[scilab.git] / scilab / modules / external_objects_java / help / en_US / 01-getting-started.xml

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<refentry xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink"
          xmlns:svg="http://www.w3.org/2000/svg" xmlns:mml="http://www.w3.org/1998/Math/MathML"
          xmlns:db="http://docbook.org/ns/docbook" version="5.0-subset Scilab" xml:lang="en"
          xml:id="jims-getting-started">
    <refnamediv>
        <refname>Getting started - Beginning</refname>
        <refpurpose>How to use the Java Scilab binding?</refpurpose>
    </refnamediv>
    <refsection>
        <title>Description</title>
        <para>The goal is this module is to allow the load and interaction on Java objects and datatypes.
        </para>
    </refsection>
    <refsection>
        <title>Basic</title>
        <para>
            Before starting, it is convenient to know the most important functions and what they do.
            These frequently used functions are the following Scilab functions:
            <itemizedlist>
                <listitem>
                    <link linkend="jimport">jimport</link>: Import a Java class
                </listitem>
                <listitem>
                    <link linkend="jimport">jinvoke</link>: Invoke a method of a Java object
                </listitem>
            </itemizedlist>
        </para>
        <para>
            jimport is the function that mirrors the functionality of the java statement 'import',
            and loads the specified class definition/template of a given class in memory.
            When loaded, this definition is used to accesses static methods/members,
            and create new objects.
        </para>
        <para>
            jinvoke is a function that calls (invokes) a specified method on a java class or object.
            This invoke has an optional set of parameters that must coincide with the actual member
            signature. This means you must pass the same number of arguments, and these arguments
            must have the correct type.
        </para>
    </refsection>
    <refsection>
        <title>Example 1: Creating a basic class and calling a simple method</title>
        <para>
            In this first example, the three basic pillars of working with Java are treated.
            The first is to load a class, then an instance is constructed and finally the invocation
            or calling of one of this methods or members.
        </para>
        <para>
            Consider a basic class as presented in the example <literal>HelloWorld</literal>.
            It has a default constructor generating a message upon construction and one
            public method which also shows a message when it has been called.
            This class should now be compiled into java byte-code. When developing your own code,
            then this section is usually handled by your IDE (integrated development environment).
            If you plan to use external libraries, these will available in precompiled form
            (packed in a JAR).
        </para>
        <programlisting role="java"><![CDATA[
// Save under the name HelloWorld.java
package com.foo;
public class HelloWorld {
   public HelloWorld() {
      System.err.println("HelloWorld constructed!");
  }

  public void message() {
      System.err.println("Hello world!");
  }
}
      ]]></programlisting>
        <programlisting role="example"><![CDATA[
// How to compile the Java code from Scilab
javacode=mgetl(fullfile(TMPDIR, 'HelloWorld.java'));
jcompile("com.foo.HelloWorld",javacode);
      ]]></programlisting>
        <para>Once the compiled version of this Java class exists, we can start Scilab and try
            to get Scilab to show us the various messages.
            Now the HelloWorld class can be imported into our workspace. This is done using the
            already mentioned jimport:
            <screen>
--> jimport com.foo.HelloWorld

--> HelloWorld
HelloWorld  =
class com.foo.HelloWorld

--> whos -name HelloWorld
Name                     Type           Size           Bytes
HelloWorld               _EClass        ?              168
</screen>
            <para>
                Upon competition, an additional variable named HelloWorld has been created.
                This is equivalent to a Class object in java. From this class object,
                new objects of the HelloWorld type can be created.
            </para>
            <para>
                Creating such an object instance can be done by invoking <link linkend="new">new</link>
                on the class definition. The arguments to this function are the parameters that are
                delegated to the Java constructor.
                The result of this operation is a new Java object reference which can be stored
                in a variable for later use.
            </para>
            <screen>
--> object = HelloWorld.new();
HelloWorld constructed!

--> object
object  =
com.foo.HelloWorld@49aacd5f

--> whos -name object
Name                     Type           Size           Bytes
object                   _EObj          ?              160
</screen>
            <para>
                What one sees when the <link linkend="new">new</link> operator is called on the
                JClass, it transparently invokes the Java constructor, and our
                "HelloWorld constructed!" message appears. The resulting HelloWorld object is
                stored in the "object" variable. When the variable name is reentered in the
                command line, the details of its reference are shown. This message can be
                customized by overriding the toString method in the HelloWorld class.
            </para>
            <para>
                Now that a specific HelloWorld object has been created, one can try to call
                the public method that has been declared; <literal>HelloWorld\#message()</literal>.
                The same natural technique as with <link linkend="new">new</link> can then be
                applied to invoke the method:
            </para>
            <screen>
--> object.message();
Hello world!
</screen>
            <para>
                The dot operator (dot between object and message) is actually a handy shortcut
                and expands the following snippet of Scilab code. The use of this shortcut
                makes it simpler and cleaner to invoke methods/get member variables.
            </para>
            <screen>
--> jinvoke(object, 'message');
Hello world!
</screen>
        </para>
    </refsection>
    <refsection>
        <title>Example 2: Exchanging Scilab and Java primitives</title>
        <para>
            This example treats the way you can exchange primitive data types and strings between
            Scilab and Java. We will be passing various types of objects between these two languages.
        </para>
        <para>
            For this an example class (see Class Inspector) has been defined that takes and
            returns objects.
            There are two methods defined. The first takes a double does some arithmetic and
            returns a result: Inspector#eval(double). The other methods takes any object,
            shows some basic information and returns it: Inspector#inspect(Object).
        </para>
        <programlisting role="java"><![CDATA[
// Save under the name Inspector.java
package com.foo;
public class Inspector {
    public double eval(double x) {
        return x / 2.;
    }

