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Reuse

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identifier

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names

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subscopes

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obscuration

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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.

According to §6.4.2, "Obscuring," of the JLS [JLS 2013],

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.

  That is, the identifiers in the current scope render those defined elsewhere inaccessible.  While the JLS 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 the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], [Section 6.3.2|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.2], "Obscured Declarations"
{quote}
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.
{quote} 

This implies that a variable can [obscure|BB. Definitions#obscure] a type or a package, and a type can obscure a package name. [Shadowing|BB. Definitions#shadow], 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|BB. Definitions#hide], 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.

h2. 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. 

{code:bgColor=#FFcccc}
class MyVector {
  private int val = 1;
  private void doLogic() {
    int val;
    //...   
  }
}
{code}

h2. Compliant Solution (Field Shadowing)

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

{code:bgColor=#ccccff}

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 val to newValue:

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

h2. Noncompliant Code Example 

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Code Block
bgColor #FFcccc
class MyVector { private int i = 0; of {{i}} 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++) {/* ... */} } } h2.

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Code Block
bgColor #ccccff
block. {code:bgColor=#ccccff} class MyVector { private void doLogic() { for (int i = 0; i < 10; i++) {/* ... */} for (int i = 0; i < 20; i++) {/* ... */} } } {code} h2. Risk Assessment Name reuse makes code more difficult to read and maintain. This can result in security weaknesses. || Guideline || Severity || Likelihood || Remediation Cost || Priority || Level || | EXP15-J | low | unlikely | medium | {color:green}{*}P2{*}{color} | {color:green}{*}L3{*}{color} | h3. Automated Detection An automated tool can easily detect reuse of names in containing scopes. h2. Related Guidelines C Secure Coding Standard: [seccode:DCL01-C. Do not reuse variable names in subscopes] C+\+ Secure Coding Standard: [cplusplus:DCL01-CPP. Do not reuse variable names in subscopes] h2. 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 \[[FindBugs 2008|AA. Bibliography#FindBugs 08]\] ---- [!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|MET17-J. Do not increase the accessibility of overridden or hidden methods]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|VOID 05. Scope (SCP)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|OBJ17-J. Do not expose sensitive private members of an outer class from within a nested class]

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 identifiers in containing scopes.

Automated Detection

Bibliography

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