
1. [Raison D'Etre](#raison-detre)
1. [Example](#example)
1. [Differences From Classes](#differences)
1. [Methods With Implementations](#implemented-methods)
1. [Implementing Multiple Interfaces](#multiples)
1. [Polymorphism](#Polymorphism)
1. [Inheritance](#inheritance)
1. [Conclusions](#conclusions)

---

# Overview

## Concept

An **interface**, like a class, is a Java reference type.

```java
public interface Foo {

/* some stuff here */

}
```

---

## Similarity to classes

Like classes, interfaces can contain...

- properties

- methods

```java
public interface Foo {

// a property
  public int x = 10;

// a method
  public static void y() {
    System.out.println("Hello!");
  }

}
```

---

## False advertising

But all is not as it seems.

- Interfaces are intended to _solve a very different problem_ than classes.

- Because of their intended use, there are very specific _limitations_ on what kinds of properties and methods can be placed within an interface.

---

# Raison d'être

## Concept

Interfaces serve as a sort of contract.

- An interfaces defines a **public interface** - a set of behaviors - that any class can agree to adhere to.

- Java will _not compile_ the code if any class that agreed to adhere to an interface does not properly implement its public interface.

---

## Contractual agreement

At their core, interfaces specify a set of **method signatures**.

- Any class can declare that it _implements_ an interface, meaning it agrees to implement the behaviors the interface specifies.

- This contractually _guarantees_ that the class will implement the methods specified in the interface.

- Other code can rely on the fact that the class will be able to perform the behaviors the interface specifies.

---

# Example

## A facetious interface

For example, imagine the following interface:

```java
// your mileage may vary
public interface IdealEmployee {
  public abstract void enjoyEatingMealsInCubicle();
  public abstract void rarelyLeaveBuildingOrCampus();
  public abstract void playSillyGamesInOffice();
  public abstract void readilyAcceptAFancyJobTitleInsteadOfIncreasedPay();
}
```

Any class can "sign on" to this interface using the **implements** keyword.

```java
public class RecentGrad implements IdealEmployee {
  /* some stuff here */
}
```

---

## Abiding by the contract

Classes that implement an interface _must_ implement the methods declared within the interface. The code won't compile otherwise.

```java
public class RecentCollegeGrad implements IdealEmployee {
  public void enjoyEatingMealsInCubicle() {
    // implementation goes here
  }
  public void rarelyLeaveBuildingOrCampus() {
    // implementation goes here
  }
  public void playSillyGamesInOffice() {
    // implementation goes here
  }
  public void readilyAcceptAFancyJobTitleInsteadOfIncreasedPay() {
    // implementation goes here
  }
}
```

- The class may contain other properties and methods in addition to those specified in the interface.

---

## Ensuring consistent behavior across classes

The value of an interface is that it can _ensure the same set of behaviors_ across several different classes.

```java
public class OfficePuppy implements IdealEmployee {
  public void enjoyEatingMealsInCubicle() {
    // implementation goes here
  }
  public void rarelyLeaveBuildingOrCampus() {
    // implementation goes here
  }
  public void playSillyGamesInOffice() {
    // implementation goes here
  }
  public void readilyAcceptAFancyJobTitleInsteadOfIncreasedPay() {
    // implementation goes here
  }
}
```

- The expectation would be that the implementation of these behaviors is significantly different among the different classes that implement them.

---

# Differences from classes

## Concept

Unlike classes, interfaces are intended to be **abstract** and not include implementation details of the behaviors they specify.

- all methods are _abstract_ by default, even if that is not written explicitly into the code.

- all properties and methods in an interface are _public_, even if that is not written explicitly into the code.

- all properties are furthermore _static and constant_, even if that is not written explicitly into the code.

---

# Methods With Implementations

## Default methods

It is possible to include an _instance method with an implementation_ in an interface. This is called a **default** method and serves one specific use case, and no other.

- Imagine our example interface is already in use - developers have writen code abiding by its contractual rules.

