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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 The Java Language Specification (JLS), §6.3.2, "Obscured Declarations" [JLS 2005]. This can result in , 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 type-import-on-demand declaration (see the JLS, §7.5.2, "Type-Import-on-Demand Declaration" [JLS 2005]) 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. This , 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.

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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");
    }
  }
}

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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
}

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], Item 16, "Prefer interfaces Interfaces to abstract classesAbstract Classes." Changing the original class into an interface would permit class MyVector to declare that it implements the hypothetical Vector interface. This would permit With this technique, client code that intended to use MyVector to would remain compatible with code that uses the original implementation of Vector.

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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

Bibliography

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