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In a JVM a class is identified by its fully-qualified class name AND its classloader. A class with the same name but different package name is different, and a class with the same fully-qualified name but which has been loaded with a different classloader is also different. "Two classes are the _same class_ (and therefore the _same type_) if they are loaded by the same class loader and they have the same fully qualified name" \[JVMSpec 99 §2§2.8.1\].

You may frequently want to know whether a given object has a specific class, or whether 2 objects have the same class, for example, in implementing the equals() method. If the comparison is performed incorrectly, your code might assume that 2 objects are of the same class when they're not.

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In this non-compliant solution, the code compares the name of the class of object h to the string "DefaultAuthenticationHandler", and proceeds according to whether this comparison succeeds or not.

  // determine whether object h has required/expected class name
if (h.getClass().getName().equals("com.example.application.auth.DefaultAuthenticationHandler")) {
        // code assumes it's an authorized class
}

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In this compliant solution, we compare the class object of h itself to the class object of the class that the current class loader would load with our required name.

  // determine whether object h has required/expected class name
if (h.getClass() == this.getClassLoader().loadClass("com.example.application.auth.DefaultAuthenticationHandler")) {
        // code determines authorized class loaded by same classloader
}

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This code compares the names of the 2 classes of objects x and y and behaves accordingly.

  // determine whether objects x and y have same class name
if (x.getClass().getName().equals( y.getClass().getName() )) {
        // code assumes objects have same class
}

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This compliant solution correctly compares the 2 objects' classes

  // determine whether objects x and y have same class
if (x.getClass() == y.getClass()) {
        // code determines objects have same class
}

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1. Wiki Markup
   \[[JVMSpec 99|AA. Java References#JVMSpec 99]\] [§2§2.8.1 Class Names|http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html]

2. Wiki Markup
   \[[Christudas 05|AA. Java References#Christudas 05]\]

3. Wiki Markup
   \[[Mcgraw 98|AA. Java References#Mcgraw 98]\]

4. Wiki Markup
   \[[Wheeler 03|AA. Java References#Wheeler 03]\] [Java|http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/java.html]

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OBJ33-J. Limit extensibility of classes and methods      06. Objects Orientation (OBJ)      OBJ35-J. Use checked collections against external code