- Now, imagine the creators of the interface decide to add a new abstract method

```java
public abstract void acceptCutsToCompensationWithNoComplaints();
```

- If the developers using the old interface upgraded to the new version, their _code would immediately break_ and not compile.

- A **default method** solves this problem by including a new method in an interface that includes its own default implementation - a placeholder until the developers can write their own.

---

## Default methods (continued)

For example, version 2 of our interface might add the new method, including a default implementation.

```java
public interface IdealEmployee {
  // assume all the methods specified in v1 of the interface are still placed here

// the new method, with a default implementation so existing class code doesn't break
  public default void acceptCutsToCompensationWithNoComplaints() {
    System.out.println("Thank you so much for giving me this opportunity!");
  }

}
```

- In this way, developers using `v1` of the interface can upgrade to `v2` with no worry about their code breaking.

---

## Static methods

Interfaces can also contain implementations of static methods.

```java
public interface IdealEmployee {
  // assume all the methods previously discussed are still placed here

// a behavior shared by all IdealEmployee objects
  public static void speakDeferrentially(String message) {
    String protocol = "I'm so sorry to disturb you... I know you're very busy... But";
    String output = String.format("%s %s", protocol, message);
  }

}

```

---

# Implementing Multiple Interfaces

## Direct implementation of multiples

Unlike with class-based inheritance, it is possible for a class to implement more than one interface.

- Imagine we had a second interface called `IndependentThinker`.

- A single class could implement both the `IdealEmployee` and `IndependentThinker` interfaces.

```java
public class RecentCollegeGrad implements IdealEmployee, IndependentThinker {

// all abstract methods from IdealEmployee must be implemented in this class

// all abstract methods from IndependentThinker must also be implemented in this class

}
```

---

## Indirect implementation of multiples

Interfaces can also inherit from one-another.

- For example, let's imagine a simple parent interface:

```java
public interface Thinker {
  public abstract void pauseBeforeRespondingToQuestion();
  public abstract void considerAllPossibleImplications(BigEvent bigEvent);
}
```

- And a child interface:

```java
public interface IndependentThinker extends Thinker {
  public abstract void lookAskanceTowardsHorizon();
  public abstract void mumbleToOneself();
}
```

- As with class-based inheritance, a child interface inherits all the methods and properties of the parent, including abstract methods.

---

## Indirect implementation of multiples (continued)

A class that implements an interface has to implement any abstract methods in the interface, including those passed down from ancestor interfaces.

```java
public class OfficePuppy implements IndependentThinker {

// must implement all abstract methods from IndependentThinker interface
  public void lookAskanceTowardsHorizon() {
    // implementation goes here
  }
  public void mumbleToOneself() {
    // implementation goes here
  }

// must also implement all abstract methods from Thinker interface
  public void pauseBeforeRespondingToQuestion() {
    // implementation goes here
  }
  public void considerAllPossibleImplications() {
    // implementation goes here
  }

}
```

---

# Polymorphism

## Concept

Objects that implement a given interface in Java can polymorphically be considered to be of the interface type.

- For example, a `RecentCollegeGrad` object could be referenced by a `IdealEmployee`-typed variable.

```java
// instantiate an object and reference with variable of interface type
IdealEmployee pat = new RecentCollegeGrade("Pat", 21, "Computer Science");
```

---

## Batch operations

As with all polymorphism, this can be useful for batch operations.

- Create an array data structure with a bunch of objects that all implement the interface

```java
IdealEmployee[] employees = {
  new RecentCollegeGrade("Pat", 21, "Computer Science"),
  new OfficePuppy("Fido", "German Shepherd", 4)
};
```

- Loop through them and trigger some behaviors they all have in common.

```java
// iterate through each object
for (IdealEmployee emp : employees) {
  emp.acceptCutsToCompensationWithNoComplaints();
}
```

---

# Conclusions

As you have seen, interfaces in Java offer an alternative form of inheritance in Java with some similarities - and many differences - with class-based inheritance.

- Thank you. Bye.