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<code>NullPointerException</code> in Java — Causes and Fixes

Updated on 2026-04-21 Reviewed by the Java With Us editorial team

NullPointerException (NPE) is the most frequent runtime exception in Java. It's thrown whenever you try to use a reference that holds null: call a method on it, read a field, index an array, unbox it into a primitive, or throw it.

The five patterns that cause 95% of NPEs

// 1. Method call on null
String s = null;
s.length();                              // NPE

// 2. Field access on null
User u = null;
u.name;                                  // NPE

// 3. Array indexing on null
int[] a = null;
a[0] = 1;                                // NPE

// 4. Auto-unboxing null wrapper
Integer i = map.get("missing");          // null — map has no such key
int    v = i;                            // NPE on unbox

// 5. Returning null from a method that autoboxes
int count = users.size() == 0 ? null : users.size(); // compiles, NPE if true branch runs

Helpful NPE messages (Java 14+)

Modern JVMs print the exact expression that was null:

Exception in thread "main" java.lang.NullPointerException:
    Cannot invoke "String.length()" because "order.customer.name" is null
    at com.example.Service.process(Service.java:42)

Enable with -XX:+ShowCodeDetailsInExceptionMessages (on by default since Java 15). Saves minutes of bisection.

The right fixes

1. Don't return null — use Optional

// ❌
public User findById(long id) {
    return db.lookup(id);   // might be null
}

// ✅
public Optional<User> findById(long id) {
    return Optional.ofNullable(db.lookup(id));
}
// Caller is forced to handle the empty case:
service.findById(42).ifPresent(u -> send(u));

2. Fail fast at boundaries

public void register(User u, String email) {
    this.user  = Objects.requireNonNull(u,     "user");
    this.email = Objects.requireNonNull(email, "email");
    ...
}

3. Prefer collection factories over nullable returns

return matches.isEmpty() ? null : matches;  // ❌ callers must null-check
return matches;                               // ✅ empty list is a valid answer

4. Use Objects.equals for null-safe equality

Objects.equals(a, b);    // handles a == null or b == null

5. Annotate nullability

Tools like Spotless, ErrorProne, NullAway, IntelliJ inspections enforce @Nullable / @NonNull annotations at compile time.

Common mistakes

  • Catching NPE — it's a bug, not a recoverable condition. Fix the cause.
  • Returning null from a public API — forces every caller to remember the null-check. Return Optional or an empty collection.
  • Checking null after use — s.length(); if (s != null) — the NPE already happened.

Related

Pillar: Java exceptions. Siblings: try/catch, IllegalArgumentException. Debug quickly with the Java Online Compiler.

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