NullPointerException ☕ Java

java.lang.NullPointerException

Code called a method or accessed a field on a reference that is null.

Seen on: Java Android

Meaning

Since Java 14, helpful NPE messages tell you exactly what was null: Cannot invoke "String.length()" because "name" is null. Read that message and the first stack frame in your own package.

Common causes

  • Uninitialized field or variable
  • Method returned null (map lookup, repository find)
  • Dependency injection didn’t populate a field (@Autowired on a new-created object)
  • Auto-unboxing a null Integer to int
  • Android: view not found (findViewById returns null) or used after destroy

⚡ Quick fix

  1. Read the “because X is null” part of the message
  2. Find the first stack frame in your code
  3. Return Optional or empty collections instead of null
  4. Validate inputs with Objects.requireNonNull(x, "x")

Detailed fix by platform

Java

  1. Optional instead of null checks:
    java
    String city = Optional.ofNullable(user)
        .map(User::getAddress)
        .map(Address::getCity)
        .orElse("Unknown");

Android

  1. In Kotlin, prefer non-null types and ?./?:; in Java, annotate with @Nullable/@NonNull so lint warns you.

Code examples

Helpful NPE message (Java 14+)

text
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "name" is null
    at com.example.App.greet(App.java:12)

How to diagnose

  1. Message — What is null (Java 14+ tells you)?
  2. Frame — First frame in your package
  3. Origin — Where should the value have been set?

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