DCL51-JG. Do not shadow or obscure identifiers in subscopes

Reuse of identifier names in subscopes leads to obscuration or shadowing. The reused identifiers in the current scope render those defined elsewhere inaccessible. Although 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 exacerbated when the reused name is defined in a different package.

According to §6.4.2, "Obscuring," of the Java Language Specification [JLS 2011],

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 nonprivate) 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. The resulting 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 the 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 defined in the scope of the doLogic() method.

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

Applicability

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

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

Bibliography