---
title: Private Class Data
category: Structural
language: en
tag:
    - Abstraction
    - Encapsulation
    - Security
---

## Also known as

* Data Hiding
* Encapsulation

## Intent

The Private Class Data design pattern aims to restrict access to the internal state of an object by providing controlled access through methods, thereby increasing security and reducing accidental data corruption.

## Explanation

Real world example

> Imagine you are cooking a stew for your family dinner. You want to stop your family members from tasting the stew while you're still preparing it. If they do, there might not be enough stew left for dinner.

In plain words

> Private class data pattern prevents manipulation of data that is meant to be immutable by separating the data from the methods that use it into a class that maintains the data state.

Wikipedia says

> Private class data is a design pattern in computer programming used to encapsulate class attributes and their manipulation.

**Programmatic Example**

Taking our stew cooking example from above. First, we have a `Stew` class where its data is not protected by private class data, making the stew's ingredient mutable to class methods. 

```java
@Slf4j
public class Stew {
  private int numPotatoes;
  private int numCarrots;
  private int numMeat;
  private int numPeppers;
  public Stew(int numPotatoes, int numCarrots, int numMeat, int numPeppers) {
    this.numPotatoes = numPotatoes;
    this.numCarrots = numCarrots;
    this.numMeat = numMeat;
    this.numPeppers = numPeppers;
  }
  public void mix() {
    LOGGER.info("Mixing the stew we find: {} potatoes, {} carrots, {} meat and {} peppers",
        numPotatoes, numCarrots, numMeat, numPeppers);
  }
  public void taste() {
    LOGGER.info("Tasting the stew");
    if (numPotatoes > 0) {
      numPotatoes--;
    }
    if (numCarrots > 0) {
      numCarrots--;
    }
    if (numMeat > 0) {
      numMeat--;
    }
    if (numPeppers > 0) {
      numPeppers--;
    }
  }
}
```

Now, we have `ImmutableStew` class, where its data is protected by `StewData` class. The methods in `ImmutableStew` are unable to manipulate the data of the `StewData` class.

```java
public class StewData {
  private final int numPotatoes;
  private final int numCarrots;
  private final int numMeat;
  private final int numPeppers;
  public StewData(int numPotatoes, int numCarrots, int numMeat, int numPeppers) {
    this.numPotatoes = numPotatoes;
    this.numCarrots = numCarrots;
    this.numMeat = numMeat;
    this.numPeppers = numPeppers;
  }
  public int getNumPotatoes() {
    return numPotatoes;
  }
  public int getNumCarrots() {
    return numCarrots;
  }
  public int getNumMeat() {
    return numMeat;
  }
  public int getNumPeppers() {
    return numPeppers;
  }
}
@Slf4j
public class ImmutableStew {
  private final StewData data;
  public ImmutableStew(int numPotatoes, int numCarrots, int numMeat, int numPeppers) {
    data = new StewData(numPotatoes, numCarrots, numMeat, numPeppers);
  }
  public void mix() {
    LOGGER
        .info("Mixing the immutable stew we find: {} potatoes, {} carrots, {} meat and {} peppers",
            data.getNumPotatoes(), data.getNumCarrots(), data.getNumMeat(), data.getNumPeppers());
  }
}
```

Let's try creating an instance of each class and call their methods:

```java
var stew = new Stew(1, 2, 3, 4);
stew.mix();   // Mixing the stew we find: 1 potatoes, 2 carrots, 3 meat and 4 peppers
stew.taste(); // Tasting the stew
stew.mix();   // Mixing the stew we find: 0 potatoes, 1 carrots, 2 meat and 3 peppers
var immutableStew = new ImmutableStew(2, 4, 3, 6);
immutableStew.mix();  // Mixing the immutable stew we find: 2 potatoes, 4 carrots, 3 meat and 6 peppers
```

## Class diagram

![Private Class Data](./etc/private-class-data.png "Private Class Data")

## Applicability

Use the Private Class Data pattern when

* When you want to protect the integrity of an object’s state.
* When you need to limit the visibility of the internal data of an object to prevent unintended modification.
* In scenarios where multiple classes need to share access to some common data without exposing it directly.

## Known Uses

* Java Beans, where properties are accessed via getters and setters.
* In many Java libraries where the internal state is hidden from the user to ensure consistency and security.
* Enterprise applications where sensitive data needs to be protected from direct access.

## Consequences

Benefits:

* Enhanced Security: Reduces the risk of unintended data corruption by encapsulating the data.
* Ease of Maintenance: Changes to the internal representation of data do not affect external code.
* Improved Abstraction: Users interact with a simplified interface without worrying about the complexities of data management.

Trade-offs:

* Performance Overhead: Additional method calls (getters/setters) can introduce slight performance overhead.
* Complexity: May increase the complexity of the class design due to the additional layer of methods for data access.

## Related Patterns

* [Proxy](https://java-design-patterns.com/patterns/proxy/): Both patterns restrict access to the underlying object but Proxy controls access to the object itself, while Private Class Data controls access to the data.
* [Singleton](https://java-design-patterns.com/patterns/singleton/): Ensures that a class has only one instance and provides a global point of access to it; often used to manage shared data with controlled access.
* [Decorator](https://java-design-patterns.com/patterns/decorator/): Adds behavior to an object without altering its structure; can be combined with Private Class Data to manage additional state privately.

## Credits

* [Clean Code: A Handbook of Agile Software Craftsmanship](https://amzn.to/3UJTZJk)
* [Design Patterns: Elements of Reusable Object-Oriented Software](https://amzn.to/3w0pvKI)
* [Effective Java](https://amzn.to/4cGk2Jz)