How do I call a method within a setString

I am trying to use a setString to call a method from another class, but I get an error message that says that <variableName> cannot be resolved to a variable.

In my Tag class, I have:

import java.util.List;
import models.Book;


public class Tag {

	private String isbn_13;
	private String tag_name;
	
	public Tag(String isbn, String tagName) {
		this.isbn_13 = isbn;
		this.tag_name = tagName;
	}
	
	public Tag() {
		this.isbn_13 = null;
		this.tag_name = null;
	}
	
	public String getIsbn13() {
		return isbn_13;
	}

	public void setIsbn13(String isbn) {
		this.isbn_13 = isbn;
	}
	
	public String getTag() {
		return tag_name;
	}
	
	public void setTag(String tagName) {
		this.tag_name = tagName;
	}
	
	public void add(List<Tag> book_tags) {
		// TODO Auto-generated method stub
		
	}

}

Now, in my TagImpl class, the relevant method is

public boolean addTag(Tag tag) {
		try {
			connection = DAOUtilities.getConnection();
			String sql = "INSERT INTO Book_tags VALUES (?, ?)"; 
			stmt = connection.prepareStatement(sql);
			
			stmt.setString(1, tag.setIsbn13(isbn)); // Error here "isbn cannot be resolved to a variable"
			stmt.setString(2, tag.setTag(tagName)); // Error here "tagName cannot be resolved 
                                                    // to a variable"
			
			if (stmt.executeUpdate() != 0)
				return true;
			else
				return false;
			
		} catch (SQLException e) {
			e.printStackTrace();
			return false;
		} finally {
			closeResources();
		}
	}

I have looked at all the documentation I can think of, and the tutorials I can find, and all of them look like this is correct. The only difference is that some of them show empty parentheses, i.e. stmt.setString(1, tag.setIsbn13()); but that just gives a different error, "The method setIsbn13(String) in the type Tag is not applicable for the arguments ()" Also, it makes no difference if I change (isbn) to (isbn_13) or (tagName) to (tag_name) -- it still "cannot be resolved to a variable.

You have to get the isbn_13 and tag_name from tag and set into stmt.

Replace

stmt.setString(1, tag.setIsbn13(isbn)); 
stmt.setString(2, tag.setTag(tagName));

with

stmt.setString(1, tag.getIsbn13()); 
stmt.setString(2, tag.getTag());

I'm not sure what you are trying to do with the setIsbn13 call, there. I think that you just want tag.getIsbn13(). Are you looking to get the Isbn13 value out of the tag and send it to the stmt.setString() function? Presumably, your method was passed the tag, which has already been filled with values, and you want to use those values in your preparedStatement.

You don't have anything named isbn in your hand, which is the problem. But even if you did, you'd still be in trouble, because setIsbn13 method returns a void, which you then are trying to pass the result to stmt.setString. That will be an error, too.

I assume you're trying to do this. You could inline the call, but I made a new local variable to make it more clear what's happening.

public boolean addTag(Tag tag) {
    try {
        connection = DAOUtilities.getConnection();
        String sql = "INSERT INTO Book_tags VALUES (?, ?)"; 
        stmt = connection.prepareStatement(sql);

        String isbn = tag.getIsbn13();
        String tagName = tag.getName();  // You named this getTag, but please change it.
        
        stmt.setString(1, isbn); 
        stmt.setString(2, tagName);
        
        if (stmt.executeUpdate() != 0)
            return true;
        else
            return false;
        
    } catch (SQLException e) {
        e.printStackTrace();
        return false;
    } finally {
        closeResources();
    }
}