Question

Consider following 2 programs giving same error
First calss:

public class Testing {    
    Testing t=new Testing();        
    public static void main(String args[]){   
        testing t1=new testing();  
    }
}

Second class:

class foo{
    void baz(){
        new testing();
    }
}
public class testing {
    testing t=new testing();    
    public static void main(String args[]){
    foo f=new foo();
        f.baz();
    }       
}

how does above code giving following error?
I know instance of class is creating recursively but I want to know how?

Exception in thread "main" java.lang.StackOverflowError
    at com.Testing.<init>(Testing.java:4)
    at com.Testing.<init>(Testing.java:4)

also Why doesn’t this happen if we do

    public class testing { 
    testing t2=new testing(); 
    testing t1=new testing(); 
    public static void main(String args[])
    {//anything}
    }

as t1 will require t2 object to be initalized and vice-versa?

Answer 1

This is the problem:

public class Testing {    
    Testing t=new Testing();  
}

This is broadly the same as:

public class Testing {
    Testing t;

    public Testing() {
        t = new Testing();
    }
}

That makes the recursion clearer – the constructor is unconditionally calling itself.

Each instance of Testing contains a reference to another instance of Testing. That needn’t be a problem – but it is here, because it’s creating a new instance of Testing. So creating the “outermost” instance immediately triggers the creation of another one… which triggers the creating of another one, etc.

Your two classes aren’t significantly different in that respect – creating any instance of Testing will cause a stack overflow.

EDIT: Regarding this code (names and formatting improved):

public class Testing { 
    Testing t1 = new Testing(); 
    Testing t2 = new Testing(); 

    public static void main(String[] args) {
        //anything
    }
}

will still throw the same exception if you try to create an instance of Testing. You’re wrong to state that “t1 will require t2 object to be initialized” though – there’s no interdependence between the variables here. Each variable initializer creates a separate object… but each of those objects would in turn try to create two more instances of Testing.

Note that an instance of Testing is not required in order to run the main method – but if that main method either calls new Testing directly, or calls something else which calls new Testing(), then you’ll get the stack overflow.