Unit 28: Immutability

Learning Objectives

Students should

understand when a class is immutable or not.
be able to create an immutable class.

Avoiding Change

So far in this course, we have been focusing on three ways of dealing with software complexity: (i) by encapsulating and hiding the complexity behind abstraction barriers, (ii) by using a language with a strong type system and adhering to the subtyping substitution principle, and (iii) applying the abstraction principles and reusing code written as functions, classes, and generics types.

Another useful strategy to reduce bugs when code complexity increases is to avoid change altogether. This can be done by making our classes immutable. We create an instance of an immutable class, the instance cannot have any visible changes outside its abstraction barrier. This means that every call of the instance's method must behave the same way throughout the lifetime of the instance.

Immutable Class

An immutable class is a class for which there cannot be any visible changes outside of its abstraction barrier.

There are many advantages of why we want to make our class immutable when possible. To start, let's revisit a common bug due to aliasing. Recall the following example from Unit 9, where we create two circles c1 and c2 centered at the origin (0, 0).

Point p = new Point(0, 0);
Circle c1 = new Circle(p, 1);
Circle c2 = new Circle(p, 4);

Let's say that we have the moveTo method in both Circle and Point, to move the circle and point respectively.

class Point {
  private double x;
  private double y;

    :

  public void moveTo(double x, double y) {
    this.x = x;
    this.y = y;
  }
}

class Circle {
  private Point c;
  private double r;

  public Circle (Point c, double r) {
    this.c = c;
    this.r = r;
  }

    :

  public void moveTo(double x, double y) {
    c.moveTo(x, y);
  }
}

Suppose we want to move c1 and only c1 to be centered at (1,1).

c1.moveTo(1, 1);

The line of code above surprisingly moved the center of both c1 and c2, due to both circles c1 and c2 sharing the same point. We have explored a solution below:

Point p1 = new Point(0, 0);
Circle c1 = new Circle(p1, 1);

Point p2 = new Point(0, 0);
Circle c2 = new Circle(p2, 4);

c1.moveTo(1, 1);

This approach avoids sharing references by creating copies of our points so that no two references point to the same instance, avoiding aliasing altogether. This partial fix, however, comes with extra costs in computational resources as the number of objects may proliferate.

This is also not a complete solution because surprisingly, we can move c2 without calling c2.moveTo(1, 1) but by calling the code below.

p2.moveTo(1, 1);

Let's now see how immutability can help us resolve our problem.

Immutable Points and Circles

Immutable Points

Let's start by making our Point class immutable. We start by making the fields final to signal our_ intention_ that we do not intend to assign another value to them. Now that the x and y cannot be re-assigned (a new value or even the same value), to move a point, we shouldn't re-assign to the fields x and y anymore. Instead, we return a new Point instance to prevent mutating the current instance, as follows:

final class Point {
  private final double x;
  private final double y;

  public Point(double x, double y) {
    this.x = x;
    this.y = y;
  }

  public Point moveTo(double x, double y) {
    return new Point(x, y);
  }
    :

  @Override
  public String toString() {
    return "(" + this.x + "," + this.y + ")";
  }
}

Note that, to avoid (likely malicious or ignorant) subclasses of Point overriding the methods to make it appears that the point has mutated, it is necessary that we declare immutable classes as final to disallow inheritance.

Analysis of Points

It is always good to check if the Point class above are really immutable. Assume that the public methods available are:

Point Point::moveTo(double, double)
String Point::toString()
double Point::getX()
double Point::getY()

To see why Point is indeed is immutable, we look at the following code snippet.

Point p = new Point(1.0, 1.0);
p.getX(); // 1.0
  :       // any sequence of invocation of methods in Point`
p.getX(); // 1.0

No methods in Point can actually change the field x or y. First, notice that x and y are double. So the only way they can be changed is by assignment to the fields. But the fields are declared final, so there can be no assignment done on them.

The keyword final on the fields are not necessary because there is no assignment at all to the fields. However, it is still a good practice to have them. Just in case someone in the future try to perform an assignment on the field and change the Point class from immutable to mutable.

