1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<!-- THIS PAGE IS AUTOMATICALLY GENERATED. DO NOT EDIT. -->
<!-- Mon Oct 22 14:35:40 2007 -->
<!-- USING HT2HTML 2.0 -->
<!-- SEE http://ht2html.sf.net -->
<!-- User-specified headers:
Title: Subclassing Java Classes in Jython
-->
<head>
<title>Subclassing Java Classes in Jython</title>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<meta name="generator" content="HT2HTML/2.0">
<style type="text/css">
body { margin: 0px; }
</style>
</head>
<body bgcolor="#ffffff" text="#000000"
marginwidth="0" marginheight="0"
link="#0000bb" vlink="#00000"
alink="#ff0000">
<!-- start of page table -->
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<!-- start of banner row -->
<tr>
<!-- start of corner cells -->
<td width="150" valign="middle" bgcolor="#cccccc" class="corner">
<center>
<a href="http://www.jython.org/">
<img border="0" src="./images/jython-new-small.gif"></a></center> </td>
<td width="15" bgcolor="#cccccc"> </td><!--spacer-->
<!-- end of corner cells -->
<!-- start of banner -->
<td width="90%" bgcolor="#cccccc" class="banner">
<!-- start of site links table -->
<table width="100%" border="0"
cellspacing="0" cellpadding="2"
bgcolor="#ffffff">
<tr>
<td bgcolor="#cccccc">
<a href="http://www.jython.org/">Home</a>
</td>
<td bgcolor="#cccccc">
<a href="http://www.python.org/">www.python.org</a>
</td>
</tr><tr>
<td bgcolor="#cccccc">
<a href="http://www.jython.org/download.html">Download</a>
</td>
<td bgcolor="#cccccc">
<b>Documentation</b>
</td>
</tr>
</table><!-- end of site links table -->
</td><!-- end of banner -->
</tr><!-- end of banner row -->
<tr><!-- start of sidebar/body row -->
<!-- start of sidebar cells -->
<td width="150" valign="top" bgcolor="#cccccc" class="sidebar">
<!-- start of sidebar table -->
<table width="100%" border="0" cellspacing="0" cellpadding="3"
bgcolor="#ffffff">
<tr><td bgcolor="#666699"><b><font color="#ffffff">
Documentation
</font></b></td></tr>
<tr><td bgcolor="#cccccc">
<a href="./index.html">Overview</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="whatis.html">Executive Summary</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="interpreter.html">Invoking Jython</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="registry.html">Jython Registry</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="embedding.html">Embedding</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="compile.html">Compiling Jython from source</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<tr><td bgcolor="#666699"><b><font color="#ffffff">
Working with Java
</font></b></td></tr>
<tr><td bgcolor="#cccccc">
<a href="usejava.html">The Basics</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="properties.html">JavaBean Properties</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="jarray.html">Java arrays</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<b>Subclassing</b>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="jythonc.html">Building applets, servlets, beans...</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="jreload.html">Reloading java classes</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="zxjdbc.html">zxJDBC</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<tr><td bgcolor="#666699"><b><font color="#ffffff">
Python Docs (exits)
</font></b></td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.python.org/doc/tut/tut.html">Python Tutorial</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.python.org/doc/lib/lib.html">Library Reference</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<tr><td bgcolor="#666699"><b><font color="#ffffff">
Other
</font></b></td></tr>
<tr><td bgcolor="#cccccc">
<a href="differences.html">Jython vs. CPython</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.jython.org/cgi-bin/faqw.py?req=index">Jython FAQ (exit)</a>
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.geocrawler.com/archives/3/7017/2000/">List Archives</a> (exit)
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.python.org/workshops/1997-10/proceedings/hugunin.html">JPython paper</a> (exit)
</td></tr>
<tr><td bgcolor="#cccccc">
<tr><td bgcolor="#666699"><b><font color="#ffffff">
Contact
</font></b></td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://lists.sourceforge.net/lists/listinfo/jython-users">Questions on Jython?<br>jython-users</a>
</td></tr>
<tr><td bgcolor="#cccccc">
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://www.python.org/">
<center>
<img border="0" src="./images/PythonPoweredSmall.gif"></center>
</a>
</td></tr>
<tr><td bgcolor="#cccccc">
</td></tr>
<tr><td bgcolor="#cccccc">
<a href="http://sourceforge.net/">
<center>
<img src="http://sourceforge.net/sflogo.php?group_id=12867" width="88" height="31" border="0" alt="SourceForge Logo"></center>
</a>
</td></tr>
</table><!-- end of sidebar table -->
</td>
<td width="15"> </td><!--spacer-->
<!-- end of sidebar cell -->
<!-- start of body cell -->
<td valign="top" width="90%" class="body"><br>
<h3>Subclassing Java Classes in Jython</h3>
<h3>A Short Example</h3>
The example below should both demonstrate how this subclassing is
performed and why it is useful. At first glance, the code looks
exactly like subclassing any other Python class. The key difference
in this example is that awt.event.ActionListener is a Java class, not
a Python one. In the 4th line from the end,
"b.addListener(SpamListener())",
a Java method is being called that
requires an instance of the Java class ActionListener. By providing a
Python subclass of this Java class, everybody is happy.