    public Object inspect(Object prototype) {
        System.err.println("Inspecting: '" + prototype + "'");
        System.err.println("Class: " + prototype.getClass().getSimpleName());
        return prototype;
    }
}
      ]]></programlisting>
        <para>
            As in the previous example, this code must be compiled to Java byte-code before
            it can be used directly.
        </para>
        <programlisting role="example"><![CDATA[
// How to compile the Java code from Scilab
javacode= mgetl(fullfile(TMPDIR, 'Inspector.java'));
jcompile("com.foo.Inspector",javacode);
      ]]></programlisting>
        To start, import the Inspector class and create an Inspector object:
        <screen>
--> jimport('com.foo.Inspector');

--> myInspector = Inspector.new()
myInspector  =
com.foo.Inspector@2a788315
</screen>
        Now information between the two systems can be passing along.
        When passing along any of the Scilab data types into Java, it is automatically wrapped
        (see <link linkend="jwrap">jwrap</link>) to its Java equivalent.
        First, an example using the most used data type in Scilab; reals (constant) is given.
        When passing along a real, this type gets automatically mapped to the Scilab type double.
        Let's try;
        <screen>
--> result = myInspector.eval(12.5)
result  =
6.25

--> result * 2
ans  =
12.5

--> whos -name result
Name                     Type           Size           Bytes
result                   constant       1 by 1         24
</screen>
        <para/>
        The automatic convert is controlled by the jautoUnwrap function.
        Without this function, all the conversions have to be done explicitly.
        <screen>
--> result = myInspector.eval(12.5)
result  =
6.25

--> result * 2
ans  =
null

--> whos -name result
Name                     Type           Size           Bytes
result                   _EObj          ?              160
        </screen>
        <para/>
        The result that is returned seems to be correct at first glance ($12.5/2=6.25$).
        However upon closer inspection, notice that what is returned from our function call
        is not a number. What we received is another Java object (Double in this case).
        To be able to use the given data in again in Scilab, the already mentioned
        <link linkend="junwrap">junwrap</link> functionality can be used if the jautoUnwrap
        is not set to true.
        This transforms Java types back to there equivalent Scilab form. From then onward we
        have a normal number again:
        <screen>
--> result = junwrap(result)
result  =
6.25

--> whos -name result
Name                     Type           Size           Bytes
result                   constant       1 by 1         24

--> result * 2
ans  =
12.5
</screen>
        <para/>
        From this example is clear how doubles get automatically transformed into a Double,
        which is used by the Java VM and returned back.
        When calling <link linkend="junwrap">junwrap</link> on the returned variable, it is
        transformed back into a native Scilab type. But how do other types work? Let's inspect
        several of the other primitive types;
        <screen>
--> jautoUnwrap(%f) // Make sure we disable the auto Unwrap

--> result = myInspector.inspect("Hello world!");
Inspecting: 'Hello world!'
Class: String

--> whos -name result
Name                     Type           Size           Bytes
result                   _EObj          ?              160

--> result = junwrap(result)
result  =
Hello world!

--> whos -name result
Name                     Type           Size           Bytes
result                   string         1 by 1         72

// An Integer
--> result = myInspector.inspect(int32(150));
Inspecting: '150'
Class: Integer

--> result = junwrap(result)
result  =
150

--> whos -name result
Name                     Type           Size           Bytes
result                   int32          1 by 1         40

// A boolean
--> result = myInspector.inspect(%t);
Inspecting: 'true'
Class: Boolean

--> result = junwrap(result)
result  =
T

--> whos -name result
Name                     Type           Size           Bytes
result                   boolean        1 by 1         16
</screen>
        <para/>
        As can be seen, all relevant data types are can be transformed transparently between
        Scilab and Java type. However this also extends without any additional effort to matrices;
        <screen>
--> jautoUnwrap(%t) // Make sure we come back in the default mode where Scilab auto unwrap all calls

--> result = myInspector.inspect(1:5)
Inspecting: '[D@b05236'
Class: double[]
result  =
1.    2.    3.    4.    5.

--> whos -name result
Name                     Type           Size           Bytes
result                   constant       1 by 5         56

--> result = myInspector.inspect(testmatrix('magi',3))
Inspecting: '[[D@11d13272'
Class: double[][]
result  =
8.    1.    6.
3.    5.    7.
4.    9.    2.

--> whos -name result
Name                     Type           Size           Bytes
result                   constant       3 by 3         88
</screen>
        <para/>
        When looking at the class of these wrapped matrices, it is clear that Java stores them
        as arrays of the appropriate size. When working with 2D matrices, the data in these
        equivalent Java arrays can be stored in are column-major (default) or row-major mode.
        In column-major mode, the first array contains a pointer to each of the columns.
        Whereas in row-major mode, the first array contains the pointers to each row of data.
        For more information see <link linkend="jautoTranspose">jautoTranspose</link>.
    </refsection>
    <refsection>
        <title>History</title>
        <revhistory>
            <revision>
                <revnumber>5.5.0</revnumber>
                <revremark>
                    Function introduced. Based on the 'JIMS' module. The main difference in the
                    behavior compared to the JIMS module is that
                    <link linkend="jautoUnwrap">jautoUnwrap</link> is enabled by default.
                </revremark>
            </revision>
        </revhistory>
    </refsection>
</refentry>