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copied!<p>When you instantiate a generic type with different type arguments, the two instantiations are not type compatible. Different instantiations of a generic type are not type-equivalent in the way that a class that inherits from another class is type-equivalent to that class, even when the type arguments themselves may be related by inheritance. Genericity and polymorphism are two separate language features.</p> <p>Your code is equivalent to the following:</p> <pre><code>// Java defaults unspecified type arguments to Object. fileHandler<Object, Object> file; if (...) { // Error: fileHandler<String, String> is not equivalent to fileHandler<Object, Object> file = new fileHandler<String, String>(); } else { // Error: fileHandler<String, String[]> is not equivalent to fileHandler<Object, Object> file = new fileHandler<String, String[]>(); } </code></pre> <p>If you truly want to isolate a polymorphic interface that is independent of the types used to instantiate your fileHandler class, then I would suggest you use an interface:</p> <pre><code>interface IFileHandler { // If this interface needs to talk about keys and values, it // does so using only java.lang.Object. } public class fileHandler<Key extends Object, Value extends Object> implements IFileHandler { // ... } public class A extends fileHandler<String, String> { // ... } public class B extends fileHandler<String, String[]> { // ... } IFileHandler file; if (...) { file = new A(); } else { file = new B(); } </code></pre>

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