Question

I’m curious as to why abstract methods MUST be overridden by the first concrete implementing class, and not one further down the hierarchy change.

I’m not suggesting I want to do this, but I’m curious as to why it has to be the first class

Consider this example

abstract class Upper
{
    abstract void doSomething();
}

class Middle extends Upper
{
    void doSomething()
    {
       // I'm forced to be implemented here
    }
}

abstract class Lower extends Middle
{

}

class Bottom extends Lower
{
    void doSomething()
    {
        // I'm valid, but I'm too far down the hierarchy
    }
}

Answer 1

By definition a normal class must implement all abstract methods. If you would declare Middle abstract, then you would not have to implement the methods in Middle.

A normal class can be instantiated, whereas a abstract class cannot. Think about what would happen if you try to call the methods that are not implemented in the class.