How do I prevent the modification of a private field in a class?

java

defensive-coding

immutability

null-safety

byNikita Barsukov·Oct 12, 2024

To protect a private field enforce its declaration with final and by omitting setters. For mutable collections such as lists or arrays, adopt a defensive copying strategy in your getters:

public class CoronaSafe {
    private final List<String> vaccineList;

    public CoronaSafe(List<String> vaccineList) {
        // deep copying incoming list to keep our vaccines safe
        this.vaccineList = new ArrayList<>(vaccineList);  
    }

    public List<String> getVaccineList() {
        // Honey, I'm home! Just kidding - you're cloning me again?
        return new ArrayList<>(vaccineList);  
    }
}

Above method ensures immutability and non-modifiability of our vaccineList.

The magic potion of immutability

For arrays or collections, meticulous measures will fortify the protection of your private fields:

Putting handcuffs on lists

Use a read-only scope for your lists to thwart any unwelcome modifications. The magic tool? Collections.unmodifiableList().

public List<String> getVaccineList() {
    // Sorry buddy, no peeping inside!
    return Collections.unmodifiableList(new ArrayList<>(vaccineList));
}

This lock-down shields the list from being adulterated.

Arrays and their bodyguards

For arrays, clone them or resort to Arrays.copyOf() to ensure a fresh copy is served in getters:

private final String[] virusGenome;

public String[] getVirusGenome() {
    // Cloning... maybe this is how Dolly the sheep felt?
    return Arrays.copyOf(virusGenome, virusGenome.length);
}

This creates a new body double each time, leaving the original untouched.

Null values: The unwanted guests

When implementing defensive copies, it's vital to build robustness against null references:

public List<String> getVaccineList() {
    // No vaccines? No problem - have a placebo
    return vaccineList == null ? Collections.emptyList() : Collections.unmodifiableList(new ArrayList<>(vaccineList));
}

Efficiency: A love for Arrays

If memory conservation is crucial, Collections.unmodifiableList(Arrays.asList(arr)) serves an immutable presentation without making a duplicate of the underlying array.

Protecting array data: Tactics and Patterns

Ensuring property safety extends beyond mere insight and necessitates strategizing:

Immutable Objects

Reflect upon Guava's ImmutableList or comparable library resources for deeper immutability, often providing more optimized solutions than pure Java.

Defensive Copying vs. Immutable Views

There's a subtle contrast between creating a copy of the complete structure (defensive copying) and providing a read-only mask (immutable view). The former yields an entirely separate structure, the latter cloaks the existing one.