How produce two's complement of a Uint16?

I have two bytes of data. I converted each of them to Uint8 then I produced a Uint16 from them.

How I can produce two's complement of this Uint16 number?

I've tried uInt16 = ~uInt16 + 1 but the code produces 32bit integer and I want it to stay a 16bit Integer.

    byte firstByte, secondByte;
    int firstUint8, secondUint8, uInt16;
    firstByte = buffer[index];//get first byte from buffer
    secondByte = buffer[index + 1];//get second byte from buffer


    firstUint8=firstByte & 0xFF;//produce Uint8
    secondUint8 = secondByte & 0xFF;//produce Uint8

    uInt16 = 256 * firstUint8 + secondUint8;//create Uint16 from these to    Uint8

    twosComplementOfUInt16=~number+1; //produce 32 bit integer but I want int16 

Java is not the best programing language to work with bits. But if you want you can read the documentation to see how the number are represented in java; how to work with bytes or you can do a tutorial.

As observation the (~ and +) returns an integer

    public static void main(String[] args) {
        int uint8 = 0xff;
        int uint16 = 0xffff;
        long uint32 = 0xffffffff;

        int one = 0x0001;
        int ten = 0x000A;
        int twoComplementOfTen = 0xFFF6;
        int computedTwoComplementOfTen = ~ten + one;
        int revertTwoComplementOfTen = ~twoComplementOfTen + one;

        System.out.printf("One = 0x%04X \n", one);
        System.out.printf("ten = 0x%04X \n", ten);
        System.out.printf("~ten + one = 0x%04X \n", twoComplementOfTen);
        System.out.printf("Computed ~ten + one = 0x%04X \n", computedTwoComplementOfTen);
        System.out.printf("~twoComplementOfTen + one = 0x%04X \n", revertTwoComplementOfTen);

        System.out.printf("Computed ~ten + one with uint16 mask = 0x%04X \n", uint16 & computedTwoComplementOfTen);
        System.out.printf("~twoComplementOfTen + one with uint16 mask  = 0x%04X \n", uint16 & revertTwoComplementOfTen);
    }

Output:

One = 0x0001 
Ten = 0x000A 
~ten + one = 0xFFF6 
Computed ~ten + one = 0xFFFFFFF6 
~twoComplementOfTen + one = 0xFFFF000A 
Computed ~ten + one with uint16 mask = 0xFFF6 
~twoComplementOfTen + one with uint16 mask  = 0x000A