Android Question UDP Checksum

while(checksum >> 16) {}

while(checksum >>> 16) {}

while((checksum >>> 16) > 0)

#Region Project Attributes 
    #MainFormWidth: 600
    #MainFormHeight: 600 
#End Region

Sub Process_Globals
    Private fx As JFX
    Private MainForm As Form
    
    Private CRC As CalcCRC
End Sub

Sub AppStart (Form1 As Form, Args() As String)
    MainForm = Form1
    'MainForm.RootPane.LoadLayout("Layout1") 'Load the layout file.
'    MainForm.Show
    CRC.Initialize
    CRC.CalculateCRC(0,"FFFC",2)
'    CRC.CalculateCRC(0,"9801331b980e5e4b",16)
'    CRC.CalculateCRC(0,"9801331b980e5e4b0011000a",8)
    ExitApplication
End Sub

'Return true to allow the default exceptions handler to handle the uncaught exception.
Sub Application_Error (Error As Exception, StackTrace As String) As Boolean
    Return True
End Sub

Sub Class_Globals
    Private NativeMe As JavaObject
End Sub

'Initializes the object. You can add parameters to this method if needed.
Public Sub Initialize
    NativeMe = Me
End Sub

#IF JAVA
/*
 * The converted algorithm is simple, using a 32 bit accumulator (checksum),
 * we add sequential 16 bit words to it, and at the end, fold back all
 * the carry bits from the top 16 bits into the lower 16 bits.
 */
 
/*
 * We start with:
 * uint16_t Sys_Checksum(uint16_t init, uint8_t* buff, int32_t len)
 */
 
public int CalculateCRC(int init, String buff, int len)
{
    /* uint32_t i = 0; */

    int words = len;
    int        checksum;
    
    /* We need a counter */
    int        i = 0;
    int            temp;
    
    /* uint32_t checksum = init; */
    checksum = init;

    /* variables for debuging */
    String  s;
    BA.Log(buff);    
    
    /* while(len >= 2) */
    while(len >= 2){
    
        s = buff.substring(i,i+2);
        BA.Log(s.toUpperCase());
        
        /* checksum += ((buff[i] << 8) & 0xff00) | buff[i + 1]; */
        
        /* Get MSB value, clear overflow bit, and clear LSB bits */
        temp = ((HexToDecimal(buff.substring(i,i+1))<< 8) & HexToDecimal("FF00"));
        BA.Log("MSB = " + Integer.toHexString(temp).toUpperCase());
        
        /* Add LSB value */
        temp = temp | (HexToDecimal(buff.substring(i+1,i+2)));
        BA.Log(s.toUpperCase() + " = " + Integer.toHexString(temp).toUpperCase());
        
        /* Add result to checksum */
        checksum = checksum + temp; 
        BA.Log("Checksum = " + Integer.toHexString(checksum).toUpperCase());
        
        //checksum = checksum + HexToDecimal(s);
        // BA.Log(String.valueOf(checksum));
        // BA.Log(i + " - " + Integer.toHexString(checksum));
                
        // move to next byte
        i += 2;
        len -= 2;
    }
    
    BA.Log(Integer.toString(i) + " - " + Integer.toString(len));
    
    BA.Log("Checksum 1 ---> " + Integer.toHexString(checksum).toUpperCase());
    
    /* if(len) */
    
    if (len > 0){
        /* Get MSB value for last byte, clear overflow bit, and clear LSB bits */
        /*  checksum += (buff[i] << 8) & 0xff00; */
        temp = ((HexToDecimal(buff.substring(i,i+1))<< 8) & HexToDecimal("FF00"));
        BA.Log("MSB = " + Integer.toHexString(temp).toUpperCase());
        /* Add result to checksum */
        checksum = checksum + temp; 
        BA.Log("Checksum = " + Integer.toHexString(checksum).toUpperCase());
    }
    
    /* while(checksum >> 16) { 
     * This is special and write as 'checksum >>> 16' in Java which generate a error
     * So we use OliverA solution 
      
     */
    while((checksum >>> 16) > 0) { 
        /* right shift and truncate to 16 bits */
          checksum = (checksum >> 16) + (checksum  & HexToDecimal("ffff"));
    }
    
    BA.Log("Checksum " + Integer.toHexString(checksum).toUpperCase());
      
    /* return ~checksum; 
     * This is 'the bitwise complement operator' Bitwise NOT which doesn't exist in Java
     * "note that, in all cases, ~x equals (-x)-1"
     */   
     
    temp = (0 - checksum) - 1;
    BA.Log("Checksum " + Integer.toHexString(temp).toUpperCase());
    return temp;
}

public static int HexToDecimal(String hex){  
    int decimal=Integer.parseInt(hex,16);
    //System.out.println(decimal); 
    return decimal;
}
#End If

Public Sub CalculateCRC(Init As Int, Buffer As String, len As Int)
'    NativeMe.InitializeContext
    Dim s As String = NativeMe.RunMethod("CalculateCRC", Array(Init, Buffer, len))
    Log(s)
End Sub