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The Java standard library provides many useful Do not reuse the names of publicly visible identifiers, public utility classes, interfaces, and packages. Do not use the identifiers representing these items to refer to some distinct item.

If a developer uses an identifier that reuses the name of a public class, such as {{Vector}}, a subsequent maintainer might not be aware that this identifier does not actually refer to {{java.util.Vector}}, and might incorrectly use the custom {{Vector}} when their intention was to use the original {{java.util.Vector}} class. The custom type {{Vector}} can [shadow|BB. Definitions#shadow] a class name from {{java.util.Vector}}), as specified by Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], [Section 6.3.2.|http://java.sun.com/docs/books/jls/third_edition/html/packages.html#6.3.2]. This can cause unexpected program behavior.

or packages in the Java Standard Library.

When a developer uses an identifier that has the same name as a public class, such as Vector, a subsequent maintainer might be unaware that this identifier does not actually refer to java.util.Vector and might unintentionally use the custom Vector rather than the original java.util.Vector class. The custom type Vector can shadow a class name from java.util.Vector, as specified by The Java Language Specification (JLS), §6.3.2, "Obscured Declarations" [JLS 2005], and unexpected program behavior can occur.

Well-defined import statements can resolve these issues. However, when reused name definitions are imported from other packages, use of the Wiki MarkupWell-defined import statements can 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 \[[JLS 2005|AA. Bibliography#JLS 05]\], [Section 7.5.2|http://java.sun.com/docs/books/jls/third_edition/html/packages.html#7 (see §7.5.2], "Type-Import-on-Demand Declaration"[JLS 2005]) 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. can complicate a programmer's attempt to determine which specific definition was intended to be used. Additionally, a common practice that can lead to errors is to produce the import statements after writing the code, often via automatic inclusion of import statements by an IDE, which creates further ambiguity with respect to the names. When a custom type is found earlier than the intended type in the Java include path, no further searches are conducted. Consequently, the wrong type is silently adopted.

Noncompliant Code Example (Class Name)

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. Unexpected behavior can arise if a maintainer confuses the isEmpty() method with the java.util.Vector.isEmpty() method.

class Vector {
  private int val = 1;

  public boolean isEmpty() {
    if (val == 1) {   // comparesCompares with 1 instead of 0
      return true;
    } else {
      return false;
    }
  }
  // otherOther 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 (Class Name)

This compliant solution uses a different name for the class, preventing any potential shadowing. of the class from the Java Standard Library:

class MyVector {
  //otherOther code
}

Wiki MarkupWhen the developer and organization control the original shadowed class, 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}}.

Risk Assessment

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

], 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. With this technique, client code that intended to use MyVector would remain compatible with code that uses the original implementation of Vector.

Risk Assessment

Public identifier reuse decreases the readability and maintainability of code.

Automated Detection

An automated tool can easily detect reuse of the set of names representing public classes or interfaces from the Java Standard Library.

Related Guidelines

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Bibliography

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\[[JLS 2005|AA. Bibliography#JLS 05]\] [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"
\[[FindBugs 2008|AA. Bibliography#FindBugs 08]\]
\[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 67: All Strung Out
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 16: Prefer interfaces to abstract classes

[Bloch 2005]	Puzzle 67, "All Strung Out"
[Bloch 2008]	Item 16, "Prefer Interfaces to Abstract Classes"
[FindBugs 2008]
[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"

 

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Image Added Image Added Image AddedMET17-J. Do not increase the accessibility of overridden or hidden methods      Image Removed      OBJ17-J. Do not expose sensitive private members of an outer class from within a nested class