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<title> Jace Developer's Guide - A Mapping Example </title>
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<b>Chapter 10</b>
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<b>A Mapping Example</b>
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<table bgcolor="#EEEEEE" width="100%"> <tr> <td> Using Maps </td> </tr> </table>
<p>
This example demonstrates the usage of a Java Map from C++. This example doesn't introduce a lot of new concepts. It just enforces the existing examples, and draws attention to a few interesting details. Like most of the other examples, this example, <a href="../../examples/map_example">map_example</a> only has a single source file, <a href="../../examples/map_example/source/main.cpp">main.cpp</a>, which contains the code we'll be examining:
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<font face="Verdana" size="+1" color="#000066"><code><pre>
include "jace/JNIHelper.h"
using jace::OptionList;
#include "jace/StaticVmLoader.h"
using jace::StaticVmLoader;
#include "jace/proxy/java/util/Set.h"
using jace::proxy::java::util::Set;
#include "jace/proxy/java/lang/System.h"
using jace::proxy::java::lang::System;
#include "jace/proxy/java/lang/Object.h"
using ::jace::proxy::java::lang::Object;
#include "jace/proxy/java/lang/Integer.h"
using jace::proxy::java::lang::Integer;
#include "jace/proxy/java/lang/String.h"
using jace::proxy::java::lang::String;
#include "jace/proxy/java/util/Map.h"
using jace::proxy::java::util::Map;
#include "jace/proxy/java/util/HashMap.h"
using jace::proxy::java::util::HashMap;
#include "jace/proxy/java/util/Map.Entry.h"
using jace::proxy::java::util::Map_Entry;
#include "jace/proxy/java/util/Iterator.h"
using jace::proxy::java::util::Iterator;
#include "jace/javacast.h"
using jace::java_cast;
#include <vector>
using std::vector;
#include <iostream>
using std::cout;
using std::endl; </pre></code></font>
<p> By now, you should be familiar with all of these #includes. However, the include and using directive for <a href="../../examples/map_example/proxies/include/jace/proxy/java/util/Map.Entry.h">Map_Entry</a> should catch your attention. Map_Entry is actually an inner class, Entry, for the outer class java.util.Map. In Java notation, it is referred to as java.util.Map$Entry. When Jace generates the proxies for nested classes, it generates them as normal C++ classes. Because, '$' characters are illegal in C++ identifiers, Jace translates all '$' characters to '_' characters. So, for example, a nested, nested class, Foo$Bar$Baz, would be generated as Foo_Bar_Baz. However, when naming the files, Jace uses '.' characters instead of '$' characters or '_' characters. This works well, because '$' characters are typically illegal in file names, and the use of '.' characters instead of '_' characters helps Jace to distinguish between an outer class named Foo_Bar and a nested class named Foo$Bar.
</p>
<font face="Verdana" size="+1" color="#000066"><code><pre>
int main() {
try {
StaticVmLoader loader( JNI_VERSION_1_2 );
jace::helper::createVm( loader, OptionList(), false );
for ( int i = 0; i < 1000; ++i ) { </pre></code></font>
<p> As before, we run this code in a loop to demonstrate that Jace manages references in all situations. </p>
<font face="Verdana" size="+1" color="#000066"><code><pre>
Map map = HashMap(); </pre></code></font>
<p>
Nothing new here. Following good coding style guidelines, we declare our variables to be of an interface type, rather than a concrete type.
</p>
<font face="Verdana" size="+1" color="#000066"><code><pre>
map.put( Integer( "1" ), String( "Hello 1" ) );
map.put( Integer( "2" ), String( "Hello 2" ) );
map.put( Integer( "3" ), String( "Hello 3" ) ); </pre></code></font>
<p>
Here, we're just adding three entries to the Map. It's necessary to explicitly specify Integer and String, because there is no meaningful conversion from int or char* to jace::proxy::java::lang::Object - the argument types of Map.put.
</p>
<font face="Verdana" size="+1" color="#000066"><code><pre>
Set entrySet( map.entrySet() );
for ( Iterator it( entrySet.iterator() ); it.hasNext(); ) {
Map_Entry entry = jace::java_cast<Map_Entry>( it.next() );
Integer key = jace::java_cast<Integer>( entry.getKey() );
String value = jace::java_cast<String>( entry.getValue() );
cout << "key: <" << key << "> value: <" << value << ">" << endl;
} </pre></code></font>
<p>
We're just iterating through and printing out the Map's entrySet here. Normally, in Java code, you would type-cast from the return value of it.next() to Map_Entry, but you can't just use C style casts to perform casting of Java objects. Rather, to execute a Java 'type-cast' using Jace, you use the java_cast<> template function. You can use java_cast to cast between two Java types, or between a Java type and a JNI handle. If the cast fails, java_cast will throw a JNIException. You can also use the instanceof<> template function in the same way you'd use the instanceof operator in Java to determine if it is safe to perform a cast.
</p>
<font face="Verdana" size="+1" color="#000066"><code><pre>
}
catch ( std::exception& e ) {
cout << e.what() << endl;
return -1;
}
return 0;
} </pre></code></font>
<p>
As always, we make sure to catch any exceptions that might occur.
</p>
<table bgcolor="#EEEEEE" width="100%"> <tr> <td> Building and running </td> </tr> </table>
<p>
Like the other examples, you can build this example by running ANT on <a href="../../examples/map_example/build.xml">build.xml</a>. Other than having the JVM in your library path, there are no special requirements for running this example.
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