SQL查询未返回预期结果，移至其他类正常，疑似构造器/返回值问题

Hey Nick, awesome work narrowing down the problem to either a constructor exception or an ignored return value—you’re already halfway there! Let’s dig into both scenarios and walk through actionable checks to fix this:

1. Check for Uncaught Constructor Exceptions #

It’s super common for constructor errors to get swallowed silently, especially if you’re not explicitly handling them in your database query flow. Here’s what to look for:

• Does your Speler constructor throw exceptions? If you’re initializing a Speler from the ResultSet but the constructor throws (e.g., invalid data types, missing required fields), this will skip setting your speler variable to a valid object—and if you don’t have a catch block, you might never see the error.
• Add explicit error handling when creating Speler instances: Wrap the constructor call in a try-catch to log any issues, like this:

ResultSet rs = query.executeQuery();
if (rs.next()) {
    try {
        // Replace with your actual constructor parameters
        speler = new Speler(rs.getString("username"), rs.getString("password"));
    } catch (Exception e) {
        e.printStackTrace(); // This will show exactly what's breaking the constructor
        System.err.println("Failed to create Speler from result set: " + e.getMessage());
    }
}

• Verify your try-with-resources has a catch block: If your current method doesn’t catch exceptions, any error (including constructor issues) will bubble up to the caller. If the caller isn’t handling it, you might think the method is returning null when it’s actually throwing an uncaught exception.

2. Rule Out Ignored or Misused Return Values #

If the constructor is working fine, the next culprit is likely how you’re using the method’s return value in your original class:

• Are you actually storing the return value? Double-check the code that calls geefSpeler—it’s easy to accidentally call the method without assigning it to a variable:

// ❌ Wrong: Method is called but return value is ignored
geefSpeler("myUsername");
// ✅ Correct: Assign the returned Speler to a variable
Speler activeSpeler = geefSpeler("myUsername");

• Is there a method overload causing confusion? Make sure you’re not calling a different version of geefSpeler in your original class (e.g., a method with the same name but different parameters that returns null by default).
• Could the method be overridden? If your original class is a subclass that overrides geefSpeler, you might be executing a different implementation entirely—check for @Override annotations or mismatched method signatures.

Quick Debugging Tips to Confirm #

• Add print statements to trace execution: Stick simple logs in your geefSpeler method to confirm it’s running and what it’s returning:

public Speler geefSpeler(String username) {
    System.out.println("Running geefSpeler for username: " + username);
    Speler speler = null;
    // ... rest of your code ...
    System.out.println("Returning Speler object: " + speler);
    return speler;
}

• Validate your database connection: Ensure Connectie.JDBC_URL is the same in both the working class and the problematic one—you might be connecting to a different database instance without realizing it!

内容的提问来源于stack exchange，提问作者Nick L