how to control active window buttons

[ashwani]

Hello
I have a application which need some inputs from user, then user will
press buttons like start, pause, stop, cancel, Ok etc on the
application. Now i need to write another application which will start
this existing application and based on some inputs it automatically
press certain buttons on existing application and existing application
behaves like user is pressing these buttons manually.
For this application which programming language i need to study and
till what extent? Do you have any book, link or existing software to
do this.

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