<HTML>
<TITLE>
The java::for Command
</TITLE>
<BODY>
<HR>
<H3>
The java::for Command
</H3>
<HR>
<DL>
<H3>
Usage:
</H3>
<DD><B>java::for</B> <I>type_var</I> <I>collection</I> <I>script</I>
<P>
The <B>java::for</B> command provides a Tcl mapping to the
enhanced Java for statement that appeared in JDK 1.5.
The enhanced Java for statement can be used to iterate
over a collection or over the elements of an array. The
<B>java::for</B> command is like Tcl's <B>foreach</B>
command, except that <B>java::for</B> works with
a Collection object or a Java array object instead of a
Tcl list or array. The <B>java::for</B> command
does not require JDK 1.5 or newer, it works in any
supported JDK version.
<p>
The <I>type_var</I> argument is a Tcl list that contains two elements.
The first element is the type of the class, interface, or primitive type
for the iteration variable. The second element is the name of the
iteration variable.
</p>
<p>
The <I>collection</I> object is a handle to a Java
array or a Java object that implements the <code>java.util.Collection</code>
interface.
</p>
<p>
The <I>script</I> argument is evaluated once for each
element in the collection. The iteration variable is
set to an element in the collection before each
evaluation of the <I>script</I> argument.
</p>
</DL>
<DL>
<H3>
Examples:
</H3>
<DD>
<H4>
Array Example:
</H4>
The following example compares how one would iterate
over an array in Java, in Tcl using a Java array handle,
and in Tcl using the <B>java::for</B> command.
<P>
A Java programmer would iterate over an array as follows:
<code>
<pre>
String[] a = {"str1", "str2", "str3"};
for (int i=0; i < a.length; i++) {
String elem = a[i];
System.out.println("elem is " + elem);
}
</pre>
</code>
In Tcl, one could iterate over an array with:
<code>
<pre>
set a [java::new {String[]} {3} {str1 str2 str3}]
for {set i 0} {$i < [$a length]} {incr i} {
set elem [$a get $i]
puts "elem is $elem"
}
</pre>
</code>
Using the enhanced Java for syntax, one could iterate
over an array with the following code:
<code>
<pre>
String[] a = {"str1", "str2", "str3"};
for (String elem : a) {
System.out.println("elem is " + elem);
}
</pre>
</code>
The same iteration could be implemented in Tcl
using the <B>java::for</B> command:
<code>
<pre>
set a [java::new {String[]} {3} {str1 str2 str3}]
java::for {String elem} $a {
puts "elem is $elem"
}
</pre>
</code>
</P>
<H4>
Collection Example:
</H4>
<P>
The <B>java::for</B> command also supports iteration over
any type of collection object.
Assume for a moment that the user has defined a Java class.
This class contains a static method that returns a
collection object. The JDK 1.5 code to iterate over this
collection would be:
<code>
<pre>
Collection<String> c = MyJavaClass.getCollection();
for (String elem : c) {
System.out.println("elem is " + elem);
}
</pre>
</code>
This same iteration could be implemented in Tcl, using
the <b>java::for</b> command.
<code>
<pre>
set c [java::call MyJavaClass getCollection]
java::for {String elem} $c {
puts "elem is $elem"
}
</pre>
</code>
The reader will note that the type of the iteration variable
is passed into the <b>java::for</b> command, just like the
Java for statement. The <b>java::for</b> command uses
this type information to check that the Java object extracted
from the collection is of the expected type.
</P>
<H4>
Multi-Dimensional Arrays:
</H4>
<P>
Multiple <B>java::for</B> commands can be used together
to iterate over all the elements of a multi-dimensional
array object.
<code>
<pre>
set a [java::new {int[][]} {3 3} {{1 2 3} {4 5 6} {7 8 9}}]
java::for {int[] asub} $a {
java::for {int e} $asub {
puts $e
}
}
</pre>
</code>
The code above will print the following to the console:
<code>
<pre>
1
2
3
4
5
6
7
8
9
</pre>
</code>
</P>
<H4>
Multiple Collections:
</H4>
<P>
Multiple <B>java::for</B> commands can be used together
to iterate over multiple collection objects.
<code>
<pre>
java::import -package java.util Vector
set v1 [java::new Vector]
set v2 [java::new Vector]
$v1 addElement [java::new String "Black"]
$v1 addElement [java::new String "White"]
$v2 addElement [java::new Integer 0]
$v2 addElement [java::new Integer 1]
$v2 addElement [java::new Integer 2]
java::for {String s} $v1 {
java::for {int i} $v2 {
puts "$s $i"
}
}
</pre>
</code>
The code above will print the following to the console:
<code>
<pre>
Black 0
Black 1
Black 2
White 0
White 1
White 2
</pre>
</code>
</P>
<H4>
Elements in a Hashtable:
</H4>
<P>
The <B>java::for</B> command can be used to iterate
over elements in a Hashtable or any other Map implementation.
<code>
<pre>
java::import -package java.util Hashtable Map
set t [java::new Hashtable]
$t put "One" "onestr"
$t put "Two" "twostr"
java::for {Map.Entry e} [$t entrySet] {
set key [[$e getKey] toString]
set value [[$e getValue] toString]
puts "$key -> $value"
}
</pre>
</code>
The code above will print the following to the console:
<code>
<pre>
One -> onestr
Two -> twostr
</pre>
</code>
</P>
<P>
The next <B>java::for</B> example shows how
one could iterate over just the keys in a HashMap.
The key could then be used to lookup the value.
<code>
<pre>
java::import -package java.util HashMap
set t [java::new HashMap]
$t put "s1" "one"
$t put "s2" "two"
java::for {String key} [$t keySet] {
set value [[$t get $key] toString]
puts "$key -> $value"
}
</pre>
</code>
The code above will print the following to the console:
<code>
<pre>
s2 -> two
s1 -> one
</pre>
</code>
</P>
<H4>
Automatic Type Conversion:
</H4>
<P>
The <B>java::for</B> command supports automatic conversion
of a Java primitive wrapper in a collection to a Tcl
value. For example, a Vector of <code>java.lang.Integer</code>
objects can be iterated over as Tcl integers with the
following code:
<code>
<pre>
set v [java::new java.util.Vector]
$v addElement [java::new Integer 0]
$v addElement [java::new Integer 1]
$v addElement [java::new Integer 2]
java::for {int e} $v {
puts $e
}
</pre>
</code>
One could also iterate over the contents of this same Vector
as <code>java.lang.Integer</code> objects with the following code:
<code>
<pre>
set v [java::new java.util.Vector]
$v addElement [java::new Integer 0]
$v addElement [java::new Integer 1]
$v addElement [java::new Integer 2]
java::for {Integer e} $v {
puts [$e toString]
}
</pre>
</code>
Both examples given above will write the same
output to the console.
</P>
</DL>
<br>
<PRE>
<A HREF="../license.html">Copyright</A> © 1997-1998 Sun Microsystems, Inc.
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