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Reuse of identifier names in subscopes leads to obscuration or shadowing, that is, the names . Reused identifiers in the current scope mask can render those defined elsewhere . Name reuse creates ambiguity and burdens code maintenanceinaccessible. 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 entity with the reused nameinaccessible one. The problem is aggravated exacerbated when the reused name is required to be defined in a different package.unmigrated-wiki-markup

According to the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\] section 6.3.2 "Obscured Declarations"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 masking of variables, fields, types, method parameters, labels and exception handler parameters in a subscope. Both these differ from hiding wherein , 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 non-privatenonprivate) that should have been inherited by a subclass is forgone in lieu of replaced by a locally declared subclass member that assumes the same name .

In general, do not

...

but has a different, incompatible method signature.

Noncompliant Code Example (Field Shadowing)

This noncompliant code example implements a class that reuses the name of the class java.util.Vector. It attempts to introduce a different condition for the isEmpty() method for interfacing with native legacy code, by overriding the corresponding method in java.util.Vector.

A future programmer may not know about this extension and may incorrectly use the custom Vector class when his intention was to use the original java.util.Vector class. The custom type Vector can obscure a class name from another package (e.g. java.util.Vector), as specified by JLS 6.3.2 (see above). Should this occur, it may cause undesirable effects by violating the programmer's assumptions.

Wiki MarkupWell defined import statements resolve these issues. However, when the definitions of the reused name are imported from other packages, use of the _type-import-on-demand declaration_ (see Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\] section 7.5.2 "Type-Import-on-Demand Declaration") can lead to unexpected import of a class that was not intended. Moreover, a common --- and potentially misleading --- tendency is to produce the import statements _after_ writing the code, often via automatic inclusion of import statements by an IDE. This creates further ambiguity with respect to the names; when a custom type is found earlier in the Java include path than the intended type, no further searches are conducted. val instance field in the scope of an instance method.

class VectorMyVector {
  private int val = 1;

  publicprivate booleanvoid isEmptydoLogic() {
    int if(val == 1) {   //compares with 1 instead of 0
      return true;
    } else {
      return false;
    }
  }
  //other functionality is same as java.util.Vector
}

// import java.util.Vector; omitted

public class VectorUser {
  public static void main(String[] args) {
    Vector v = new Vector();
    if(v.isEmpty()) {
      System.out.println("Vector is empty");
    }
  }
}

Compliant Solution

This compliant solution declares the class Vector with a different name.

class MyVector {
  //other code
}

Note: When the developer and her organization control the original hidden class in addition to the code being written, it may be preferable to change the design strategy of the original in accordance with Bloch's _Effective Java_ \[[Bloch 2008|AA. Bibliography#Bloch 08]\] "Item 16: Prefer interfaces to abstract classes." Changing the original class into an interface would permit class {{MyVector}} to declare that it implements the hypothetical {{Vector}} interface. This would permit client code that intended to use {{MyVector}} to remain compatible with code that uses the original implementation of {{Vector}}.

Noncompliant Code Example

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.

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

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

Compliant Solution

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

private void doLogic() {
  int newValue;
  //...   
}

Noncompliant Code Example

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 MyVector class:Method shadowing in different scopes becomes possible when two or more packages are used. Method shadowing differs from method overloading in that subclasses are allowed to inherit overloadings defined in the base class. It differs from hiding in that the methods need not be declared static. It is also distinct from method overriding as exemplified in this noncompliant code example.

package x;
public class AMyVector {
  voidprivate doLogic()int {i // default accessibility= 0;
    // print 'A'  
  }  
  public static void main(String[] argsprivate void doLogic() {
    Afor a(i = new y.C();
    a.doLogic(); // invokes doLogic() of class x.B and prints 'B'
  }
}

package x;
public class B extends A {
  void doLogic() { // default accessibility
    // print 'B'  
  } 
}

package y; // different package
public class C extends x.B { // public accessibility 
  public void doLogic() { 
    // print 'C'
  } 
}

Note that class y.C is accessible from the package x and so is its doLogic() method. However, if the main() method defined in class A tries to polymorphically invoke y.doLogic() as shown, the override corresponding to class B in package x takes precedence. This is because the doLogic() methods in classes x.A and x.B are not visible from class y.C due to the default access specifier. As a result, the class x.C is not considered a part of the overriding hierarchy. Note, however, that the code behaves as expected if the access specifiers of all of the methods are changed to public.

Compliant Solution

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:Use a different name to indicate that the class residing in another package is not intended to be part of the overriding chain. A programmer can use dotted notation to explicitly specify which class defines the method to be invoked. Avoid reusing names even when all the relevant classes define the affected methods with a public access specifier; future evolution of one or more of the classes may reduce method accessibility, leading to unexpected results.

package x;
public class AMyVector {
  private void doLogic() { 
    // print 'A'  
  }  

  public static void main(String[] args) {
    // explicitly invokes doSequence() of class y.C and prints 'C'
    y.C.doSequence(); 
  }
}

package x;
public class B { for (int i = 0; i < 10; i++) {/* ... */ }

package y; // different package
public class C extends x.B {  
  public void doSequence() { // now renamed
    // print 'C' 
  } 
}

Exceptions

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

for for (int i = 0; i < 1020; i++) {/* ... */} 
for(int i = 0; i < 20; i++) {  }
}

...

Applicability

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

Automated Detection

An automated tool can easily detect reuse of names whose earlier definition appears somewhere in the Java include path. FindBugs, for example, detects at least four sub-instances of this guideline.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Other Languages

This guideline appears in the C Secure Coding Standard as DCL01-C. Do not reuse variable names in subscopes.

This guideline appears in the C++ Secure Coding Standard as DCL01-CPP. Do not reuse variable names in subscopes.

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] 6.3.2 "Obscured Declarations", 6.3.1 "Shadowing Declarations", 14.4.3 "Shadowing of Names by Local Variables"
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Puzzle 67: All Strung Out
\[[Kabanov 2009|AA. Bibliography#Kabanov 09]\]
\[[Conventions 2009|AA. Bibliography#Conventions 09]\] 6.3 Placement
\[[FindBugs 2008|AA. Bibliography#FindBugs 08]\]:
Nm: Class names shouldn't shadow simple name of implemented interface  
Nm: Class names shouldn't shadow simple name of superclass
MF: Class defines field that masks a superclass field
MF: Method defines a variable that obscures a field

identifiers in containing scopes.

Automated Detection

Bibliography

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Image Added Image Added Image AddedSCP01-J. Do not increase the accessibility of overridden or hidden methods      05. Scope (SCP)      SCP03-J. Do not expose sensitive private members of the outer class from within a nested class