StackOverflowError in toString()

Why This Happens

When two classes reference each other and both include the other in toString(), printing either object creates infinite recursion. This commonly happens with bidirectional JPA relationships (Parent has List<Child>, Child has Parent) where Lombok @ToString or IDE-generated toString includes all fields.

The Problem

class Parent {
    List<Child> children;
    public String toString() { return "Parent{children=" + children + "}"; }
}
class Child {
    Parent parent;
    public String toString() { return "Child{parent=" + parent + "}"; } // Infinite loop!
}

The Fix

class Parent {
    List<Child> children;
    public String toString() { return "Parent{children=" + children + "}"; }
}
class Child {
    Parent parent;
    public String toString() { return "Child{parentId=" + parent.getId() + "}"; } // No recursion
}

Step-by-Step Fix

1. 1

   Identify the recursive toString

   Look at the stack trace for repeated toString() calls between two or more classes.

2. 2

   Find the circular reference

   Identify bidirectional relationships where both classes include each other in toString().

3. 3

   Break the cycle

   Exclude the back-reference from toString(), use only IDs, or use @ToString.Exclude in Lombok.