Encapsulation

One of the most important features of Object-Oriented Programming is encapsulation.

encapsulation

Hiding internal state and requiring all interaction to be performed through an object's methods.¹

Encapsulation insulates changes from propagating throughout your program.

• This is good because if a private part is changed, that change is constrained to the class in which it was defined.

To increase encapsulation, you should:

• Give fields and methods the most restricted access possible.

• Make all fields private and provide non-private methods to access and modify (or mutate) them.

mutate

A fancy word for change. A method that sets a private field could be called a mutator. Similarly, a method that retrieves the value of a private field could be called an accessor. Or we could just call them a setter and a getter.

The goal of encapsulation is to keep users from changing data you don't want them to change, or changing it in a way we don't want them to.

Practice Exercise

Allowing other code to access fields directly could create an unwanted dependency. This means other code expects our code to work a certain way.

If we change a field's visibility later, we risk breaking other code that depends on our field being visible.

Practice good encapsulation from the start.

Drill

Encapsulation/com.example.encapsulation.drills.BankApp

Encapsulation/com.example.encapsulation.drills.Account

You have already made the accountId and balance fields private in Account. * Add four public methods to Account. * getAccountId() to allow users to retrieve the accountId. * setAccountId(String id) to allow users to set the accountId field by passing a new value. * A get- and set- method for the balance. * Call the methods from BankApp.

(Solution:BankApp2.java, Account2.java)

Drill

Encapsulation/com.example.encapsulation.drills.Student

public class Student{
 
 int[] scores;
 private double average;
 
 public int[] getScores(){ return scores; }
 
 public double getAverage(){ return average; }
 
 private void computeAverage(){
     // valid code to compute average
     // average = update average value
 }

 public Student(){
     computeAverage();
 }
}

What two things can we do to improve the encapsulation of this class (choose two)? * [ ] Make the class private. * [ ] Make the scores instance field private. * [ ] Make getScores() protected. * [ ] Make computeAverage() public. * [ ] Change getScores to return a copy of the scores list:

public int[] getScores() {
  int[] sCopy = new int[scores.length];
  for (int i=0; i < scores.length; i++) {
    sCopy[i] = scores[i];
  }
  return sCopy;
}

(Solution: Student.java)

[1] https://docs.oracle.com/javase/tutorial/java/concepts/object.html

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