Java is one of the most popular object-oriented programming languages in use today. With its wide adoption across enterprises and focus on portability, Java has become many developers’ language of choice for building robust and scalable applications.One of the key concepts in Java that developers must understand is that of mutable and immutable objects. The ability to change or not change the state of an object is a fundamental distinction in Java that affects how code is written.

Understanding the difference between mutable and immutable objects allows developers to write safer, more efficient code. However, knowing when to use each type of object is also important for balancing flexibility and performance.

This comprehensive guide will provide an in-depth look at mutable and immutable objects in Java. It will cover the definitions of mutability and immutability, discuss how to work with each type of object, examine use cases and performance tradeoffs, and provide actionable insights for Java developers. Code examples will be included where relevant to illustrate the concepts.

Defining Mutability and Immutability

To start, let’s define what exactly mutable and immutable mean in the context of Java objects:

Mutable Objects Have Changeable State

A mutable object is an object whose state can be modified after it is initialized or created. In other words, you can change one or more properties of a mutable object after it already exists.

For example, consider a simple MutableClass in Java:

public class MutableClass {

  private int number;

  public MutableClass(int number) {
    this.number = number;
  }

  public int getNumber() {
    return number;
  }

  public void setNumber(int number) {
    this.number = number; 
  }

}

Here the number property can be changed at any time using the setNumber() method. So MutableClass objects are mutable – their state can be modified after creation.

Some common mutable object examples in Java include:

• Arrays
• Collections like ArrayList, HashSet, etc.
• Most custom model objects
• java.util.Date

Immutable Objects Have Fixed State

Conversely, an immutable object is an object whose state cannot be changed after initialization. Once an immutable object is created, you cannot modify its properties.

For example:

public final class ImmutableClass {

  private final int number;

  public ImmutableClass(int number) {
    this.number = number;
  }

  public int getNumber() {
    return number;
  }

}

Here number is marked final so it can only be set during construction. There is no setNumber() method. So once an ImmutableClass is constructed, its state can never change.

Some common immutable examples in Java include:

• String
• The wrapper classes like Integer, Long, etc.
• java.time.LocalDate
• Any class marked final with only final fields

So in summary:

• Mutable objects can be changed after creation
• Immutable objects cannot be changed after creation

Understanding this key difference allows developers to reason about the potential behavior of objects in Java systems.

Next let’s look at how we actually work with mutable and immutable objects in Java code.

Working with Mutable and Immutable Objects in Java

Now that you understand the core concepts of mutability and immutability, let’s see how that translates into real code.

Making Mutable Classes with Setters and Getters

To create a mutable object in Java, you need to provide methods that allow the state to be changed after creation. This usually involves exposing setters and getters.

For example, consider this MutablePoint class:

public class MutablePoint {

  private int x;
  private int y;

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

  public int getX() {
    return x;
  }

  public void setX(int x) {
    this.x = x;
  }

  public int getY() {
    return y;
  }

  public void setY(int y) {
    this.y = y;
  }

}

Here we have getX()/setX() and getY()/setY() methods to modify the x and y coordinates after construction:

MutablePoint point = new MutablePoint(10, 20); // x = 10, y = 20

point.setX(5); // x is now 5

point.setY(15); // y is now 15 

This allows the state of the object to change over time, meeting the criteria for a mutable object.

Other ways to make mutable objects include having public fields or exposing methods that directly modify state like incrementCount(). But having proper encapsulation with getters and setters is recommended.

Making Immutable Classes with Final Fields

To create an immutable object in Java, you need to prevent state from being changed after initialization. This typically involves:

• Declaring the class final so it can’t be inherited
• Making fields private and final so they can only be set once
• Not exposing any setters that modify state

For example:

public final class ImmutablePoint {

  private final int x;
  private final int y;

  public ImmutablePoint(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public int getX() {
    return x;
  }

  public int getY() {
    return y;
  }

}

Here x and y can only be set in the constructor. And no setters allow changing them afterwards. So ImmutablePoint cannot be modified after creation.

We can construct instances:

ImmutablePoint point = new ImmutablePoint(10, 20); // x = 10, y = 20

But cannot change their state:

“`java
point.setX(5); // Compile error – no setter available!

This enforces immutability in Java.

Some key principles for creating immutable classes:

• Don’t provide setters
• Make all fields final
• Make class final to prevent subclassing
• If mutable fields are needed, return defensive copies from getters rather than originals

Adhering to these principles allows you to ensure instances of the class remain immutable.

Use Cases and Performance Tradeoffs

Now that you’ve seen how to create mutable and immutable objects in Java code, let’s explore some of the use cases and performance tradeoffs to consider when deciding between the two.

Mutable Objects Allow In-Place Changes But Require Synchronization

Mutable objects have the advantage of allowing in-place changes. You can modify the state of a single mutable object as much as needed rather than creating a new object each time.

For example, imagine you need to update a Player object representing state in a game:

Player player = new Player();
player.setPosition(10, 20);

// …later in game loop
player.setPosition(15, 25)

By mutating the player we can reuse a single object rather than allocating a new one each frame.

However, mutable objects require synchronization when used across threads to ensure state changes safely. Two threads accessing the same mutable object at the same time could cause race conditions.

So performance optimizations like lock-free programming are harder with mutable objects.

Immutable Objects Are Simpler and Faster But Require New Allocations

Immutable objects have the advantage of being simpler to reason about since their state never changes. This makes them inherently thread-safe without synchronization.

But modifying an immutable object requires creating a whole new object:

ImmutablePoint point = new ImmutablePoint(10, 20);

// To 'modify' it, need to construct new instance
point = new ImmutablePoint(15, 25);

This can allocate more objects over time than mutating a single one.

However, modern JVM optimizations can reduce this cost through techniques like escape analysis and scalar replacement. The JVM can avoid allocating actual objects in cases where immutability allows.

So in performance critical scenarios, benchmark both mutable and immutable approaches to see which works best for your system.

Generally immutable objects allow for cleaner, more maintainable code. But there are cases where mutable objects make sense too.

When to Use Each Approach

Given the pros and cons, here are some guidelines on when to use mutable vs immutable objects in Java:

• Use immutable objects by default – Immutability makes code safer and more predictable. Only use mutability when you need it.
• Use mutable objects for changing state – When an object needs to change state over time, mutability avoids extra allocations.
• Use mutable objects for performance – In some cases mutating existing objects is faster than reallocating. Profile to be sure.
• Use immutable objects for concurrency – Immutable objects are always thread-safe.
• Use immutable objects as map keys – Mutable keys can cause hard to track bugs.
• Use immutable objects to prevent changes – When state should not change, immutability acts as an enforcer.

Think carefully about the needs of your specific application and architecture when deciding between mutable and immutable classes.

Conclusion

Mutability and immutability are key concepts in Java that have a big impact on how classes are designed and used.Mutable objects allow their state to be modified after creation, while immutable objects do not. Understanding this distinction helps developers write safer, more efficient code.

Mutable objects are useful when state needs to change, like game state updating each frame. But they require synchronization in concurrent code.Immutable objects provide thread-safety by default and simplify reasoning about state. But they require creating new objects instead of reusing existing ones.Modern JVMs can optimize some immutable object allocations. But in performance sensitive code, benchmark tradeoffs between mutable and immutable approaches.

Use immutable objects by default, and only leverage mutability when its benefits are needed. Think about the concurrency needs, architectural constraints, and performance characteristics of your specific problem space.By gaining a deep understanding of mutable and immutable objects in Java, you can build systems that are safer, faster, and more maintainable. Master these core concepts to improve your Java code and deliver robust solutions