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InstanceCreator (Gson 1.7.1 API)
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SUMMARY: NESTED | FIELD | CONSTR | <A HREF="#method_summary">METHOD</A></FONT></TD>
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DETAIL: FIELD | CONSTR | <A HREF="#method_detail">METHOD</A></FONT></TD>
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<H2>
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com.google.gson</FONT>
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<BR>
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Interface InstanceCreator<T></H2>
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<DL>
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<DT><DT><B>Type Parameters:</B><DD><CODE>T</CODE> - the type of object that will be created by this implementation.</DL>
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<HR>
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<DL>
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<DT><PRE>public interface <B>InstanceCreator<T></B></DL>
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</PRE>
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<P>
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This interface is implemented to create instances of a class that does not define a no-args
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constructor. If you can modify the class, you should instead add a private, or public
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no-args constructor. However, that is not possible for library classes, such as JDK classes, or
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a third-party library that you do not have source-code of. In such cases, you should define an
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instance creator for the class. Implementations of this interface should be registered with
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<A HREF="../../../com/google/gson/GsonBuilder.html#registerTypeAdapter(java.lang.reflect.Type, java.lang.Object)"><CODE>GsonBuilder.registerTypeAdapter(Type, Object)</CODE></A> method before Gson will be able to use
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them.
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<p>Let us look at an example where defining an InstanceCreator might be useful. The
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<code>Id</code> class defined below does not have a default no-args constructor.</p>
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<pre>
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public class Id<T> {
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private final Class<T> clazz;
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private final long value;
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public Id(Class<T> clazz, long value) {
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this.clazz = clazz;
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this.value = value;
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}
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}
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</pre>
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<p>If Gson encounters an object of type <code>Id</code> during deserialization, it will throw an
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exception. The easiest way to solve this problem will be to add a (public or private) no-args
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constructor as follows:</p>
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<pre>
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private Id() {
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this(Object.class, 0L);
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}
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</pre>
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<p>However, let us assume that the developer does not have access to the source-code of the
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<code>Id</code> class, or does not want to define a no-args constructor for it. The developer
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can solve this problem by defining an <code>InstanceCreator</code> for <code>Id</code>:</p>
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<pre>
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class IdInstanceCreator implements InstanceCreator<Id> {
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public Id createInstance(Type type) {
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return new Id(Object.class, 0L);
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}
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}
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</pre>
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<p>Note that it does not matter what the fields of the created instance contain since Gson will
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overwrite them with the deserialized values specified in Json. You should also ensure that a
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<i>new</i> object is returned, not a common object since its fields will be overwritten.
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The developer will need to register <code>IdInstanceCreator</code> with Gson as follows:</p>
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<pre>
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Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
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</pre>
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<P>
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<P>
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<DL>
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<DT><B>Author:</B></DT>
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<DD>Inderjeet Singh, Joel Leitch</DD>
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</DL>
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<HR>
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<P>
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<!-- ========== METHOD SUMMARY =========== -->
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<B>Method Summary</B></FONT></TH>
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<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
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<CODE> <A HREF="../../../com/google/gson/InstanceCreator.html" title="type parameter in InstanceCreator">T</A></CODE></FONT></TD>
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<TD><CODE><B><A HREF="../../../com/google/gson/InstanceCreator.html#createInstance(java.lang.reflect.Type)">createInstance</A></B>(<A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Type.html?is-external=true" title="class or interface in java.lang.reflect">Type</A> type)</CODE>
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<BR>
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Gson invokes this call-back method during deserialization to create an instance of the
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specified type.</TD>
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<!-- ============ METHOD DETAIL ========== -->
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<A NAME="createInstance(java.lang.reflect.Type)"><!-- --></A><H3>
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createInstance</H3>
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<PRE>
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<A HREF="../../../com/google/gson/InstanceCreator.html" title="type parameter in InstanceCreator">T</A> <B>createInstance</B>(<A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Type.html?is-external=true" title="class or interface in java.lang.reflect">Type</A> type)</PRE>
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<DL>
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<DD>Gson invokes this call-back method during deserialization to create an instance of the
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specified type. The fields of the returned instance are overwritten with the data present
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in the Json. Since the prior contents of the object are destroyed and overwritten, do not
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return an instance that is useful elsewhere. In particular, do not return a common instance,
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always use <code>new</code> to create a new instance.
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<P>
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<DD><DL>
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<DT><B>Parameters:</B><DD><CODE>type</CODE> - the parameterized T represented as a <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Type.html?is-external=true" title="class or interface in java.lang.reflect"><CODE>Type</CODE></A>.
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<DT><B>Returns:</B><DD>a default object instance of type T.</DL>
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