Feature flags concept in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does a feature flag affect Spring Boot application behavior?

Consider a Spring Boot app with a feature flag controlling a new payment method. What happens when the flag is OFF?

Spring Boot

public boolean isNewPaymentEnabled() {
    return featureFlagService.isEnabled("newPayment");
}

if (isNewPaymentEnabled()) {
    // use new payment method
} else {
    // use old payment method
}

A The app uses the old payment method only.

B The app uses both old and new payment methods simultaneously.

C The app throws an error because the feature flag is off.

D The app disables all payment methods.

💡 Hint

Think about what the flag controls and what happens when it is false.

When the feature flag is OFF, the app skips the new payment method and uses the old one as fallback.

📝 Syntax

intermediate

Which code snippet correctly checks a feature flag in Spring Boot?

You want to check if a feature flag named "betaFeature" is enabled using a FeatureFlagService. Which option is correct?

A if (featureFlagService.isEnable("betaFeature")) { /* enabled */ }

B if (featureFlagService.enabled("betaFeature")) { /* enabled */ }

C if (featureFlagService.isEnabled("betaFeature")) { /* enabled */ }

D if (featureFlagService.check("betaFeature")) { /* enabled */ }

💡 Hint

Look for the common naming pattern for boolean checks in Java.

The method is usually named isEnabled to indicate a boolean check. Other options are invalid method names.

❓ state_output

advanced

What is the output when toggling a feature flag at runtime?

Given a Spring Boot app where a feature flag "darkMode" is initially OFF, what happens if you enable it at runtime and then reload the page?

Spring Boot

boolean darkMode = featureFlagService.isEnabled("darkMode");
System.out.println("Dark mode enabled: " + darkMode);

A Prints true initially and false after reload.

B Always prints false because flags can't change at runtime.

C Throws an exception when toggling the flag.

D First prints false, then after enabling and reload, prints true.

💡 Hint

Feature flags can be dynamic if implemented properly.

When the flag is toggled on, the app reads the new value on reload and reflects the change.

🔧 Debug

advanced

Why does the feature flag check always return false?

You wrote this code but the feature flag check always returns false even when enabled: @Service public class FeatureFlagService { private Map flags = Map.of("newUI", false); public boolean isEnabled(String flag) { return flags.getOrDefault(flag, false); } } What is the likely cause?

A The flags map is immutable and never updated after initialization.

B The isEnabled method has a typo in the flag name.

C The service is not annotated with @Component.

D The flags map keys are case sensitive and mismatched.

💡 Hint

Think about how Map.of works in Java.

Map.of creates an immutable map. You cannot update flags later, so the flag stays false.

🧠 Conceptual

expert

What is the main benefit of using feature flags in Spring Boot applications?

Choose the best explanation for why developers use feature flags in Spring Boot projects.

A They automatically fix bugs in the code at runtime.

B They allow turning features on or off without redeploying the app.

C They improve database query performance by caching results.

D They replace the need for unit testing by isolating features.

💡 Hint

Think about how feature flags help with releasing new features safely.

Feature flags let teams enable or disable features dynamically, avoiding full redeploys and reducing risk.