Jython 2.7.4

Document “Properties”

Properties

Jython uses JavaBean properties to make it easier to interact with most Java classes. These properties can be used as normal object attributes, and can also be specified to the class constructor as keyword arguments (this idea is stolen from TkInter where it seems to work extremely well).

These properties are generated automatically using the JavaBean Introspector which identifies properties either from common design patterns, or from explicitly specified BeanInfo.

As a first example, consider the case where you wish to create a button that is disabled.

The first example shows how you would do this in the typical Java fashion:

b = awt.Button()
b.setEnabled(0)

The second example shows how enabled can be set as a property:

b = awt.Button()
b.enabled = 0

The final example sets this property at instantiation time using a keyword argument:

b = awt.Button(enabled=0)

Tuples

If the value of a property is specified as a tuple, then the property will be created by applying the constructor for the type of the property to the tuple. This is particularly handy for specifying sizes:

frame = awt.Frame(size=(500,100))

It can also be handy for specifying color as an RGB triple:

frame.background = 255,255,0

will set the background color of the frame to yellow.

Event Properties

In standard Java, the event handlers for a widget are specified by passing in an instance of a class that implements the appropriate interface. This is the only reasonable approach to take in a language that doesn't have first-class functions. In Jython, for every event listener supported by a class, there will be a property added to the class for each method supplied by the event listener class. These properties can be set to give a function to be called when the appropriate event occurs.

The standard Java style for setting an event listener is shown below:

class action(awt.event.ActionListener):
    def actionPerformed(self,event):
        java.lang.System.exit(0)

button = awt.Button("Close Me!")
button.addActionListener(action())

This can be written in a more Pythonesque (and compact) style by using event properties as follows:

def exit(event):
    java.lang.System.exit(0)

button = awt.Button("Close Me!", actionPerformed=exit)

Methods, Properties and Event Properties

Jython have only one namespace for these three class attributes. Java can be seen as having a unique namespace for each of the three types. As a consequense, there can be conflicts between methods, properties and event properties. These conflicts are resolved so that:

properties < event-properties < fields < methods

This means that a method will override a field with the same name. Some carefull handling of properties and static fields allow for the existence of, and access to, both an instance property and a static field with the same name.