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    <p><a href="http://java.sun.com/javase/6/docs/api/java/lang/Class.html#isAssignableFrom%28java.lang.Class%29" rel="nofollow noreferrer"><code>Class.isAssignableFrom()</code></a> essentially checks the <a href="http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.10" rel="nofollow noreferrer">subtyping relation</a>. "subtype" and "subclass" are two different concepts. The class hierarchy (i.e. subclassing) is only a part of subtyping.</p> <p><a href="http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.10.1" rel="nofollow noreferrer">Primitive types</a> and <a href="http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.10.3" rel="nofollow noreferrer">array types</a> have special cases for subtyping.</p> <p>The rules for subtyping of array types are like this (note that "><sub>1</sub>" means "is a directy subtype of"):</p> <blockquote> <ul> <li>If <code>S</code> and <code>T</code> are both reference types, then <code>S[]</code> ><sub>1</sub> <code>T[]</code> iff <code>S</code> ><sub>1</sub> <code>T</code>.</li> <li><code>Object</code> ><sub>1</sub> <code>Object[]</code></li> <li><code>Cloneable</code> ><sub>1</sub> <code>Object[]</code></li> <li><code>java.io.Serializable</code> ><sub>1</sub> <code>Object[]</code></li> <li>If <code>p</code> is a primitive type, then: <ul> <li><code>Object</code> ><sub>1</sub> <code>p[]</code></li> <li><code>Cloneable</code> ><sub>1</sub> <code>p[]</code></li> <li><code>java.io.Serializable</code> ><sub>1</sub> <code>p[]</code></li> </ul></li> </ul> </blockquote> <p>The important part for your question is the very first item: an array type <code>X[]</code> is a subtype of an array type <code>Y[]</code> if and only if the component type <code>X</code> is a subtype of the component type <code>Y</code>.</p> <p>Also note that strictly speaking neither <code>Object[]</code> nor <code>String[]</code> are classes. They are "only" types. While every class implicitly is a type, the reverse is not true. Another example of types that are not classes are the primitive types: <code>boolean</code>, <code>byte</code>, <code>char</code>, <code>short</code>, <code>int</code>, <code>long</code>, <code>float</code> and <code>double</code> are types, but they are <em>not</em> classes.</p> <p>Another cause for confusion is the fact that you can easily get <code>java.lang.Class</code> objects representing those <em>types</em>. Again: This does <em>not</em> mean that those types are classes.</p>
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    1. POWhy Object[].class.isAssignableFrom(String[].class) == true?
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    1. COwow, where did you get the rules? can you give me a link? it seems String[] is subclass of Object[]. thank you a lot
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    2. CONo, String[] +is not a subclass+ of Object[]! That's what I'm saying all the time! It's a +subtype+, but that's a dfferent thing! I've had the links to the rules in my answer all the time, btw.
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