Class without Encapsulation

Question

I'm having a bit of trouble understanding encapsulation in Java. What I know is that encapsulation allows for information hiding (using private variables) from anywhere else in the program. But, could anyone show me how creating a class with public data fields could cause problems? I'm not understanding how the data is hidden if the classes have getter and setter methods provided.

   public class Student{

       private int id;
       private String name;
       private int grade;

  public Student(){
  }

  public Student(int id; String name, int grade){
      this.id = id;
      this.name = name;
      this.grade = grade;
  }

  public int getId(){
       return id;
  }

  public void setId(int id){
       this.id = id;
  }

  //more code

  }

Answer 1

Here's an example where a public field can make its class completely unreliable. An alternative private version can guarantee that its information is always valid, and is untouchable by undesirable actors.

Public field class (insecure):

class VeryImportantCounterPublic  {
   public int currentCount;
   public VeryImportantCounterPublic(int initial_count)  {
      currentCount = initial_count;
   }
   public void incrementCount()  {
      currentCount++;
   }
   public int getCurrentCount()  {
      return  currentCount;
   }
}

Private field class (secure):

class VeryImportantCounterPrivate  {
   private int currentCount;
   public VeryImportantCounterPrivate(int initial_count)  {
      currentCount = initial_count;
   }
   public void incrementCount()  {
      currentCount++;
   }
   public int getCurrentCount()  {
      return  currentCount;
   }
}

Main:

public class CounterTest  {
   public static final void main(String[] ignored)  {
      VeryImportantCounterPublic counter = new VeryImportantCounterPublic(3);
      counter.incrementCount();
      counter.incrementCount();
      counter.incrementCount();

      //Should not be able to do this!!
      //But it's a public field, so you can  :(
      counter.currentCount = -203847382;

      System.out.println("[PUBLIC] Current count is " + counter.getCurrentCount());


      VeryImportantCounterPrivate counter2 = new VeryImportantCounterPrivate(3);
      counter2.incrementCount();
      counter2.incrementCount();
      counter2.incrementCount();

      //Can't do this. Compiler error:
      //counter2.currentCount = -203847382;

      System.out.println("[PRIVATE] Current count is " + counter2.getCurrentCount());
   }
}

Output:

[PUBLIC] Current count is -203847382
[PRIVATE] Current count is 6

---

Full source:

public class CounterTest  {
   public static final void main(String[] ignored)  {
      VeryImportantCounterPublic counter = new VeryImportantCounterPublic(3);
      counter.incrementCount();
      counter.incrementCount();
      counter.incrementCount();
      counter.currentCount = -203847382;
      System.out.println("[PUBLIC] Current count is " + counter.getCurrentCount());


      VeryImportantCounterPrivate counter2 = new VeryImportantCounterPrivate(3);
      counter2.incrementCount();
      counter2.incrementCount();
      counter2.incrementCount();
      //Can't do this. Compiler error:
      //counter2.currentCount = -203847382;
      System.out.println("[PRIVATE] Current count is " + counter2.getCurrentCount());
   }
}


class VeryImportantCounterPublic  {
   public int currentCount;
   public VeryImportantCounterPublic(int initial_count)  {
      currentCount = initial_count;
   }
   public void incrementCount()  {
      currentCount++;
   }
   public int getCurrentCount()  {
      return  currentCount;
   }
}

class VeryImportantCounterPrivate  {
   private int currentCount;
   public VeryImportantCounterPrivate(int initial_count)  {
      currentCount = initial_count;
   }
   public void incrementCount()  {
      currentCount++;
   }
   public int getCurrentCount()  {
      return  currentCount;
   }
}