Stack Overflow Exception in Java

Question

I am new to Thread. I have created two classes name A and B as follow-

public class A {

    private B b;

    public void setB(B b) {
        this.b = b;
    }

    synchronized void foo() {
        b.foo();
        System.out.println("Hi A");
    }
}

public class B {

    private A a;

    public void setA(A a) {
        this.a = a;
    }

    synchronized void foo() {
        a.foo();
        System.out.println("Hi B");
    }
}

Now i have created two other classes which implements Runnable interface.

public class AThread implements Runnable{
    private A a;
    public AThread(A a){
        this.a = a;
    }
    @Override
    public void run() {
        a.foo();
    }

}

public class BThread implements Runnable {
    private B b;
    public BThread(B a){
        this.b = b;
    }
    @Override
    public void run() {
        b.foo();
    }

}

In main method, i have written the following code-

public static void main(String args[]) {
        A a = new A();
        B b = new B();
        a.setB(b);
        b.setA(a);
        Runnable r1 = new AThread(a);
        Runnable r2 = new BThread(b);
        Thread t1 = new Thread(r1);
        t1.start();

    }

When i am running this code, i got the following exception.

Exception in thread "Thread-0" java.lang.StackOverflowError
    at student.B.foo(B.java:21)
    at student.A.foo(A.java:21)..

Can any one explain what is the route cause of it and how can i solve it?

Answer 1

What did you expect?

You have a foo() method in A calling the foo() method in B that called the foo() method in A, and so on, until the stack overflows.

You can solve it by avoiding circular method calls.