I want to execute a simple series of instructions to implement a bitcoin script in java using bitcoinj API. All I am looking for is performing arithmetic operations for negative integers or integers greater than 128. An example bitcoin script for the task would look like this:

150 120 OP_ADD 270 OP_CHECKNUMEQUAL

This script should return True eventually. To implement this in java, to push integer values like 150, 270 etc i have to first convert them to byte arrays. I have followed answers given in https://stackoverflow.com/questions/1936857/convert-integer-into-byte-array-java but none of them works in here probably because of differently defined arithmetic operations in bitcoinj API script. Can anyone please help me in this.