Does Java Support Multiple Inheritance?

Java Inheritance: Single Inheritance vs. Multiple Inheritance

During a job interview, a candidate was asked if Java supported multiple inheritance. The candidate responded with "No," citing that each class in Java extends Object and extending another class like "Class A extends Class B" means Class A inherits from both Class B and Object, which is considered multi-level inheritance, not multiple inheritance.

However, the interviewer challenged this answer, arguing that since Class B extends Object, extending Class B in Class A results in Class A inheriting from both Class B and Object, essentially constituting multiple inheritance.

Clarification

The candidate's answer was largely correct in the context of the interviewer's specific example. Multiple inheritance refers to a situation where a class inherits from two or more unrelated bases, creating a "diamond" structure in the inheritance hierarchy.

Java's Single Inheritance with Multiple Levels

In Java, however, inheritance is single-level with multiple levels. Class A extends Class B, which in turn extends Object. This creates a chain of inheritance, but Class A only inherits directly from Class B and indirectly from Object. This is not multiple inheritance.

Interfaces and "Default" Methods

While Java does not support traditional multiple inheritance, it does have interfaces and "default" methods on interfaces (introduced in Java 8). This feature allows classes to implement multiple interfaces and inherit their respective default methods.

However, this is still not true multiple inheritance because:

• The "super" type is determined by the base class (single-lineage)
• Interfaces do not have constructors to manage initialization
• Constructors from base classes are not inherited more than once
• If multiple default methods with the same signature are inherited from different interfaces, it results in a compile-time or runtime error.

Conclusion

Java's inheritance model allows for single inheritance with multiple levels through the "extends" keyword and interfaces with default methods provide a form of "multiple inheritance light." However, it is important to distinguish these concepts from true multiple inheritance, where a class can inherit from multiple unrelated bases.

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