Stackoverflow error

问题:

The code given below shows a Stackoverflow error when run.But if I make another class CarChange to create objects of Car ,it runs sucessfully. I am a beginner ,doing this code to understand the importance of upcasting in java.

public class Car {

    int i;
    Car[] c=new Car[2];

    Car() {
        c[0] = new Polo();
        i=0;
    }



    void drive(){
        c[i].testdrive(); //the overloaded method in subclasses polo and swift
    }

    void change() {
        if(i==0) { 
            i++; 
            c[i] = new Swift();
        }
    }

    public void testdrive() {
        //overloaded method in subclasses polo and swift
        System.out.println(" test drive car");
    }



//class Tester {
     //main
     Car c= new Car();
     c.drive();
     c.change();
     c.drive();

回答1:

A stackoverflow usually means you have an infinite loop.

The reason you're receiving this is because you're calling drive from the testdrive method and in that method you're calling drive again.

回答2:

Car() {
    c[0] = new Polo();
    i=0;
}

As Polo is a subclass of Car() - it must be to fit in the Car[] - it will call the Car's constructor when being constructed itself. The Car constructor tries to create a new Polo().

As Polo is a subclass of Car() - it must be to fit in the Car[] - it will call the Car's constructor when being constructed itself. The Car constructor tries to create a new Polo().

As Polo is a subclass of Car() - it must be to fit in the Car[] - it will call the Car's constructor when being constructed itself. The Car constructor tries to create a new Polo().

... you get the picture?

回答3:

It sounds like you might have some infinite recursion happening.

drive() calls testdrive() which class drive() which calls testdriver()...forever, or until you run out of memory, hence your stack overflow error.