16-2: JAVA super constructor and abstract class

1.다음 코드에서 생성자로 인한 오류를 찾아내어 이유를 설명하고 오류를 수정하라.

class A {
   private int a;
   protected A(int i) { a = i; }
}
class B extends A {
   private int b;
   public B() { b = 0; }
}

• Javac builds default constructor when developer didnt write about the constructor of the class.
• Even child class should have default super constructor when developer didnt write the super constructor in child class.
• The only constructor in class A needs parameter int a. But in the default super constructor in class B doesnt have any parameter.
• So javac is not able to compile this code.

//
// simple solution
//
class A {
   private int a;
   protected A(int i) { a = i; }
}
class B extends A {
   private int b;
   //public B() { b = 0; }		//removed
   public B() {
       super(0);				//parameter int 0 added in super constructor
       b = 0; 
   }
}

2.다음 추상 클래스의 선언이나 사용이 잘못된 것을 있는 대로 가려내고 오류를 지적하라.

(1)abstract class A {
      void f();
   }
//   
(2)abstract class A {
      void f() { System.out.println("~"); }
   }
//   
(3)abstract class B {
      abstract void f();
   }
   class C extends B {
   }
//   
(4)abstract class B {
      abstract int f();
   }
   class C extends B {
      void f() { System.out.println("~"); }
   }

• (1): f() must have method body in the class A.
• (2): OK.
• (3): class C needs to implements the method body of f().
• (4): void f() must be int f() to override the abstract method f() in class B.