InterfaceCannotBeImplemented.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>


<overview>
<p>
An interface that contains methods whose return types clash with protected
methods on <code>java.lang.Object</code> can never be implemented, because
methods cannot be overloaded based simply on their return type.</p>

</overview>
<recommendation>

<p>
If the interface is useful, name methods so that they
do not clash with methods in <code>Object</code>. Otherwise you should delete the interface.
</p>

</recommendation>
<example>

<p>In the following example, the interface <code>I</code> is useless because the 
<code>clone</code> method must return type <code>java.lang.Object</code>:</p>

<sample src="InterfaceCannotBeImplemented.java" />

<p>
Any attempt to implement the interface produces an error:
</p>

<pre>
InterfaceCannotBeImplemented.java:6: clone() in C cannot override
  clone() in java.lang.Object; attempting to use incompatible return
  type
found   : int
required: java.lang.Object
  public int clone() {
             ^
1 error
</pre>

</example>
<references>


<li>
Help - Eclipse Platform:
<a href="https://help.eclipse.org/2020-12/advanced/content.jsp?topic=/org.eclipse.jdt.doc.user/reference/preferences/java/compiler/ref-preferences-errors-warnings.htm">Java Compiler Errors/Warnings Preferences</a>.
</li>
<li>
Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-9.html#jls-9.2">9.2 Interface Members</a>.
</li>


</references>
</qhelp>