1. 1. Type System: The key difference between Hack and Java lies in their type systems. Hack incorporates a gradual typing system, allowing for gradually adding type annotations to an existing codebase. On the other hand, Java follows a static typing system, requiring explicit type declarations for variables and parameters before they can be used. This distinction provides flexibility in Hack by introducing optional types and a smoother transition for dynamic languages.
2. 2. Null Safety: Another major difference is the approach to null safety. In Hack, the type system provides built-in support for handling nullable and non-nullable types. This means that programmers can specify whether a variable can have a null value or not, reducing the chance of null pointer exceptions. In Java, null safety is not built into the type system, leading to a higher risk of null pointer exceptions if not handled adequately.
3. 3. Error Handling: Hack and Java also differ in their error handling mechanisms. In Hack, error handling is primarily done through exceptions, allowing the programmer to catch and handle specific types of exceptions as needed. Java follows a similar approach but additionally includes checked exceptions, which need to be declared in the method signature or handled explicitly. This difference adds an extra layer of error handling complexity in Java.
4. 4. Interoperability: Hack and Java offer different levels of interoperability. Hack is built on the top of PHP, so it seamlessly integrates with existing PHP codebases. This allows Hack code to call PHP functions and vice versa. In contrast, Java operates on its own runtime environment, and while it supports interoperability with other languages through Java Native Interface (JNI), it doesn't have the same level of integration with PHP as Hack does.
5. 5. Collections and Generics: Hack and Java have some differences in their approach to collections and generics. Hack provides collections with immutable semantics by default, whereas Java offers mutable collections by default. Additionally, Hack introduces shape types, which allow defining structures with specific named fields, providing more expressive and strict type checking compared to Java's generics.
6. 6. Development Tools and Ecosystem: The development tools and ecosystem associated with Hack and Java also vary. Java has a mature set of integrated development environments (IDEs) like Eclipse and IntelliJ IDEA, with extensive plugin support and a large community. Hack, being a relatively newer language, has fewer IDE options with Hack-specific features. However, it benefits from the wider PHP ecosystem and tooling, making it suitable for PHP developers seeking enhanced type safety and gradual typing.

In Summary, Hack and Java differ in their type systems, null safety approach, error handling mechanisms, interoperability with other languages, collections and generics implementation, and the development tools and ecosystem they offer.