Java Immutable Objects

An object is considered immutable if its state cannot change after it is constructed. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state making them useful in concurrent applications.

For example String objects in Java are immutables. 

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class ImmutableTest {
    @Test
    public void testImmutableObjs() {
        String firstName = "yogen";
        String lastName = "rai";

        // concatenate two strings
        String fullName = firstName.concat(" ").concat(lastName);

        assertEquals("yogen", firstName);
        assertEquals("rai", lastName);
        assertEquals("yogen rai", fullName);
    }
}

The concatenation of firstName on " " (whitespace) and lastName is not going to modify either of them, rather create new object which will be referenced by fullName.

String- Immutables

Figure: Strings are immutables

Defining Custom Immutable Objects

The following rules define a simple strategy for creating immutable objects:

  1. Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
  2. Make all fields final and private.
  3. Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
  4. If the instance fields include references to mutable objects, don't allow those objects to be changed:

    • Don't provide methods that modify the mutable objects.
    • Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

Defining Custom Immutable Object with Date object

Date object in java.util.* is not thread immutable.

It is good news that new java.time.* API after JDK 8 has Date objects as immutable objects. For example, the Student object in example below is immutable even if the property dateJoined is Date object.

import java.time.LocalDate;

final class Student {
    private final String name;
    private final int age;
    private final LocalDate dateJoined;

    public Student(String name, int age, LocalDate dateJoined) {
        this.name = name;
        this.age = age;
        this.dateJoined = dateJoined;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public LocalDate getDateJoined() {
        return dateJoined;
    }
}

public class TestImmutable {
    @Test
    public void testImmutableObject() {
        Student original = new Student("Yogen", 23, LocalDate.of(2016, 5, 1));

        LocalDate modifiedLocalDate = original.getDateJoined().plusYears(2);

        Student expected = new Student("Yogen", 23, LocalDate.of(2016, 5, 1));
        assertEquals(expected, original);
    }
}

The code from above example is available on Github.

Benefits of Immutable objects

  • Immutable objects are simpler to construct, test, and use as they are side-effect free.
  • They are much easier to cache since the modification on object is not going affect it's original state.
  • Truly immutable objects are always thread-safe
  • Identity mutability problem is avoided
  • They help to avoid temporal coupling