why cant we have a parameterized constructor of parent class when no value is being passed in the child class?

class a{
    a(int age){}
}
class b extends a{}
public class HelloWorld {
    public static void main(String[] args) {
        b var1=new b();
        System.out.println("Try programiz.pro");
    }
}

above code doesnt work.if i include the statement super(10) in child class constructor, it works.

but without any constructor in a ,it works like below

class a{}
class b extends a{}
public class HelloWorld {
    public static void main(String[] args) {
        b var1=new b();
        System.out.println("Try programiz.pro");
    }
}

if that is because we are required to give default constructor of super class in 2nd code we havent specified the constructor and still it works.can you explain why?

Solution

In Java, the compiler inserts a default constructor if and only if no explicit constructor has been defined for the class¹. This default constructor:

Has zero parameters.
Has the same visibility as the class
Invokes the superclass's zero-parameter constructor².

That last bit is why your first code example fails to compile. Your A class has defined an explicit constructor which has one parameter, thus it does not have a zero-parameter constructor to invoke from the default constructor of subclass B.

First case

When you have:

public class A {

  // Explicit constructor
  public A(int arg) {}
}

public class B extends A {}

That is the same as writing:

public class A extends Object {

  // Note: No default constructor.

  // Explicit constructor
  public A(int arg) {
      super(); // OKAY: The 'Object' class has a no-arg constructor.
  }
}

public class B extends A {

  // Default constructor
  public B() {
      super(); // ERROR: The 'A' class does not have a no-arg constructor.
  }
}

Second case

When you have:

public class A {}

public class B extends A {}

That is the same as writing:

public class A extends Object {

  // Default constructor
  public A() {
    super(); // OKAY: The 'Object' class has a no-arg constructor.
  }
}

public class B extends A {

  // Default constructor
  public B() {
    super(); // OKAY: The 'A' class has a no-arg constructor.
  }
}

^{1. See §8.8.7. Constructor Body and §8.8.9. Default Constructor of the Java Language Specification.}

^{2. Note every constructor has an implicit call to super() unless there is an explicit call to a super constructor or an explicit delegation to another constructor in the same class (i.e., this(...)). See §8.8.7. Constructor Body of the Java Language Specification.}