How do I identify if a class contains a foreign key field?

Question

I'm trying to create a method in Java to create a table in a MySQL database. I have two classes:

User.java

public class User {
    private int userId;
    private String userName;
    private String userEmail;
    private String userPassword;

    // getters and setters
}

Post.java

public class Post {
    private int postId;
    private String postCategory;
    private String postTitle;
    private String post;

    private int userId; // foreign key
    
    // getters and setters
}

createTable() method

public String createTable(Class<?> users) throws IllegalAccessException {
        String message = "";
        String className = printClassName(User.class);

        StringBuilder sql = new StringBuilder("CREATE TABLE IF NOT EXISTS " + className);
        sql.append("(");

        // getting fields
        Field[] fields = users.getDeclaredFields();
        for ( int i = 0; i < fields.length; i++ ) {
            if ( fields[i].getType() == int.class && fields[i].getName().equals("userId")) {
                sql.append(fields[i].getName()).append(" INT AUTO_INCREMENT NOT NULL");
            } else if ( fields[i].getType() == int.class ) {
                sql.append(fields[i].getName()).append(" INT NOT NULL");
            } else if ( fields[i].getType() == String.class ) {
                sql.append(fields[i].getName()).append(" VARCHAR(255) NOT NULL");
            }

            if ( i < fields.length - 1 ) {
                sql.append(", ");
            }

            if ( i == fields.length - 1 ) {
                sql.append("PRIMARY KEY (").append(fields[0].getName()).append(")");
            }
        }
        

        sql.append(");");
        System.out.println("Executing Sql Query: "+ sql.toString());

        try {
            int i = 0;
            dataBaseConnection.pstmt = dataBaseConnection.conn.prepareStatement(sql.toString());
            i = dataBaseConnection.pstmt.executeUpdate();

            if ( i > 0 ) {
                message = "table created with name: " + className + " successfully...";
            } else {
                message = "something went wrong...";
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return message;
    }

The problem is to how to know which table has a foreign key and which table does not?

This method works perfectly for creating tables without foreign keys, but I don't get the logic for ones with a foreign key. I want to create a method createTable() which works well for both kinds of tables. I can achieve this task by creating annotations, but I don't want annotation because every time the annotation needs to be processed first. I am looking for an alternate solution.

Answer 1

Since you're already using reflection, you can do the same here to check the name of the field to see if it contains id. Change your primary key for each class to just be called id, so all other ___Id's are easily identified as foreign keys.

public class Post {
    private int id;
    // ...
}

public class User {
    private int id;
    // ...
}

(Alternatively, you could check the name of the class using reflection and ignore the field that contains the name of the class.)

Field[] fields = users.getDeclaredFields();
for ( int i = 0; i < fields.length; i++ ) {
    if (fields[i].getName().contains("Id")) {
        // This field is a foreign key
    }
}