CODEWITHSUNDEEP
java
Rupesh Yadav
Rupesh Yadav

Reputation: 12312

Java: StackOverflowError

I am working on Inner Class concepts and write the below code_

public class InnerClassConcepts {
private int x = 11;

public static void main(String... args) {
    // method local 'x' variable
    int x = new InnerClassConcepts().new InnerA().new InnerB().x;

    InnerClassConcepts in = new InnerClassConcepts();
    InnerA InnerA = in.new InnerA();
    InnerA.InnerB xx = in.new InnerA().new InnerB();

    System.out.println("InnerClassConcepts`s x = " + in.x);
    System.out.println("InnerB`s x = " + xx.x);
    System.out.println("main`s x(local variable) = " + x);
    // System.out.println("InnerA`s x = " + InnerA.x);
    System.out.println("InnerA`s y = " + InnerA.y);
}

/**
 * Local inner class.
 */
class InnerA {
    int y = 10;

    /*
     * java.lang.StackOverflowError coming over here... I don`t
     * understand why?
     */
    // int x=new InnerClassConcepts().new InnerA().new InnerB().x;//Line-29
    /**
     * Inner class inside an another Inner class.
     */
    class InnerB {
        private int x = 22;
        int z = InnerA.this.y;

    }

 }
}

Output_

InnerClassConcepts's x = 11
InnerB's x = 22
main's x(local variable) = 22
InnerA's y = 10

I am wondering why StackOverflowError is coming on line-29 when i uncomment line-29. Where as the same is working in main method.

Can somebody please help me where i am wrong or what concept is behind this?

Upvotes: 1

Views: 194

Answers (5)

mwhs
mwhs

Reputation: 5977

Inner classes should be static. Or do you need a new class declaration per instance?

See this answer to explain why: whats-the-advantage-of-making-an-inner-class-as-static-with-java

Also see this findbugs bug description: finbugs: inner class should be static

If you would make your inner classes static you could see that it doesn't compile the way you wrote it.

Upvotes: -2

René Link
René Link

Reputation: 51473

When an instance of InnerA is created the variable x gets initialized which causes an InnerA to be created and another x gets initialized and so on.

Thus the StackOverflowError

// int x=new InnerClassConcepts().new InnerA().new InnerB().x;//Line-29

In the stack trace of the StackoverFlowError you should see that the Constructor of InnerA is invoked again and again.

Something like this

at InnerClassConcepts$InnerA.<init>(InnerClassConcepts.java:31)
at InnerClassConcepts$InnerA.<init>(InnerClassConcepts.java:31)
at InnerClassConcepts$InnerA.<init>(InnerClassConcepts.java:31)

Upvotes: 5

Jesper
Jesper

Reputation: 206996

Let's make the example a bit simpler, to explain what happens:

class Example {
    private Example example = new Example();
}

What happens when you create a new Example object?

The new object is created, and then its member variables are initialized. Class Example has a member variable that is an Example. So, to initialize the member variable, a new Example object is created. That one in turn has a member variable of type Example that needs to be initialized. So, again a new Example object is created. And that one again has an Example member variable that needs to be initialized. This goes on forever.

You have an infinite recursive cycle there, which leads to a StackOverflowError.

You have exactly the same thing in your class InnerA when you uncomment line 29: member variable x is initialized with a new InnerA, which has its own member variable that is initialized with a new InnerA, etc.

Upvotes: 5

Matthias
Matthias

Reputation: 3592

You are initializing InnerA's member x by creating a new InnerA instance. So this will never end :)

Upvotes: 0

micha
micha

Reputation: 49612

The line 

int x = new InnerClassConcepts().new InnerA().new InnerB().x;

is executed every time you create a new instance of InnerA. Since this line creates another instances of InnerA it will be called again and again..

Upvotes: 0

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