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  1. PO
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    <p>since you can read chinese you can look at my post <a href="http://www.hetaoblog.com/myblogs/post/%E8%AF%B4%E4%B8%80%E8%AF%B4java%E9%87%8C%E9%9D%A2%E7%9A%84hashcode-string-hashcode.jhtml">http://www.hetaoblog.com/myblogs/post/%E8%AF%B4%E4%B8%80%E8%AF%B4java%E9%87%8C%E9%9D%A2%E7%9A%84hashcode-string-hashcode.jhtml</a></p> <p>see a test method, basically, so long as you match, a1*31+b1 = a2*31 +b2, which means (a1-a2)*31=b2-b1</p> <pre><code>public void testHash() { System.out.println("A:" + ((int)'A')); System.out.println("B:" + ((int)'B')); System.out.println("a:" + ((int)'a')); System.out.println(hash("Aa".hashCode())); System.out.println(hash("BB".hashCode())); System.out.println(hash("Aa".hashCode())); System.out.println(hash("BB".hashCode())); System.out.println(hash("AaAa".hashCode())); System.out.println(hash("BBBB".hashCode())); System.out.println(hash("AaBB".hashCode())); System.out.println(hash("BBAa".hashCode())); } </code></pre> <p>you will get </p> <pre><code>A:65 B:66 a:97 2260 2260 2260 2260 2019172 2019172 2019172 2019172 </code></pre> <p><strong>edit: someone said this is not straightforward enough. I added below part</strong></p> <pre><code> @Test public void testN() throws Exception { List&lt;String&gt; l = HashCUtil.generateN(3); for(int i = 0; i &lt; l.size(); ++i){ System.out.println(l.get(i) + "---" + l.get(i).hashCode()); } } AaAaAa---1952508096 AaAaBB---1952508096 AaBBAa---1952508096 AaBBBB---1952508096 BBAaAa---1952508096 BBAaBB---1952508096 BBBBAa---1952508096 BBBBBB---1952508096 </code></pre> <p>below is the source code, it might be not efficient, but it work:</p> <pre><code>public class HashCUtil { private static String[] base = new String[] {"Aa", "BB"}; public static List&lt;String&gt; generateN(int n) { if(n &lt;= 0) { return null; } List&lt;String&gt; list = generateOne(null); for(int i = 1; i &lt; n; ++i) { list = generateOne(list); } return list; } public static List&lt;String&gt; generateOne(List&lt;String&gt; strList) { if((null == strList) || (0 == strList.size())) { strList = new ArrayList&lt;String&gt;(); for(int i = 0; i &lt; base.length; ++i) { strList.add(base[i]); } return strList; } List&lt;String&gt; result = new ArrayList&lt;String&gt;(); for(int i = 0; i &lt; base.length; ++i) { for(String str: strList) { result.add(base[i] + str); } } return result; } } </code></pre> <p>look at String.hashCode()</p> <pre><code> public int hashCode() { int h = hash; if (h == 0) { int off = offset; char val[] = value; int len = count; for (int i = 0; i &lt; len; i++) { h = 31*h + val[off++]; } hash = h; } return h; } </code></pre>
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    1. POHow to generate strings that share the same hashcode in Java?
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    1. POHow to generate strings that share the same hashcode in Java?
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    1. COwell, that's fine if this is SO's rule or culture to provide english only link... I just want to provide more to the author; while for the problem itself, i think i've explained enough using demo codes and some words here...
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    2. CO1) Yes it is. 2) The demo codes and words don't actually answer the question. The question is about how to **generate** collisions. An explanation of how/why the collisions occur is not relevant.
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    3. COI think this is a very good answer, although the generated string is very long if N is very large.
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