If you extend a class without no-argument constructor, then you must explicitly call super(). : Constructor « Object Oriented « SCJP

1. SCJP
2. Object Oriented
3. Constructor

public class MainClass {
  public static void main(String[] argv) {
    new MySubclass();
  }
}

class MyClass {
  public MyClass(int i) {
    System.out.println("MyClass");
  }
}

class MySubclass {
  public MySubclass() {
    super(0);
    System.out.println("MySubclass");
  }
}

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The constructor Object(int) is undefined

	at MySubclass.(MainClass.java:15)
	at MainClass.main(MainClass.java:3)