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.

Well-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#7.5.2], "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. 

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}}.

\[[JLS 2005|AA. Bibliography#JLS 05]\] [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