NullPointerException

NullPointerException — NPE — is thrown when your code tries to use a reference that is null as if it pointed to an object. It is the most common runtime exception in Java, nicknamed "the billion-dollar mistake" after Tony Hoare called out the null reference concept he introduced.

Typical triggers

String s = null;
s.length();                  // NPE
int[] arr = null;
int x = arr[0];              // NPE
arr.length;                  // NPE
Map<String, User> m = getMap();
m.get("missing").getName();  // NPE if key not present (get returns null)

// Auto-unboxing a null Integer:
Integer boxed = null;
int n = boxed;               // NPE

Helpful NullPointerExceptions (Java 14+)

Run with -XX:+ShowCodeDetailsInExceptionMessages (default on recent JVMs) and the message tells you exactly which reference was null:

Cannot invoke "String.length()" because "s" is null

Avoiding NPE

• Return Optional instead of null from methods that might not find a value.
• Objects.requireNonNull(arg) at method entry to fail fast.
• getOrDefault / computeIfAbsent on Maps instead of get.
• Nullability annotations (@Nullable, @NonNull from JetBrains or JSpecify) + static analysis.
• Don't return null collections — return an empty List.of() / Set.of() instead.

Catching NPE

Don't. NPE almost always indicates a bug — catching it hides the bug. The correct fix is to ensure the variable is not null before dereferencing, or use Optional.