To get an int64 value from a Java method that returns a long, you can use the longValue() method of the Long class to convert the Long object to a long value, and then cast it to an int64 value. Here's an example: 

% Load the Java class that contains the method you want to call
jclass = javaObject('com.example.MyClass');

% Call the method that returns a long value
jresult = jclass.myLongMethod();

% Convert the Long object to a long value and cast it to an int64 value
matlabResult = int64(jresult.longValue());

In this example, replace com.example.MyClass with the fully qualified name of the Java class that contains the method you want to call, and replace myLongMethod() with the name of the method that returns a long value. The jresult variable will contain a Long object, which you can convert to a long value using the longValue() method. Finally, you can cast the long value to an int64 value using the int64() function in MATLAB.