StackOverflowError when try create new object of inherit class in superclass

问题

I am newbie in Java. Can anyone explain to me why it show StackOverflowError ?

public class MainClass {

    static Start st = new Start();

    public static void main(String[] args) {        
        st.start();
    }
}


public class Start {

    Generator objGenerator = new Generator();

    void start() {      
        objGenerator.generator();
    }
}


public class Generator extends Start {

    void generator() {
        //...
    }
}

If Generator class is not inherited from class Start, everything is ok, but why ?

回答1:

Generator inherits from Start

class Generator extends Start

And Start creates a Generator on construction:

Generator objGenerator = new Generator();

Which is the same as the following:

public class Start {

    Generator objGenerator;

    public Start() {
         objGenerator = new Generator();
    }
}

---

1. Start has constructor that runs objGenerator = new Generator().

2. This calls the constructor for Generator.

3. The first thing that the constructor for Generator does is call super().

4. super() is default constructor for Start.

5. GOTO 1.

回答2:

When an Instance of Generator is created, the constructor of Start is called because Generators extends Start. This is called constructor chaining.

However, when you call the constructor of start you also have a variable thats call new Generator...

You create a Generator that is a Start that creates a Generator that is a Start... and its goes on until your stack overflows

回答3:

The reason for the StackOverflow error is because the recursive instantiation of objects does not terminate. The recursive object intstantiation is as follows.

Step 1: static Start st = new Start();

Step 2: Instantiating a Start object requires instantiating a Generator object due to the initialization of the member variable objGenerator.

Generator objGenerator = new Generator();

Step 3: Since Generator is a subclass of Start, instantiating a Generator object requires instantiating Start object, which is going back to step 2.

So in effect you are in a infinite loop going back and forth between step 2 and step 3. Upon hitting the stack limit, the StackOverflow exception is thrown.

来源：https://stackoverflow.com/questions/49709691/stackoverflowerror-when-try-create-new-object-of-inherit-class-in-superclass