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Subclassing Java Classes in JythonA Short ExampleThe 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.Note: This example can be accomplished much more elegantly by using JavaBeans properties (and event properties).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) Calling Methods in Your SuperclassIn Python, if I want to call the foo method in my superclass, I use the form: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.SuperClass.foo(self) ExampleThe 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:
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 ContinuedTo 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 ConstructorYou 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. ExampleThis example shows how the superclass's constructor can be effectively called in order to explictly choose a non-empty version.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()
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