Unit 28: Immutability

Learning Objectives

After completing this unit, students should be able to:

• Explain immutability as a design principle and articulate how it reduces bugs arising from aliasing, mutation, and unintended side effects.
• Design and implement immutable classes in Java, including the correct use of final, factory methods, and copy-on-write semantics.
• Distinguish between final and true immutability, and identify common pitfalls where final fields do not guarantee immutability.
• Reason about safe sharing of objects and internal representations, including when and why structural sharing is correct and efficient.
• Recognize the role of immutability in program reasoning and concurrency, and explain why immutable objects are inherently thread-safe.

Overview

In earlier units, we saw how abstraction, typing, and reuse help manage software complexity. In this unit, we introduce another powerful strategy: avoiding change.

Many subtle bugs arise from mutation, especially when objects are aliased and updated through multiple references. When an object can change over time, reasoning about program behavior becomes significantly harder.

Immutability avoids this problem by ensuring that an object’s observable state never changes after creation¹. Updates instead produce new objects, eliminating aliasing bugs and enabling safe sharing without defensive copying.

Avoiding Change

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 observable changes outside its abstraction barrier. This means that every call to the instance's method must behave the same way throughout the lifetime of the instance. An object can be logically immutable even if it mutates private, unobservable state, as long as its externally visible behavior remains unchanged.

There are many advantages to making classes 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 separate copies of the 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 we can still 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

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 prevent subclasses from overriding methods in a way that breaks immutability, it is recommended that we declare immutable classes as as final to disallow inheritance.

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), r);
  }
}

With both Point and Circle immutable, we can be sure that 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.

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 values 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.

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.

Enabling Safe Sharing of Objects

Making a class immutable allows us to safely share instances of the class, 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's 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 final static Point ORIGIN = new Point(0, 0);

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

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

  // other methosd omitted
}

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!

Enabling Safe Sharing of Internals

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

final class ImmutableSeq<T> {
  private final T[] array;

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

  private ImmutableSeq(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:

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

@SafeVarargs.   Since the varargs is implemented as 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:

ImmutableSeq<Integer> a = ImmutableSeq.of(10, 20, 30, 40, 50, 60);
ImmutableSeq<Integer> b = a.subarray(2, 4); // b is [30, 40, 50]
b.get(0) // returns 30
ImmutableSeq<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 ImmutableSeq 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 ImmutableSeq<T> {
  private final int start;
  private final int end;
  private final T[] array;

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

  private ImmutableSeq(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 bounds");
    }
    return this.array[this.start + index];
  }

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

Enabling Safe Concurrent Execution

We will explore concurrent execution of code towards the end of the course, but making our classes immutable goes a long way in reducing bugs related to concurrent execution. Without going into details (you will learn this 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 help us avoid accidentally making things easily mutable and the actual concept of an 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 to the fields and copying all return values. Assume that all classes have a correctly implemented clone() method. Then the following Circle is immutable even with a 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 does not mean 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.

Performance Trade-offs of Immutability

While immutability offers significant benefits in correctness, reasoning, and safety, it is not without cost. Because immutable objects cannot be modified in place, updates typically require creating new objects, which may increase memory allocation and garbage collection overhead.

In performance-critical code, such as tight loops, low-level data processing, or numerical computations, this additional allocation can be expensive compared to mutating an existing object. You have seen an example of this issue when we discussed wrapper classes, which are immutable.

In such cases, a carefully designed mutable implementation may be more efficient. Hence, immutability should be viewed as a design trade-off, not a universal rule. When correctness, simplicity, and safe sharing are priorities, immutability is often the better choice. When performance is critical and mutation can be tightly controlled within a well-defined abstraction barrier, mutability may be justified.

In practice, many systems combine both approaches: using mutable objects internally for efficiency, while exposing immutable interfaces to clients.

---

1. Note that this is a looser definition than some other definitions of immutability. Java tutorial, for instance, defines immutability as preventing any change to the object, including private state. Our definition allows private state to change as long as the observable behavior remains unchanged.

   In this unit, we learn how to design immutable classes in Java and why immutability is a key tool for writing simpler, safer, and more robust programs. ↩