Java不支持未签名的解决方法

我有一个类，有一个名为的变量buffer，其中包含一个字节数组。我有一个称为的方法readShort，该方法从缓冲区读取两个字节并将其返回为short。但是，有时它返回负值，我不知道为什么。

这是我的课：

public class InPacket {

    private int index;
    private byte[] buffer;

    public InPacket(byte[] data) {
        this.index = 0;
        this.buffer = data;
    }

    public int getLength() {
        return buffer.length;
    }

    public int getAvailable() {
        return buffer.length - index;
    }

    public int getPosition()
    {
        return index;
    }

    public  void setPosition(int value) throws Exception {
        if (value < 0 || value > buffer.length) {
            throw new Exception("Value out of range.");
        }

        this.index = value;
    }

    private void checkLength(int length) {
        if (index + length > buffer.length || length < 0) {
            //throw new Exception("Length out of range.");
        }
    }

    public boolean readBoolean() {
        checkLength(1);
        return buffer[index++] != 0;
    }

    public byte readByte()
    {
        checkLength(1);
        return buffer[index++];
    }

    public byte[] readBytes(int count)
    {
        checkLength(count);
        byte[] temp = new byte[count];

        System.arraycopy(buffer, index, temp, 0, count);

        index += count;
        return temp;
    }

    public short readShort()
    {
        checkLength(2);

        int v1 = buffer[index++];
        v1 |= buffer[index++] << 8;

        return (short)v1;
    }

    public int readInt()
    {
        checkLength(4);

        int v1 = buffer[index++];
        v1 |= buffer[index++] << 8;
        v1 |= buffer[index++] << 16;
        v1 |= buffer[index++] << 24;
        return v1;
    }

    public long readLong()
    {
        checkLength(8);

        long v1 = buffer[index++];
        v1 |= buffer[index++] << 8;
        v1 |= buffer[index++] << 16;
        v1 |= buffer[index++] << 24;
        v1 |= buffer[index++] << 32;
        v1 |= buffer[index++] << 40;
        v1 |= buffer[index++] << 48;
        v1 |= buffer[index++] << 56;

        return v1;
    }

    public String readString() {
        short count = readShort();

        return readString(count);
    }

    public String readString(int count)
    {
        checkLength(count);

        char[] fin = new char[count];

        for (int i = 0; i < count; i++)
        {
            fin[i] = (char)readByte();
        }

        return new String(fin);
    }

    public Point readPoint() {
        return new Point(readShort(), readShort());
    }

    public byte[] readRemainingBytes() {
        return readBytes(getAvailable());
    }

    public void skip(int count)
    {
        checkLength(count);
        index += count;
    }

    public byte[] toArray() {
        byte[] fin = new byte[buffer.length];

        System.arraycopy(buffer, 0, fin, 0, buffer.length);

        return fin;
    }

    @Override
    public String toString() {
        String ret = "";

        for (int i = 0; i < buffer.length; i++) {
            ret += String.format("%02X ", buffer[i]);
        }

        return ret;
    }
}

因此，它可以一直工作到读取较高值的某一点，然后将其返回负值。（-127而不是129）。为什么会发生这种情况，我该怎么做才能解决该问题？