Why Writing to a Static Variable in an Instance Method is Considered Bad Practice?

What are the reasons writing to a static variable inside an instance method is considered a bad practice in object-oriented programming?

class Example {
    private static int staticCount = 0;
    private int instanceCount;

    public void incrementCounts() {
        staticCount++;  // Modifying static variable in instance method
        instanceCount++; // Modifying instance variable
    }
}

Modifying a static variable within an instance method can lead to unexpected behaviors, decrease code readability, and violate principles of object-oriented programming such as encapsulation and maintainability. Here’s why it's deemed a bad practice.

class SafeExample {
    private int instanceCount;

    public void increment() {
        instanceCount++;  // Safe: only affecting instance state
    }
}

Causes

• It breaks encapsulation principles by allowing instance methods to manipulate shared state across all instances.
• It leads to unpredictable results in concurrent or multi-threaded environments.
• It makes unit testing harder, as tests may have to reset static state before each execution.
• It increases the complexity of understanding the state of an application due to shared static data.

Solutions

• Utilize instance variables instead of static ones when state needs to be specific to individual object instances.
• Separate concerns by using singleton patterns for shared application state correctly, if necessary.
• Keep static variables for true constants or utility methods that don’t require state.

Mistake: Using static variables to store instance-specific data.

Solution: Always prefer instance variables for data that pertains to the specific object instance.

Mistake: Neglecting thread safety when modifying static variables from instance methods.

Solution: Use synchronization mechanisms or consider thread-local storage when dealing with multi-threaded applications.