Subclassing Java Classes in Jython

A Short Example

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.

from java import awt

class SpamListener(awt.event.ActionListener):
    def actionPerformed(self,event):
        if event.getActionCommand() == "Spam":
	    print 'Spam and eggs!'

f = awt.Frame("Subclassing Example")
b = awt.Button("Spam")
b.addActionListener(SpamListener())
f.add(b, "Center")
f.pack()
f.setVisible(1)

Note: This example can be accomplished much more elegantly by using JavaBeans properties (and event properties).

Calling Methods in Your Superclass

In Python, if I want to call the foo method in my superclass, I use the form:

SuperClass.foo(self)

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.

Example

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:

abstract int read()
int read(byte[])
int read(byte[], int, int)

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:

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

Example Continued

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:

from java.io import DataInputStream

dp = DataInputStream(io)
dp.skipBytes(1000)
print dp.readByte()
print dp.readShort()
print dp.readInt()

Invoking Your Superclass's Constructor

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.

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.

Example

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()

This example shows how the superclass's constructor can be effectively called in order to explictly choose a non-empty version.