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copied!<p>The question is vague, but I believe this excerpt from Angelika Langer's Java Generics FAQ answers it:</p> <blockquote> <h3>Which methods and fields are accessible/inaccessible through a reference variable of a wildcard parameterized type?</h3> <p>It depends on the kind of wildcard.</p> <p>Using an object through a reference variable of a wildcard parameterized type is restricted. Consider the following class: </p> <p>Example (of a generic class): </p> <pre><code>class Box<T> { private T t; public Box(T t) { this.t = t; } public void put(T t) { this.t = t;} public T take() { return t; } public boolean equalTo(Box<T> other) { return this.t.equals(other.t); } public Box<T> copy() { return new Box<T>(t); } } </code></pre> <p>When we use a reference variable of a wildcard instantiation of type Box to access methods and fields of the referenced object the compiler would reject certain invocations. </p> <p>Example (of access through a wildcard parameterized type): </p> <pre><code>class Test { public static void main(String[] args) { Box<?> box = new Box<String>("abc"); box.put("xyz"); // error box.put(null); // ok String s = box.take(); // error Object o = box.take(); // ok boolean equal = box.equalTo(box); // error equal = box.equalTo(new Box<String>("abc")); // error Box<?> box1 = box.copy(); // ok Box<String> box2 = box.copy(); // error } } </code></pre> </blockquote> <p>Essentially <code>?</code> has less information for the generic type system, and thus to enforce typesafety certain invokations must be rejected, because they're not typesafe.</p> <p>A <code>Box<?></code> may be a <code>Box<Integer></code>, a <code>Box<String></code>, or even a <code>Box<Box<?>></code>. Thus given a <code>Box<?> box</code>, <code>box.put("xyz")</code> must be rejected.</p> <h3>References</h3> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html" rel="nofollow noreferrer">Angelika Langer's Java Generics FAQ</a> <ul> <li><a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#FAQ304" rel="nofollow noreferrer">Which methods and fields are accessible/inaccessible through a reference variable of a wildcard parameterized type?</a></li> </ul></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/2776975/how-can-i-add-to-list-extends-number-data-structures">How can I add to <code>List<? extends Number></code> data structures?</a></li> </ul>

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