子类继承父类，麻烦看一下这段代码怎么改,并说明下原因，谢谢

class A
{
  public A(int x)
  {}
}
class B extends A
{
  public B()
  {}
}
public class C
{
  public static void main(String[] args)
  {
  B b=new B();
  }
}

class A
{
  public A(int x)
  {}
}
class B extends A
{
  public B(int x)
  {
  super(x);
  }
}
public class C
{
  public static void main(String[] args)
  {
  B b=new B(x);
  }
}

class A
{
int b;

public A(int x)
{
this.b = x;
}
public int getB()
{
return b;
}
}
class B extends A
{
public B(int x)
{
super(x);
}
}
public class C
{
public static void main(String[] args)
{
B b=new B(3);
System.out.println(b.getB());
}
} 
是不是这样？