Immutable Circle

Now, let's make Circle immutable:

final class Circle {
  private final Point c;
  private final double r;

  public Circle (Point c, double r) {
    this.c = c;
    this.r = r;
  }
    :

  public Circle moveTo(double x, double y) {
    return new Circle(c.moveTo(x, y), this.r);
  }
}

Analysis of Circle

With Point being immutable, we can be sure that the only way for the field c to be changed is by assigning a new value to c. However, this is again prevented by the keyword final in the field c. And again, even without such keyword, there is still no assignment on it. So we can be sure that Circle is also immutable.

In other words, once an instance is created, it remains unchanged (outside the abstraction barrier).

Point p = new Point(0, 0);
Circle c1 = new Circle(p, 1);
Circle c2 = new Circle(p, 4);
c1.moveTo(1, 1); // c1 remains unchanged

To update the variable c1, we need to explicitly reassign it.

c1 = c1.moveTo(1, 1);

Now, c1 moves to a new location, but c2 remains unchanged.

Comparison with Mutable Version

Compare our new immutable approach to the two approaches above. The first shares all the references and is bug-prone. The second creates a new copy of the instance every time and is resource-intensive. Our third approach, using immutable classes, allows us to share all the references until we need to modify the instance, in which case we make a copy. Such a copy-on-write semantic allows us to avoid aliasing bugs without creating excessive copies of objects.

Note that the final keyword prevents assigning new value to the field. Unfortunately, it does not prevent the field from being mutated. So, to ensure that the classes we create are immutable, we have to ensure that the fields are themselves immutable.

Necessity of Final Keyword on Class

We mentioned that it is necessary that we declare immutable classes as final to disallow inheritance. So let us elaborate this further. Consider the immutable Point from above. However, consider the case that the we do not declare the class as final. Now let us create a subclass of it.

class MutablePoint extends Point {
  private double x;
  private double y;

  public MutablePoint(double x, double y) {
    super(x, y);
    this.x = x;
    this.y = y;
  }

  @Override
  public Point moveTo(double x, double y) {
    this.x = x;
    this.y = y;
    return super.moveTo(x, y);
  }
    :

  @Override
  public String toString() {
    return "(" + this.x + "," + this.y + ")";
  }
}

You can see that we kept a duplicate of the internals of Point in MutablePoint, which is a bad practice. But it also shows that if we allow inheritance, we may end up with the following behavior that shows the mutability of Point.

Point p = new MutablePoint(1, 1); // at (1, 1)
p.moveTo(2, 2); // now it is at (2, 2)

Although Point is immutable, its subclass MutablePoint is actually mutable. This also breaks LSP because the subclass breaks the expectation that the superclass is immutable.

Advantages of Being Immutable

We have seen how making our classes immutable helps us remove the risk of potential bugs when we use composition and aliasing. Immutability has other advantages as well.

Ease of Understanding

Code written with immutable objects is easier to reason with and easier to understand. Suppose we create a Circle and assign it to a local variable:

Circle c = new Circle(new Point(0, 0), 8);

We pass c around to many other methods. These other methods may invoke c's methods; we may invoke c's methods locally as well. But, despite putting c through so much, unless we have explicitly re-assigned c, we can guarantee that c is still a circle centered at (0,0) with a radius of 8. This immutable property makes it significantly easier to read, understand, and debug our code.

Without this property, we have to trace through all the methods that we pass c to, and each call of c's methods to make sure that none of these codes modifies c.

Making a class immutable allows us to safely share instances of the class and therefore reducing the need to create multiple copies of the same object. For instance, the origin (0, 0) is commonly used. If the instance is immutable, we can just create and cache a single copy of the origin, and always return this copy when the origin is required.

Let modify our Point class so that it creates a single copy of the origin and returns the same copy every time the origin is required.

final class Point {
  private final double x;
  private final double y;

  private Point(double x, double y) {
    this.x = x;
    this.y = y;
  }

  private final static Point ORIGIN = new Point(0, 0);

  public static Point of(double x, double y) {
    if (x == 0 && y == 0) {
      return ORIGIN;
    }
    return new Point(x, y);
  }
    :
}

We made a few changes in the above:

We made the constructor for Point private so that one cannot call the constructor directly.
We provide a class factory method named of for the client to create a Point instance. The of method returns the same instance ORIGIN every time Point.of(0, 0) is called.

