translate example into Java

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I'm trying to translate this demo into Java http://docs.opencv.org/3.2.0/d5/dc4/t...

but I'm stuck on the lines using overloading of -= on Mat. My understanding of C++ is very limited, but I expected to see an operator-=()here http://docs.opencv.org/3.2.0/d3/d63/c... but there isn't one. What is the Java equivalent of -= on a Mat?

Note that the Java docs for 3.2.0 are not available, so consult 3.1.0 instead http://docs.opencv.org/java/3.1.0/