NullPointerException
NullPointerException is a Java runtime exception raised when code improperly dereferences or otherwise uses a null reference where an object is required.
100 Java Mistakes and How to Avoid Them
NullPointerException is the Java runtime exception raised when an operation dereferences or otherwise improperly uses a null reference.
- How it is used
- Frames null-related Java failures and prevention across APIs, varargs, initialization, and collections.
- How deep it goes
- Defines nullability concerns and develops focused failure mechanisms, stopping at prevention rules and tool guidance.
- Examples
- Uses null varargs casts, static field ordering, logger initialization, and Set.of contains(null) migration cases.
- What you can do after
- Diagnose/debug with it by tracing nullable flows, initialization order, varargs binding, and collection null policies.
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Kotlin in Action, 2E
NullPointerException is a runtime exception raised when code dereferences a null reference where a non-null object is required.
- How it is used
- Uses nullability checks to prevent runtime dereference failures in JVM backends and Android applications.
- How deep it goes
- Explains nullable types and operators with a compact example, but defers deeper runtime and interoperability edge cases.
- Examples
- Demonstrates nullable and non-nullable String access and cites reduced Google Home crashes after Kotlin adoption.
- What you can do after
- Apply nullable types and Kotlin operators to prevent or explicitly handle possible null dereferences.
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Related concepts
- !! operatorproduced by · The !! operator is Kotlin's non-null assertion operator, which treats a nullable expression as non-null and throws an exception if it is null.
- Objects.requireNonNullproduced by · Objects.requireNonNull is a Java utility method that returns a non-null reference or throws NullPointerException when its argument is null.
- nullable typeaddressed by · A nullable type is a type whose values may be either a value of the underlying type or an explicit representation of absence, such as null.
- exceptionkind of · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
- defensive programmingaddressed by · Defensive programming is a technique that anticipates invalid inputs, failures, misuse, and unsafe states and explicitly guards against their harmful effects.
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