The Java standard library provides many useful public utility classes, interfaces, and packages. Do not use the identifiers representing these items to refer to some distinct item.
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.
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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 |
A maintainer might not know about this extension and incorrectly use the custom Vector class when his intention was to use the original java.util.Vector class. The custom type Vector can obscure a class name from another package (for example, java.util.Vector), as specified by JLS 6.3.2 (see above). Should this occur, it can cause undesirable effects by violating the programmer's assumptions| Wiki Markup |
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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 Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], [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 commonâandand potentially misleadingâtendencymisleading 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.
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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");
}
}
}
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Compliant Solution (Class Name)
This compliant solution declares the class Vector with uses a different name for the class, preventing any potential shadowing.
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class MyVector {
//other code
}
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Note: When the developer and organization control the original hiddenshadowed class, in addition to the code being written, 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}}. |
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