11.6. Interface vs Abstract Class

Feature | Interface | Methods | Only abstract (Java 7); default & static (Java 8+) |

An interface is a contract that defines a set of methods that a class must implement. In Java, prior to version 7, interfaces could only contain abstract method signatures—methods without a body. With Java 7 and later, interfaces can also have default methods (methods with an implementation) and static methods. This allows interfaces not only to define behaviors but also to provide some common functionality.

Think of an interface like a set of instructions for a recipe. The recipe specifies what steps need to be followed (methods) but not how to perform them. You're given the freedom to choose how to cook your dish, just like a class implements the methods defined by an interface in its own way.

| Abstract Class | Can have both abstract and concrete methods |

An abstract class, unlike an interface, can contain both abstract methods (without implementation) and concrete methods (with implementation). This means that you can define some common behavior in the abstract class while still requiring subclasses to implement specific behaviors. This flexibility is useful when you have some shared logic along with distinct behaviors in subclasses.

Imagine an abstract class as a basic vehicle type. It can have concrete methods like 'startEngine()' that all vehicles share, while also defining abstract methods like 'drive()' that each specific vehicle must implement differently. For example, a car drives differently than a motorcycle, but both start their engines the same way.

| Fields | Public static final only | Can have any type of field |

In an interface, fields are inherently public, static, and final. This means they are constants and do not change across instances. Conversely, an abstract class can have fields of any type, which can vary across instances of its subclasses. This allows for greater flexibility in how data is modeled and managed.

Think of an interface's fields like street signs that indicate fixed rules: everyone must follow them and they don’t change. However, an abstract class's fields are like cars on the road, each with different features (color, model), but all operating on the same set of rules—the traffic laws.

| Multiple Inheritance | Yes | No |

Interfaces support multiple inheritance, meaning a class can implement multiple interfaces. This allows for greater flexibility in defining a class that can exhibit multiple behaviors. On the other hand, a class can only extend one abstract class, which limits the inheritance of behavior in a straightforward hierarchy.

If you think of interfaces as memberships in clubs, someone can be in multiple clubs (like a sports club and a book club) at once. But an abstract class is like a family tree; you can only have one direct family lineage, even though that lineage may participate in various family activities.

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