关于java：为什么第二个例子编译成功

Why second example compiles sucessfuly

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import java.util.*;

class Test
{
public static class Base
{
}

public static class Derived1
extends Base
{
}

public static class Derived2
extends Base
{
}

public static void main (String[] args)
{
//Example1.
List<? extends Base> e = new ArrayList<Base>();
e.add(new Derived1()); //this won't compile

//Example2.
List<? super Base> b = new ArrayList<Base>();
b.add(new Derived1()); //this compiles
}
}

List b可以分配为List或List