Can I Allocate memory dynamically inside of class's constructor?

Question

So, here's what I have written on my code. This is the declaration part of the code (written in JAVA).

private class DataStructure{
        private VoltageMsg[] voltageMsg;
        public DataStructure()
        {
            this.voltageMsg = new VoltageMsg[VOLTAGE_CH];
        }
        private class VoltageMsg{
            private int Decimal;
            private int Integer;
            private int ch;
            public void setter(int Decimal, int Integer, int ch){
                this.Decimal = Decimal;
                this.Integer = Integer;
                this.ch = ch;
            }
        }
}

VOLTAGE_CH is constant by the way declared in certain integer value. I tried to use the class like this,

private DataStructure datas;
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        datas = new DataStructure();
}

and here's the part when fatal exception happened.

datas.voltageMsg[0].setter(1,2,3);

here's the specific message that I copied from logcat.

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.wise.ksj.kitms_monitoring_device.DeviceStatus$DataStructure$VoltageMsg.setter(int, int, int)' on a null object reference

What did I do wrong? Can I not allocate memory dynamically inside of the constructor like I did? Is that why it keeps showing me this fatal exception? As you can probably guess by now, I'm pretty new to these whole software engineering/programming stuff. Any help would be appreciated. Thank you very much for your time.