学习java内部类时一点小疑惑

[qq_36743111]

// innerclasses/Ex6.java
// TIJ4 Chapter Innerclasses, Exercise 6, page 353
/* Create an interface with at least one method, in its own package. Create
* a class in a separate package. Add a protected inner class that
* implements the interface. In a third package, inherit from your class and
* inside a method, return an object of the protected inner class, upcasting
* to the interface during the return.
*/

// in separate package:
public interface Ex6Interface {
String say();
}

// and in a second package:
public class Ex6Base {
protected class Ex6BaseInner implements Ex6Interface {
// need public constructor to create one in Ex6Base child:
public Ex6BaseInner() { }
public String say() { return "Hi"; }
}
}


import innerclasses.ex6Interface.*;
import innerclasses.ex6Base.*;

public class Ex6 extends Ex6Base {
Ex6Interface getBaseInner() {
return this.new Ex6BaseInner();
}
public static void main(String[] args) {
Ex6 ex = new Ex6();
System.out.println(ex.getBaseInner().say());
}
}



为什么一定要写 public Ex6BaseInner() { } 这句话，不写就报错