public class TestSampleVariableAssgment{

public static void callVarAssignment(){
    List<String> sa = new List<String>();
    system.debug('Before ======>' + sa);
    CheckVariableChange.abc(sa);
    system.debug('After ======>' + sa);
}

}

public class CheckVariableChange{

    public static void abc(List<String> sampleString){
        sampleString.add('Hello');
    }

}

TestSampleVariableAssgment.callVarAssignment();