When to Use Immutable vs. Mutable Data Structures in Java

 

When to Use Immutable vs. Mutable Data Structures in Java

Choosing between immutable and mutable data structures depends on the specific use case, performance considerations, and how the data will be used.

---

1. Immutable Data Structures

Immutable data structures cannot be modified after creation. If you need to "modify" them, you create a new instance instead.

When to Use Immutable Structures?

✅ Thread Safety → No race conditions or concurrent modification issues.
✅ Safe Sharing → Can be safely shared across multiple parts of the program.
✅ Predictability → No accidental modifications.
✅ Cache-Friendly → Can be safely cached without worrying about changes.

Examples of Immutable Structures

Immutable Records (Java 14+)

javaCopyEditpublic record Person(String name, int age) {}
public class Main {
    public static void main(String[] args) {
        Person person = new Person("Alice", 30);
        System.out.println(person.name() + " is " + person.age() + " years old.");
    }
}
 

Immutable Collections (Using List.of(), Map.of())

javaCopyEditimport java.util.List;import java.util.Map;
public class Main {
    public static void main(String[] args) {
        List<String> names = List.of("Alice", "Bob");  // Immutable list        Map<String, Integer> ages = Map.of("Alice", 30, "Bob", 25); // Immutable map
        
        System.out.println(names);
        System.out.println(ages);
    }
}
 

⚠️ Modifying an immutable structure throws an exception:

javaCopyEditnames.add("Charlie");  // UnsupportedOperationException

Immutable Objects in Classes

javaCopyEditpublic final class ImmutablePerson {
    private final String name;
    private final int age;

    public ImmutablePerson(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() { return name; }
    public int getAge() { return age; }
}
 

---

2. Mutable Data Structures

Mutable data structures allow modification after creation.

When to Use Mutable Structures?

✅ Performance → Modifying an object is faster than creating a new one.
✅ Frequent Updates → When data changes frequently (e.g., counters, lists).
✅ Memory Efficiency → Avoids creating unnecessary objects.

Examples of Mutable Structures

Mutable Collections (ArrayList, HashMap)

javaCopyEditimport java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;
public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");

        Map<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 30);
        ages.put("Bob", 25);

        System.out.println(names);
        System.out.println(ages);
    }
}
 

Mutable Objects

javaCopyEditpublic class MutablePerson {
    private String name;
    private int age;

    public MutablePerson(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public void setName(String name) { this.name = name; }
    public void setAge(int age) { this.age = age; }
}
 

---

Key Differences

Feature	Immutable	Mutable
Modification	Cannot be modified	Can be modified
Thread Safety	Thread-safe	Requires synchronization for thread safety
Performance	May create new objects, slower in some cases	More efficient for frequent updates
Memory Usage	Can lead to more objects (higher GC overhead)	Lower GC overhead
Use Case	Safe for concurrent use, caching, function returns	Best for frequent updates, lists, counters

---

When to Use What?

 

Scenario	Recommended
Multi-threaded applications	✅ Immutable (avoid synchronization issues)
Data that rarely changes	✅ Immutable (safe sharing, caching)
Functional programming	✅ Immutable (pure functions, predictable)
Frequent updates (e.g., counters, lists)	✅ Mutable (better performance)
Large datasets	✅ Mutable (avoids unnecessary object creation)