abstract and final in java

Can an abstract class be final?

• If final – cannot be extended
• If abstract – must be extended
• Then final & abstract – become illegal combination of modifiers.

final abstract AbstractClass
{
	/* logic */
}

• A method cannot be abstract & final.
• Final method cannot be overridden.
• Abstract method must be overridden

abstract class Test 
{
	abstract final void fun();
}

• Abstract and static cannot be combined.
• Only specific(non static in java) methods can be abstract.
• Static method is common for all.
• We cannot combine abstract and static.

abstract class Test 
{
	abstract static void fun();
}

Why abstract method cannot be static?

• Common functionality of Object is static in java.
• Abstract methods are specific(non-static) to particular object.
• Hence both static (common) and abstract (specific) are illegal combination.

• Static methods must have definition.
• Static methods always concrete methods.
• Static methods cannot participate in inheritance.

Can a Final class extend Abstract class?

Yes allowed.

abstract class AbstractClass
{
	// logic
}
final class FinalClass extends AbstractClass
{
	// logic
}

Note: FinalClass must override all abstract methods because final class cannot be extended.

abstract class AbstractClass
{
	abstract void m1();
	abstract void m2();
}
final abstract class FinalClass extends AbstractClass
{
	// Error : Override m1() and m2() as we cannot define final class as abstract
}

If we override all specifications, then compiler will not generate any error

abstract class AbstractClass
{
	abstract void m1();
	abstract void m2();
}
final class FinalClass extends AbstractClass
{
	void m1(){ }
	void m2(){ }
}