Declaring an instance of a class inside that class

Question

This code shows error at run time:

class Animal {
    Animal object1 = new Animal();

    public static void main(String[] args) {     
        Animal obj =          


        

Answer 1

Short answer infinite recursion.

Long answer if you want recursive data structures like that, you can do something like this:

public class A {
  A object1;
  public A(A member) { 
    this.object1 = member; 
  }
  public static void main(String[] args) {
    A item = new A(new A(null)); // note the base case/termination of the recursion!
  }
}

Or:

public class B {
  B object1;
  public void init() {
    object1 = new B();
  }
  public static void main(String[] args) {
    B item = new B();
    item.init(); 
    // now item.object1 != null, but item.object1.object1 == null
  }
}

In either case, you will have “sentinel” or “leaf” nodes in your data structure, which point to a null value.