Program throws Stack Overflow Error

Question

The following program compiles correctly. What is causing the Stack Overflow Error? How is the exception pushed on to the stack?

public class Reluctant {

    private Reluctant internalInstance = new Reluctant();

    public Reluctant() throws Exception {
        throw new Exception("I’m not coming out");
    }

    public static void main(String[] args) {
        try {
            Reluctant b = new Reluctant();
            System.out.println("Surprise!");
        } catch (Exception ex) {
            System.out.println("I told you so");
        }
    }
}

Tagir Valeev · Accepted Answer · 2015-05-31 17:27:14Z

4

You have a field initialization code which is automatically added to the constructor body by javac compiler. Effectively your constructor looks like this:

private Reluctant internalInstance;

public Reluctant() throws Exception {
    internalInstance = new Reluctant();
    throw new Exception("I’m not coming out");
}

So it calls itself recursively.

answered May 31, 2015 at 17:27

Tagir Valeev

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3 Comments

Surely Over a year ago

just know this, your answer helps!

Sorter Over a year ago

Didn't know the field initialization moves to constructor. So, if its the first becomes statement like you mentioned. The exception is never thrown. Thanks to recursion.

Tagir Valeev Over a year ago

@Sorter: and if you have multiple constructors, the same field initialization code is added to every single constructor (just after superclass initialization, but before actual constructor body).

elixenide · Accepted Answer · 2015-05-31 17:25:35Z

4

This line in your main method results in infinite recursion:

Reluctant b = new Reluctant();

Every time you try to create an instance of Reluctant, the first thing you do is create another instance, which creates another instance, which creates another instance, which... You get the idea.

Voila, stack overflow!

answered May 31, 2015 at 17:25

elixenide

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1 Comment

Sorter Over a year ago

That's the line where the recursion starts.

ssantos · Accepted Answer · 2015-05-31 17:26:15Z

2

You have an infinite loop. Every instance of Reluctant class instantiates another Reluctant object on internalInstance declaration. So, when you instantiate the first Reluctant object on your main method, the program creates one instance after another, over and over again, till the stack overflows.

Replace this line.-

private Reluctant internalInstance = new Reluctant();

for

private Reluctant internalInstance;

answered May 31, 2015 at 17:26

ssantos

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1 Comment

Sorter Over a year ago

Well explained the recursion process.

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