Overview of Java's Null‑Restricted and Nullable Types Preview Feature

Java developers have long struggled with handling null references.

To address this pain point, Java is introducing a new preview feature—Null‑Restricted and Nullable types—that lets developers explicitly declare whether a type permits null values, providing stronger compile‑time and runtime type safety.

Examples:

Foo! is a non‑null restricted type—its acceptable values do not include null .

Foo? is a nullable type—its acceptable values explicitly include null .

Foo does not specify nullability—its acceptance of null is undefined.

Feature highlights:

Explicit expression of null‑value constraints: use Foo! to disallow null , Foo? to allow it, while Foo remains unspecified.

Compatibility with existing code: the new feature can be introduced gradually without breaking legacy Java code or standard libraries.

Automatic detection and warning of potential null‑pointer exceptions: the compiler inserts runtime checks when converting between nullable and non‑null types, preventing null ‑induced crashes.

Strict initialization of fields and arrays with null constraints: variables declared with null restrictions must be initialized before use, otherwise an exception is thrown (e.g., String![] labels requires each element to be initialized).

Flexible nullness conversion: types can be converted similarly to boxing/unboxing, allowing natural transitions between different nullability levels while still detecting risks at runtime.

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