[android help] how to costume android default classes(datepicker,numpicker,etc)

I want to make a custom date-picker for android and I need to change something in datepicker.class file in android.widget package. In fact, I want to change the year and month for solar hijri calendar.

I found source code of datepicker.java in http://alvinalexander.com/java/jwarehouse/android/core/java/android/widget/DatePicker.java.shtml - but I don't know how to change and replace it with default android class. Does anybody know how to do this?

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