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Reuse of identifier names in subscopes leads to obscuration or shadowing; that . That is, the names identifiers in the current scope mask render those defined elsewhere . Name reuse creates ambiguity and inaccessible. While the JLS clearly resolves any syntactic ambiguity arising from obscuring or shadowing, such ambiguity burdens code maintenance, especially when code requires access to both the original named entity and the entity with the reused nameinaccessible one. The problem is aggravated when the reused name is defined in a different package.

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This implies that a variable can obscure a type or a package, and a type can obscure a package name. Shadowing, on the other hand, refers to masking variables, fields, types, method parameters, labels, and exception handler parameters in a subscope. Both these differ from hiding one variable rendering another variable inaccessible in a containing scope. One type can also shadow another type. Both overriding and shadowing differ from hiding, wherein an accessible member (typically non-private) that should have been inherited by a subclass is replaced by a locally declared subclass member that assumes the same name but has a different and incompatible method signature.

No other identifier should obscure or shadow another identifier in a containing scope. For instance, a local variable should share not reuse the name of a class field if the other variable is in a subscope of the field. A block should not declare a variable with the same name as a variable declared in any block that contains it. Reusing variable names leads to programmer confusion about which variable is being modified. Additionally, if variable names are reused, generally one or both of the variable names are too genericor method, or the class name or package name. Likewise an inner class name should not reuse the name of an outer class or package.

Noncompliant Code Example (Field Shadowing)

This noncompliant code example reuses the name of the val instance field in the scope of an instance method. This behavior can be classified as shadowing.

class MyVector {
  private int val = 1;
  private void doLogic() {
    int val;
    //...   
  }
}

Compliant Solution (Field Shadowing)

This compliant solution eliminates shadowing by changing the name of the variable defined in method scope.

class MyVector {
  private int val = 1;
  private void doLogic() {
    int newValue;
    //...   
  }
}

Exceptions

SCP02-EX1: Reuse of names is permitted for trivial loop counter declarations in the same scope:

for (int i = 0; i < 10; i++) {/* ... */}
for (int i = 0; i < 20; i++) {/* ... */}

Risk Assessment

Name reuse makes code more difficult to read and maintain. This can result in security weaknesses.

Automated Detection

An automated tool can easily detect reuse of names in containing scopes.

Related Guidelines

C Secure Coding Standard: DCL01-C. Do not reuse variable names in subscopes

C++ Secure Coding Standard: DCL01-CPP. Do not reuse variable names in subscopes

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 6.3.2|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.2] "Obscured Declarations", [Section 6.3.1|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.1] "Shadowing Declarations", [Section 7.5.2|http://java.sun.com/docs/books/jls/third_edition/html/packages.html#7.5.2] "Type-Import-On_Demand Declaration", [Section 14.4.3|http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.4.3] "Shadowing of Names by Local Variables"
\[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 67: All Strung Out
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 16: Prefer interfaces to abstract classes
\[[Kabanov 2009|AA. Bibliography#Kabanov 09]\]
\[[Conventions 2009|AA. Bibliography#Conventions 09]\] 6.3 Placement
\[[FindBugs 2008|AA. Bibliography#FindBugs 08]\]

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