Devlog

The problem of mismatching type when you use List.

Let’s think just a really simple struture for understanding of inheritance concept.

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class A {

}

class B extends A {

}

class C {

}

Class A is a super class and Class B is extended Class A. Class C is independent thing. It doesn’t have any relationship with other classes.

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/** It's possible */
A a = new A();

/** It's possible because B is sub-class of A. */
A a = new B();

/** It's impossible because C doesn't have any relationship with A. */
A a = new C();

You could understand the above code. but let’s see the below, which i haven’t understood well just before now.

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/** It's possible. */
ArrayList<A> aList = new ArrayList<A>;

/** It's possible. */
aList.add(new B());

/** It's impossible. */
aList = new ArrayList<B>;

The last one is the main topic of this article. I usually think that code would work well. but it’s not. the reason why it isn’t work well is you should understand ArrayList is not extended ArrayList

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