What is method idempotency?

Idempotence of a method means that multiple executions of the method with the same input parameters will have the same final effect as a single execution, without additional side effects.

In simple words, repeated calls do not change the system's state after the first call.

Examples of idempotent operations:

• Reading data: Multiple reads of the same data do not change its state.
• Deleting a resource: Multiple deletions of a non-existent resource or a resource that has already been deleted do not cause errors and do not change the system's state.
• Setting a value: Setting a specific value to a variable or an object's field.

Examples of non-idempotent operations:

• Incrementing a counter: Each execution of the method increases the counter value.
• Adding an element to a list: Each execution adds a new element.

Idempotence is important for creating reliable and fault-tolerant systems, especially in distributed environments where repeated calls may occur due to network delays or failures. For example, in RESTful APIs, HTTP methods GET, PUT, and DELETE should be idempotent, while POST is usually not.

Example of an idempotent method:

// Method that sets the value of the variable income
public void setIncome(double amount) {
    this.income = amount; // Each execution sets the same value
}

Example of a non-idempotent method:

// Method that increases the value of the variable income
public void addIncome(double amount) {
    this.income += amount; // Each execution increases the income value
}