If a developer uses an identifier that has the same name as 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 unintentionally use the custom {{Vector}} instead of 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 [§6.3.2|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.2] of the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\]. This can result in unexpected program behavior.

Well-defined import statements can resolve these issues. However, when reused name definitions are imported from other packages, use of the _type-import-on-demand declaration_ (see [§7.5.2|http://java.sun.com/docs/books/jls/third_edition/html/packages.html#7.5.2], "Type-Import-on-Demand Declaration" of the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\]). Additionally, 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. 

class Vector {
  private int val = 1;

  public boolean isEmpty() {
    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");
    }
  }
}

class MyVector {
  //other code
}

When 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}}.

\[[JLS 2005|AA. Bibliography#JLS 05]\] 
[§6.3.2|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.2] "Obscured Declarations" 
[§6.3.1|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.1] "Shadowing Declarations"
[§7.5.2|http://java.sun.com/docs/books/jls/third_edition/html/packages.html#7.5.2] "Type-Import-On_Demand Declaration" 
[§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