Such a design pattern is only safe when the class is immutable. Consider the mutable version of Point -- calling Point.of(0, 0).moveTo(1, 1) would change every reference to the origin to (1, 1), causing chaos in the code!

Immutable instances can also share their internals freely. Consider an immutable implementation of our Array<T>, called ImmutableArray<T>. Let's start with a simple version first.

// version 0.1
final class ImmutableArray<T> {
  private final T[] array;

  // Only items of type T goes into the array.
  @SafeVarargs
  public static <T> ImmutableArray<T> of(T... items) {
    // We need to copy to ensure that it is truly immutable
    // Explanation will come later
    @SuppressWarnings("unchecked");
    T[] arr = (T[]) new Object[items.length];
    for (int i=0; i<items.length; i++) {
      arr[i] = items[i];
    }
    return new ImmutableArray<>(arr);
  }

  private ImmutableArray(T[] a) {
    this.array = a;
  }

  public T get(int index) {
    return this.array[index];
  }
}

There are a few things to note here.

Varargs. The parameter to the class factory method of has the form T... items. The triple . notation is a Java syntax for a variable number of arguments of the same type (T). Often called varargs, this is just syntactic sugar for passing in an array of items to a method. The method is called variadic method. We can then call of with a variable number of arguments, such as:

ImmutableArray<Integer> a;
a = ImmutableArray.of();
a = ImmutableArray.of(1, 2, 3);
a = ImmutableArray.of(1, 2, 3, 4, 5);

We can also call of with a single array as an argument!

ImmutableArray<Integer> a;
a = ImmutableArray.of(new Integer[] { 1, 2, 3 });

Another Main Function

Now that we have learnt about vargargs, we can write a different version of the main function. In this version, instead of having a parameter of an array of String written as String[], we have a parameter of an array of String written as String....

public static void main(String... args) {
    :
}

@SafeVarargs. Since the varargs is just an array, and array and generics do not mix well in Java, the compiler would throw us an unchecked warning. In this instance, however, we know that our code is safe because we never put anything other than items of type T into the array. We can use the @SafeVarargs annotation to tell the compiler that we know what we are doing and this varargs is safe.

Notice that we removed the set method and there is no other way an external client can modify the array once it is created. This, of course, assumes that we will only be inserting an immutable object into our immutable array. Unfortunately, this cannot be enforced by the compiler as the generic type T can be anything.

Now, suppose that we wish to support a subarray method, that returns a new array containing only a range of elements in the original array. It behaves as follows:

ImmutableArray<Integer> a = ImmutableArray.of(10, 20, 30, 40, 50, 60);
ImmutableArray<Integer> b = a.subarray(2, 4); // b is [30, 40, 50]
b.get(0) // returns 30
ImmutableArray<Integer> c = b.subarray(1, 2); // c is [40, 50]
c.get(1) // returns 50

A typical way to implement subarray is to allocate a new T[] and copy the elements over. This operation can be expensive if our ImmutableArray has millions of elements. But, since our class is immutable and the internal field array is guaranteed not to mutate, we can safely let b and c refer to the same array from a, and only store the starting and ending index.

class ImmutableArray<T> {
  private final int start;
  private final int end;
  private final T[] array;

  @SafeVarargs
  public static <T> ImmutableArray<T> of(T... items) {
    // We need to copy to ensure that it is truly immutable
    // Explanation will come later
    @SuppressWarnings("unchecked");
    T[] arr = (T[]) new Object[items.length];
    for (int i=0; i<items.length; i++) {
      arr[i] = items[i];
    }
    return new ImmutableArray<>(arr, 0, items.length-1);
  }

  private ImmutableArray(T[] a, int start, int end) {
    this.start = start;
    this.end = end;
    this.array = a;
  }

  public T get(int index) {
    if (index < 0 || this.start + index > this.end) {
      throw new IllegalArgumentException("Index out of bound");
    }
    return this.array[this.start + index];
  }

  public ImmutableArray<T> subarray(int start, int end) {
     return new ImmutableArray<>(this.array, this.start + start, this.start + end);
  }
}

Why Copy the Array

Now we need to explain why we need to copy the parameter items in our factory method of. Consider not copying the content of the array. Since we can invoke the factory method by passing an array as argument, we can do the following.

