类型 A 已定义错误

我试图寻找解决方案，但我发现的我不知道在这种情况下如何应用。请帮我更正我的代码。

package Exercise;

public class Ex11_11 {
public static void main(String[] args) {
    A a = new A(3);
    }
}

class A extends B { // type A is already defined, A has a red underline
    public A (int t) {
    System.out.println("A's constructor is invoked");
    }
}

class B { // type B is already defined, B has a red underline
    public B () {
        System.out.println("B's constructor is invoked");
    }  
}