java.lang.NullPointerException: Cannot invoke

JDK 14+ helpful NPE — tells you exactly which expression was null. 1. Use Optional for values that may be absent: `Optional.ofNullable(s).map(String::length)`. 2. Add `@Nullable` / `@NonNull` annotations (JSR-305, JetBrains) + enable IDE inspections. 3. For DTOs from JSON: configure Jackson with `spring.jackson.default-property-inclusion=non_null` and validate at the edges.