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Avoid Returning null from a Boolean Method

High
correctnessreliability

What is it?​

This practice is triggered by returning null from a Boolean method, which could lead to a NullPointerException if the caller assumes only true or false will be returned.

Why apply it?​

Returning null from a Boolean method without appropriate annotations leads to runtime exceptions and makes your code less robust. Using annotations informs callers that the return value could be null, and allows them to handle the result safely.

How to fix it?​

Annotate methods returning Boolean with @javax.annotation.Nullable or @javax.annotation.CheckForNull to inform callers of potential null values. Ensure callers check for null when necessary.

Examples​

Example 1:​

Negative​

The negative example returns null without annotation, leading to potential NullPointerException.

public Boolean isServerHealthy() {
  // Perform health checks
  return null;  /* Noncompliant */
}

public void performAction() {
  if (isServerHealthy()) {  // Could throw NullPointerException
    // Server is healthy; proceed with action
  }
}

Example 2:​

Positive​

The positive example uses annotations to inform the caller that a Boolean return value might be null. The caller properly checks for nullity.

@javax.annotation.Nullable
public Boolean isServerHealthy() {
  // Perform health checks
  return null;  // Indicates unknown state
}

public void performAction() {
  Boolean serverStatus = isServerHealthy();
  if (Boolean.TRUE.equals(serverStatus)) {
    // Server is healthy; proceed with action
  } else {
    // Handle unhealthy or unknown state
  }
}

Negative​

The negative example does not use any annotations, making it unclear that null might be returned.

public Boolean isFeatureEnabled() {
  // Check feature flag
  return null;  /* Noncompliant */
}

public void useFeature() {
  if (isFeatureEnabled()) {  // Could lead to NullPointerException
    // Use the feature
  }
}

Example 3:​

Positive​

The positive example uses @javax.annotation.CheckForNull to clearly indicate that the method can return null.

@javax.annotation.CheckForNull
public Boolean isFeatureEnabled() {
  // Check feature flag
  return null;  // Feature flag status is unavailable
}

public void checkFeature() {
  Boolean featureStatus = isFeatureEnabled();
  if (Boolean.TRUE.equals(featureStatus)) {
    // Feature is enabled; use feature
  } else {
    // Feature is disabled or status is unknown
  }
}