Difference Between Abstract Class and Interface

[JiaPei Jen]

I think the differences between Abstract Class and Interface are:
Abstract Classes:
1. can have constructors
2. cannot be instantiated
3. classes use 'extends' to extend abstract classes
Interfaces:
1. only have public final static data and public abstract
methods
2. all methods have void return type
3. interface methods cannot be final, static, synchronized,
or native
4. cannot have constructor
5. classes use 'implements' to implement interfaces
6. interfaces use 'extends' to extend other interfaces
Are there any other attributes can be added?

[Thomas Paul]

Abstract Classes
----------------
1. Abstract classes can not have constructors as they can never be constructed.
Interfaces
----------
2. This is incorrect. methods can have any return type.

[Bin Wang]

Originally posted by Thomas Paul:
Abstract Classes
----------------
1. Abstract classes can not have constructors as they can never be constructed.

Thomas,
I think abstract class can have constructor(s), although it doesn't make sense. See the following code, it compiles and runs well and outputs 1.
abstract class Test {
static int i=1;
Test() {i=5;}
public static void main (String[] args) {
System.out.println(Test.i);
}
}

[krutika burse]

ABSTRACT CLASSES :-
Can't have MULTIPLE INHERITANCE.
INTERFACES :-
Supports MULTIPLE INHERITANCE.

[js yang]

BTW, both abstract classes and interfaces cannot be instantiated.

[Thomas Paul]

Shows what happens when you respond before your first cup of coffee.
Abstract classes can have constructors which will be invoked when a concrete implementation of that abstract class is instantiated. Constructors themseves can not be abstract.