Reuse of identifier names in subscopes leads to obscuration or shadowing. Reused identifiers in the current scope can render those defined elsewhere inaccessible. Although the Java Language Specification (JLS) [JLS 2013] clearly resolves any syntactic ambiguity arising from obscuring or shadowing, such ambiguity burdens code maintainers and auditors, 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.
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 example, a local variable should not reuse the name of a class field or method or a 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; the method variable renders the instance variable inaccessible within the scope of the method. For example, assigning to
val from within the method does not affect the value of the instance variable, although assigning to
this.val from within the method does.
Compliant Solution (Field Shadowing)
This compliant solution eliminates shadowing by changing the name of the variable defined in the method scope from
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 the instance variable
i defined in the
Compliant Solution (Variable Shadowing)
In this compliant solution, the loop counter
i is defined in the scope of each
for loop block:
Name reuse makes code more difficult to read and maintain, which can result in security weaknesses. An automated tool can easily detect reuse of identifiers in containing scopes.
Puzzle 67, "All Strung Out"
Item 16, "Prefer Interfaces to Abstract Classes"