Java: Enhancing Scope Safety: The forget Keyword

OVERVIEW

FEATURE SUMMARY:
The forget keyword prevents further access to a variable, parameter, or field within a defined scope, attempts to access the variable in forbiden scope will result in a compile-time error.

MAJOR ADVANTAGE:
This change makes variable and resource lifetimes explicit and compiler-enforced, improving code clarity and predictability.

MAJOR BENEFITS:

• Allows explicitly removing a variable from the active context (in terms of accessibility), which is currently:
  • impossible for final variables (only comments can be used),
  • impossible for method parameters (except assigning null to non-final references),
  • impossible for fields,
  • cumbersome for local variables, requiring artificial blocks (extra lines and indentation).
• Makes it possible to explicitly declare that a variable should no longer be used or no longer represents valid data in the current scope.
• Preserves code quality over time, avoiding degradation caused by = null assignments, comments-only conventions, or artificial scoping blocks.

MAJOR DISADVANTAGE:
The introduction of a new reserved keyword introduces potential source incompatibilities with existing codebases that define identifiers named forget.

ALTERNATIVES:
Java currently provides only scope-based lifetime control (blocks and try-with-resources). It lacks a general, explicit, and compiler-enforced mechanism to terminate variable usability at an arbitrary point within an existing scope.

EXAMPLES

Simple and Advanced Examples:


forget var; // Variable is forgotten for the remainder of the current block or method (default behavior)
forget var : if; // Variable is forgotten inside the entire if statement, including else and else-if branches
forget var : for; // Variable is forgotten for the entire for-loop
forget var : while; // Variable is forgotten for the entire while-loop
forget var : try; // Variable is forgotten inside the try block (useful with resources)
forget var : label; // Variable is forgotten inside the labeled block (any loop or code section)
forget var : static; // Field is forgotten inside the static initialization block
forget var : method; // Variable is forgotten for the remainder of the enclosing method (method-wide)

forget (var1, var2, ...); // Specified variables are forgotten for the remainder of the current block

forget this.field; // Specified field are forgotten for the remainder of the current block

forget (var1, var2, ...) { /* code */ }; // Specified variables are forgotten only inside the enclosed block

void handleRequest(String request, String token) {
   if (!isTokenValid(token)) {
      throw new SecurityException("Invalid token");
   }

   authorize(request, token);

   forget token /* used & contains sensitive info */;

   process(request);

   logger.debug("token was: " + token); // Compile-time error: 'token' has been forgotten and cannot be used
}

public Product(String name) { // constructor
   this.name = name.trim().intern();
   forget name; // From now on, only use 'this.name'!

   // other constructor commands...

   if (isDuplicate(this.name)) { ... } // Always canonical, never raw input
   if (isDuplicate(name)) { ... } // Compile-time ERROR!
}

   // * Forces usage of the correctly prepared value (this.name) only.
   // * Prevents code drift, maintenance bugs, or copy-paste errors that reference the raw parameter.
   // * Makes the constructor safer: no risk of mismatches or inconsistent logic.
   // * Reads as a contract: "from here on, don't touch the original argument!"


Next Version Examples:


forget ClassName.field;
forget variable.field;
forget !(variable); // Limit allowed variables to ones that are directly specified

DETAILS

SPECIFICATION:
   forget [ Identifier | ( IdentifierList ) ] [ : Scope | { block }];

   IdentifierList:
   Identifier {, Identifier}

   Identifier:
   [ VariableIdentifier | this.FieldIdentifier ]

The forget statement forbids any further use of the specified identifier in all subsequent expressions and statements within the declared scope in which the identifier would normally be accessible.

COMPILATION:
The variable is not physically erased (except it may be if not a field); rather, it is protected from any further access after the forget statement. Additionally, retaining the variable in the scope (but inaccessible) prevents situations where a developer tries to create a new variable with the same name after removing the forget statement, thereby enforcing consistent usage and avoiding hidden bugs.

TESTING:
Testing the forget statement is equivalent to testing variable scope after exiting a block—the variable becomes inaccessible. For fields, forget enforces access control, ensuring the field cannot be used within the specified scope for the remainder of its block or method.

LIBRARY SUPPORT:
No additional library support is required.

REFLECTIVE APIs:
There is no impact on reflective APIs.

OTHER CHANGES:
No other language changes are introduced.

MIGRATION:
No automatic migration steps are needed.

COMPATIBILITY

The introduction of a new keyword (forget) may cause conflicts in codebases where forget is already used as an identifier. There are no other compatibility impacts.

REFERENCES

• earlier draft

PROBLEMS

• Backward Compatibility: Introducing forget as a new reserved keyword will cause compilation errors in existing code that already uses forget as an identifier (variable, method, class, etc).
• Tooling Lag: IDEs, static analysis tools, and debuggers must all be updated to handle the new keyword and its effects on variable visibility.
• Code Readability: Misuse or overuse of forget could make code harder to maintain or follow if not used judiciously, especially if variables are forgotten in non-obvious places.
• Teaching and Onboarding: This feature introduces a new concept that must be documented and taught to all developers, which can increase the learning curve for Java.
• Migration Complexity: Automatic migration of legacy code may be challenging, particularly for projects that rely on forget as an existing identifier or which have established conventions for variable lifetime.
• Interaction with Scoping and Shadowing: The detailed behavior when variables are forgotten, shadowed, or reintroduced in inner scopes may lead to confusion and subtle bugs if not carefully specified and implemented.
• Reflection and Debugging: While reflective APIs themselves are not impacted, developers may be surprised by the presence of variables at runtime (for debugging or reflection) that are "forgotten" in the source code.
• Consistency Across Language Features: Defining consistent behavior for forget in new contexts (e.g., lambdas, anonymous classes, record classes) may require extra specification effort.
• Edge Cases and Specification Complexity: Fully specifying the semantics of forget for all cases—including fields, parameters, captured variables in inner/nested classes, and interaction with try/catch/finally—may be complex.
• Unused Feature Risk: There is a risk that the forget keyword will see little real-world use, or will be misunderstood, if not supported and encouraged by frameworks or coding standards.

SUMMARY

The forget keyword represents a natural evolution of Java's commitment to clear, explicit, and compiler-enforced language rules. By allowing developers to mark variables, parameters, or fields as no longer usable within a defined scope, forget makes variable lifetimes and resource management visible and deliberate. This approach eliminates ambiguity in code, prevents accidental misuse, and reinforces Java’s tradition of making correctness and safety a language guarantee, not just a convention. Like Java’s robust type system and scoping mechanisms, forget enhances code clarity, maintainability, and reliability.