DCL15-J. Do not shadow or obscure identifiers in subscopes

Reuse of identifier names in subscopes leads to obscuration or shadowing. The identifiers in the current scope render those defined elsewhere inaccessible. While the Java Language Specification clearly resolves any syntactic ambiguity arising from obscuring or shadowing, such ambiguity burdens code maintainers, especially when code requires access to both the original named entity and the inaccessible one. The problem is aggravated when the reused name is defined in a different package.

According to §6.3.2, "Obscured Declarations" of the Java Language Specification [[JLS 2005]]

A simple name may occur in contexts where it may potentially be interpreted as the name of a variable, a type, or a package. In these situations, the rules of 6.5 specify that a variable will be chosen in preference to a type, and that a type will be chosen in preference to a package.

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 one variable rendering another variable inaccessible in a containing scope. One type can also shadow another type.

No identifier should obscure or shadow another identifier in a containing scope. For instance, a local variable should not reuse the name of a class field or method, or the class name or package name. Similarly, an inner class name should not reuse the name of an outer class or package.

Both overriding and shadowing differ from hiding, in which 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, incompatible method signature.

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;
    //...   
  }
}

Noncompliant Code Example (Variable Shadowing)

This example is noncompliant because the variable i defined in the scope of the second for loop block, shadows the definition of i, which is defined in the scope of the doLogic() method.

class MyVector {
  private void doLogic() {
    int i = 0;
    for (i = 0; i < 10; i++) {/* ... */}
    for (int i = 0; i < 20; i++) {/* ... */} 
  }
}

Compliant Solution (Variable Shadowing)

In this compliant solution, the loop counter i is defined in the scope of each for loop block.

class MyVector {
  private void doLogic() {
    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]]
§6.3.2 "Obscured Declarations"
§6.3.1 "Shadowing Declarations"
§7.5.2 "Type-Import-On_Demand Declaration"
§14.4.3 "Shadowing of Names by Local Variables"

[[Bloch 2005]] Puzzle 67: All Strung Out
[[Bloch 2008]] Item 16: Prefer interfaces to abstract classes
[[Kabanov 2009]]
[[Conventions 2009]] 6.3 Placement
[[FindBugs 2008]]

DCL14-J. Minimize the scope of variables      01. Declarations and Initialization (DCL)      DCL16-J. Do not reuse public identifiers from the Java Standard Library