Introduction

NullPointerException is one of the most common runtime errors in Java. It occurs when you call a method or access a field on a null reference. This guide shows typical causes and solutions.

What Is NullPointerException?

Thrown by the JVM when you dereference a null pointer. Example:

String text = null;
int len = text.length(); // throws NullPointerException

Common Causes

  1. Calling methods on uninitialized objects.
  2. Automatic unboxing of null wrapper types (e.g., Integer).
  3. Methods returning null unexpectedly.
  4. Missing checks after collection lookups or map gets.

Solution 1: Add Null Checks

if (text != null) {
  int len = text.length();
}

Solution 2: Use Optional

Optional<String> opt = Optional.ofNullable(text);
opt.ifPresent(s -> System.out.println(s.length()));

Solution 3: Proper Initialization

Always initialize objects before use:

List<String> list = new ArrayList<>();
// instead of List<String> list;

Solution 4: Apply Nullability Annotations

Use @NonNull and @Nullable to document intent:

public void process(@NonNull String input) { … }

IDEs and static analysis tools can warn you at compile time.

Solution 5: Avoid Autounboxing Null

Check wrapper types before unboxing:

Integer count = getCount();
if (count != null) {
  int c = count; // safe unboxing
}

Conclusion

NullPointerException is avoidable. Use null checks and initialize fields. Leverage Optional and nullability annotations. Adopt these practices for safer code.

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