a Marker interface doesn’t have any methods or fields declaration (an empty interface) such as “Serializable” or “Cloneable”.
a Functional Interface is an interface with just one abstract method declared in it such as Runnable interface that only has run() method.
An “interface” to iterate through and each and every element of a list (it’s not a class).
| Abstract Class | Interface | |
| Multiple inheritance | Doesn’t support | support |
| Data Member | contains | Doesn’t contain |
| Constructor declaration | contains | Doesn’t contain |
| Definition | May contain definition with no implementation | Just a pattern, an empty shell |
| Methods | Can have both abstract & non-abstract methods | Can only have abstract methods |
| Implementation | Can contain both definition and its implementation
Incomplete class, cannot be instantiated |
Can only have the signature of the functionality without any code |
| Access Modifier | Can have all access modifiers for member declaration of functions, subs and properties. | Not allowed (all methods must be implicitly defined as public).
All the members assumed as implicitly public. |
| Homogeneity | is used for implementations of the same type, behavior and status | Is used for implementations that share only method signatures |
| Declaration | Acts as a base class for all other classes, can declare/ use any variable | Not allowed to declare any variables |
| Core vs Peripheral | Core identity of a class & for objects of the same data type | Used to define Peripheral ability of a class |
| Rigid vs Supple | More supple | More rigid |
| Fields definition | Predefined fields & constants | Fields cannot be defined in interfaces |
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