Integer[] int_arr = new Integer[]{ 1, 2, 3 };
ImmutableArray<Integer> arr = ImmutableArray.of(int_arr);
arr.get(0); // 1
int_arr[0] = 9;
arr.get(0); // will be 9 if array is not copied since there is an aliasing

Enabling Safe Concurrent Execution

We will explore concurrent execution of code towards the end of the module, but making our classes immutable goes a long way in reducing bugs related to concurrent execution. Without going into details here (you will learn the details later), concurrent programming allows multiple threads of code to run in an interleaved fashion, in an arbitrary interleaving order. If we have complex code that is difficult to debug to begin with, imagine having code where we have to ensure its correctness regardless of how the execution interleaves! Immutability helps us ensure that regardless of how the code interleaves, our objects remain unchanged.

Final ≠ Immutable

When creating an immutable class, we need to be careful to distinguish between the keywords that helps us avoid accidentally making things easily mutable and the actual concept of immutable class. For instance, it is insufficient to simply declare all fields with final keywords. Just because we cannot accidentally update the field, does not mean that the field is immutable. Consider the same Circle above but with a getter for the center point and now imagine that the Point is mutable.

final class Circle {
  private final Point c;
  private final double r;

  public Circle (Point c, double r) {
    this.c = c;
    this.r = r;
  }
    :

  public Point getCenter() {
    return this.c;
  }

  public Circle moveTo(double x, double y) {
    return new Circle(c.moveTo(x, y), r);
  }
}

We can then simply retrieve the center point and mutate it externally.

Circle c = new Circle(new Point(0, 0), 1);
c.getCenter().moveTo(1, 1); // assume mutable Point

On the other hand, it is not even necessary to use the final keyword to make an immutable class. We simply have to have a class that prevents any and all kinds of sharing by copying all the parameters before assigning them into the fields and copying all return value. Assume that all classes has a correctly implemented clone() method. Then the following Circle is immutable even with getter and no final keyword on the fields. We still need the final keyword on the class to disallow inheritance.

final class Circle {
  private Point c;
  private double r;

  public Circle (Point c, double r) {
    this.c = c.clone();
    this.r = r; // primitive, no need cloning
  }
    :

  public Point getCenter() {
    return this.c.clone();
  }

  public Circle moveTo(double x, double y) {
    return new Circle(c.moveTo(x, y), r);
  }
}

That is not to say that the final keyword is not important. It helps accidental re-assignment and in some cases that is sufficient especially if the fields are of primitive type. Once we have created one immutable class, we can then create other larger immutable classes by only using immutable classes as fields.

Trivially Immutable Class

We can even have trivially immutable classes. Consider our old Pair<S, T> class.

class Pair<S,T> {
  private S first;
  private T second;

  public Pair(S first, T second) {
    this.first = first;
    this.second = second;
  }

  public S getFirst() {
    return this.first;
  }

  public T getSecond() {
    return this.second;
  }
}

Assuming that S and T are immutable, the generic class Pair<S, T> is also immutable. This is achieved by simply having no mutator. Additionally, there is no String toString() method that exposes the internal state.

Another way to have a trivially immutable class with mutator is to have no accessor to the mutated state. If we never expose the mutated internal state, then there is no way to have any visible changes outside of its abstraction barrier. The following Counter is an example.

class Counter {
  private int val;
  private int ctx;

  public Counter(int val) {
    this.val = val;
    this.ctx = 0;
  }

  public int get() {
    this.ctx += 1;
    return this.val;
  }

  @Override
  public String toString() {
    return "{" + this.val + "}";
  }
}

Note that the internal state ctx changed but there is no way for us to expose this externally. So using only its abstraction barrier (i.e., the methods get() and toString()), we can never see any visible changes.

Checklist for Immutability

The following is a general guide to help you create immutable class. Some of the items in the checklist may not be necessary but they are good to have.

Ensure that all fields have the final modifier (not necessary but good to have).
Ensure that the types of all the fields are immutable classes.
Ensure that arrays are copied before assigning to a field.
Ensure that there is no mutator.
- If there was a mutator and you are modifying the class to be immutable, then you need to return a new instance instead.
Ensure that the class has the final modifier to prevent inheritance.