<blockquote><PRE>
from java import awt
class SpamListener(awt.event.ActionListener):
def actionPerformed(self,event):
if event.getActionCommand() == "Spam":
print 'Spam and eggs!'
</PRE>
<PRE>
f = awt.Frame("Subclassing Example")
b = awt.Button("Spam")
b.addActionListener(SpamListener())
f.add(b, "Center")
f.pack()
f.setVisible(1)
</PRE></blockquote>
Note: This example can be accomplished much more elegantly by using
<A HREF="properties.html">JavaBeans properties
(and event properties)</A>.
<h3>Calling Methods in Your Superclass</h3>
In Python, if I want to call the foo method in my superclass, I use
the form:
<blockquote><PRE>
SuperClass.foo(self)
</PRE></blockquote>
This works with the majority of methods, but protected methods cannot be
called from subclasses in this way.
Instead you have to use the "self.super__foo()" call style.
<h3>Example</h3>
The following example shows how the java.io.InputStream class can be
effectively subclassed. What makes this class difficult is that the
read method is overloaded for three different method signatures:
<OL>
<LI>abstract int read()</LI>
<LI>int read(byte[])</LI>
<LI>int read(byte[], int, int)</LI>
</OL>
The first one of these methods must be overridden in a subclass.
The other two versions can be ignored. Unfortunately, Python has
no notion of method overloading based on type signatures (this might
be related to the fact that Python doesn't have type signatures
;-) In order to implement a subclass of java.io.InputStream that
overrides the "read" method, a Python method must be implemented that
handles all three possible cases. The example below shows the easiest
way to acheive this:
<blockquote><PRE>
from java.io import InputStream
class InfiniteOnes(InputStream):
def read(self, *args):
if len(args) > 0:
# int read(byte[])
# int read(byte[], int, int)
return apply(InputStream.read, (self,)+args)
return 1
io = InfiniteOnes()
for i in range(10):
print io.read(),
print
</PRE></blockquote>
<h3>Example Continued</h3>
To continue the example above, this new instance of
java.io.InputStream can be passed to any Java method that expects an
InputStream as shown below:
<blockquote><PRE>
from java.io import DataInputStream
dp = DataInputStream(io)
dp.skipBytes(1000)
print dp.readByte()
print dp.readShort()
print dp.readInt()
</PRE></blockquote>
<h3>Invoking Your Superclass's Constructor</h3>
You can explictly invoke your superclass's constructor using the
standard Python syntax of explictly calling the "__init__" method on
the superclass and passing in "self" as the first argument. If you
wish to call your superclass's constructor, you must do so within your
own "__init__" method. When your "__init__" method finishes, if your
Java superclasses have not yet been explicitly initialized, their
empty constructors will be called at this point.
<P>It's important to realize that your superclass is not initialized
until you either explictly call it's "__init__" method, or your own
"__init__" method terminates. You must do one of these two things
before accessing any methods in your superclass.
<h3>Example</h3>
<blockquote><PRE>
from java.util import Random
class rand(Random):
def __init__(self, multiplier=1.0, seed=None):
self.multiplier = multiplier
if seed is None:
Random.__init__(self)
else:
Random.__init__(self, seed)
def nextDouble(self):
return Random.nextDouble(self) * self.multiplier
r = rand(100, 23)
for i in range(10):
print r.nextDouble()
</PRE></blockquote>
This example shows how the superclass's constructor can be effectively
called in order to explictly choose a non-empty version.
<p>
</td><!-- end of body cell -->
</tr><!-- end of sidebar/body row -->
</table><!-- end of page table -->
</body></html>
|
