What Is the Diamond Problem?

The diamond problem occurs when a class inherits from two classes (or interfaces) that both inherit from the same superclass (or superinterface). This creates a "diamond" shape in the class hierarchy.

Although it's worth noticing this was only a problem introduced since Java 8 when default methods in interfaces were allowed.

🧱 Example in Java (Using Interfaces)

Java doesn't support multiple inheritance with classes, but it does allow multiple inheritance with interfaces.

interface A {
    default void greet() {
        System.out.println("Hello from A");
    }
}

interface B extends A {
    default void greet() {
        System.out.println("Hello from B");
    }
}

interface C extends A {
    default void greet() {
        System.out.println("Hello from C");
    }
}

class D implements B, C {
    // Compiler error unless greet() is overridden
    public void greet() {
        B.super.greet(); // or C.super.greet()
    }
}

🤯 The Conflict

In the diagram:

• A is at the top.

• B and C both inherit from A.

• D implements both B and C.

Now, when D calls greet(), which version should it inherit? B's or C's? Java doesn't know, so you must override it in D to resolve the ambiguity.

☕️ Why It Matters in Java

• Java avoids this problem with classes by disallowing multiple inheritance.

• With interfaces, the issue still exists but is explicitly handled by requiring the implementing class to override conflicting default methods.