How can you ensure that an object instance is created only once during program execution?

To ensure that an object instance is created only once during the execution of a program, the Singleton pattern can be used.

Main verification methods:

• Singleton Implementation: the class controls the creation of its instance by providing a static method to access it.
• Testing: you can write a test that calls the object creation multiple times and checks that all references point to the same object.

Example in Java:

public class Singleton {
    private static Singleton instance;

    private Singleton() { }

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

In the test, you can verify:

Singleton s1 = Singleton.getInstance();
Singleton s2 = Singleton.getInstance();
assert s1 == s2; // Check that references point to the same object

Thus, it is confirmed that the object is created only once.