DCL05-J. Do not reuse public identifiers from the Java Standard Library

Do not reuse identifiers, public utility classes, interfaces, and packages in the Java Standard Library.

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] a class name from java.util.Vector, as specified by §6.3.2 of the Java Language Specification [[JLS 2005]]. 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 the type-import-on-demand declaration (see §7.5.2, "Type-Import-on-Demand Declaration" of the Java Language Specification [[JLS 2005]]). 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.

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 java.util.Vector.isEmpty() method.

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");
    }
  }
}

Compliant Solution (Class Name)

This compliant solution uses a different name for the class, preventing any potential shadowing.

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]] "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 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

Bibliography

DCL04-J. Prevent class initialization cycles      01. Declarations and Initialization (DCL)      02. Expressions (EXP)