Why I'm getting StackOverflowError

Question

public class Category {

    private Category parentCategory;
    private Set<Category> childCategories;
    private String name;

    public Category() {
        childCategories = new HashSet<Category>();
    }

    public Category getParentCategory() {
        return parentCategory;
    }

    public void setParentCategory(Category parentCategory) {
        this.parentCategory = parentCategory;
    }

    public Set<Category> getChildCategories() {
        return childCategories;
    }

    public void setChildCategories(Set<Category> childCategories) {
        this.childCategories = childCategories;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Category [childCategories=" + childCategories + ", name="
                + name + ", parentCategory=" + parentCategory + "]";
    }

}


public static void main(String[] args) {
        Category books = new Category();
        books.setName("Books");
        books.setParentCategory(null);

        Category novels = new Category();
        novels.setName("Novels");
        novels.setParentCategory(books);

        books.getChildCategories().add(novels);
        //novels.setChildCategories(null);

        System.out.println("Books > " + books);
    }

The System.out.println is generating the StackOverflowError.

Answer 1

When you do your toString(), you call the toString() of the children. No problem here except that you call the toString() of the parent in here. Which will call the toString() of the children, etc.

Nice infinite loop.

The best way to get rid of it is to change your toString() method into :

@Override
public String toString() {
    return "Category [childCategories=" + childCategories + ", name="
            + name + ", parentCategory=" + parentCategory.getName() + "]";
}

This way you don't print the parentCategory but only its name, no infinite loop, no StackOverflowError.

EDIT: As Bolo said below you will need to check that parentCategory is not null, you might have a NullPointerException if it is.

---

Resources :

• Javadoc - StackOverflowError

On the same topic :

• toString() in java
• StackOverFlowError in Java postfix calculator

Answer 2

Since the error is the System.out.println the problem must be in the toString().

The problem is that toString() prints both parent and child Category object for the one your printing using their toString() method. So when you print a Category it calls toString() on the parent which calls toString() on the child which calls toString() on the parent which calls toString() on the child and so on until the stack is exhausted.