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    <p>Follow a very simple tentative with dynamic proxy: </p> <pre><code>class ImmutableBuilder { static &lt;T&gt; T of(Immutable immutable) { Class&lt;?&gt; targetClass = immutable.getTargetClass(); return (T) Proxy.newProxyInstance(targetClass.getClassLoader(), new Class&lt;?&gt;[]{targetClass}, immutable); } public static &lt;T&gt; T of(Class&lt;T&gt; aClass) { return of(new Immutable(aClass, new HashMap&lt;String, Object&gt;())); } } class Immutable implements InvocationHandler { private final Class&lt;?&gt; targetClass; private final Map&lt;String, Object&gt; fields; public Immutable(Class&lt;?&gt; aTargetClass, Map&lt;String, Object&gt; immutableFields) { targetClass = aTargetClass; fields = immutableFields; } public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (method.getName().equals("toString")) { // XXX: toString() result can be cached return fields.toString(); } if (method.getName().equals("hashCode")) { // XXX: hashCode() result can be cached return fields.hashCode(); } // XXX: naming policy here String fieldName = method.getName(); if (method.getReturnType().equals(targetClass)) { Map&lt;String, Object&gt; newFields = new HashMap&lt;String, Object&gt;(fields); newFields.put(fieldName, args[0]); return ImmutableBuilder.of(new Immutable(targetClass, newFields)); } else { return fields.get(fieldName); } } public Class&lt;?&gt; getTargetClass() { return targetClass; } } </code></pre> <p>usage:</p> <pre><code>interface Person { String name(); Person name(String name); int age(); Person age(int age); } public class Main { public static void main(String[] args) { Person mark = ImmutableBuilder.of(Person.class).name("mark").age(32); Person john = mark.name("john").age(24); System.out.println(mark); System.out.println(john); } } </code></pre> <p>grow directions:</p> <ul> <li>naming policy (getName, withName, name) </li> <li>caching toString(), hashCode()</li> <li>equals() implementations should be straightforward (although not implemented)</li> </ul> <p>hope it helps :)</p>
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    1. COThat's how I actually implemented a proof of concept on persisent data structures. I found this the simple part. What I did not find was an elegant api to "update" a field of an persistent instance, i.e. create a copy of the instance with only a certain field changed.
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    2. COsorry I've misunderstood your question! :-( By the way writing this code was very funny and an useful exercise :-)
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