将十六进制值从数组转换为ASCII字符，以及如何消除空值？

Question

I'm in the middle trying to convert the hex value that I retrieved from my method, compareHexaRGB to ASCII character which I want to know what output is going to produce. I don't know if I'm doing it wrong or I missed to code somewhere.

Code for extractMessage() method to convert hex value to ASCII:

public class extractMessage
{
private static String[][] char1;
private static String[][] char2;
private static String[][] in;
private static String[][] combine;


public static void extractMessage(String[][] inn, String[][] comb)
{
    in = inn;
    combine = comb;
}

public static void printString2DArray(String[][] in) 
{
    for (int i = 0; i < in.length; i++) 
    {
        for(int j = 0; j < in[i].length; j++)
        {
            System.out.println(in[i][j] + " ");
        }
        System.out.println();
    }
}

public static void charExtract()
{
    compareHexaRGB hexRGB = new compareHexaRGB();

    char1 = hexRGB.getCheck_hex2();
    char2 = hexRGB.getCheck_hex4();

    combine = new String[char1.length][char1[0].length];

    for(int i = 0; i < char1.length; i++)
    {
        for(int j = 0; j < char1[i].length; j++)
        {
            //concatenate string
            combine[i][j] = char1[i][j] + char2[i][j];
        }
    }
    System.out.println("Char 1 + Char 2: ");
    printString2DArray(combine);
}

public static String convertHexToString()
{
    extractMessage em = new extractMessage();
    StringBuilder sb = new StringBuilder();
    StringBuilder temp = new StringBuilder();

    String out = em.charExtract(); //error stated incompatible types: void cannot be converted to String

    int decimal;

    for(int i = 0; i < out.length(); i += 2)
    {
        String output = out.substring(i, (i + 2));
        decimal = Integer.parseInt(output, 16);
        sb.append((char)decimal);

        temp.append(decimal);
    }
    System.out.println("Output: " + temp.toString());
    return sb.toString();
}
}

Second, I still cannot eliminate the NULL value from the retrieved value. Someone said I need to add a place to hold the characters which in this case the NULL value. I've done that but when I try to run the code then, here we go again, the nullpointerexception occur. I'm still new to java and lack of experience handling with java arrays and advanced java but I'm eager to learn. Java is very interesting language and I hope one day I could master on this language.

Code for compareHexaRGB() :

public class compareHexaRGB
{
private static int w;
private static int h;
private static BufferedImage img;
private static BufferedImage img2;
private static String[][] check_hex2;
private static String[][] check_hex4;
private static String[][] not_stega2;
private static String[][] not_stega4;

public static void compareHexaRGB(BufferedImage image, BufferedImage image2, int width, int height) throws IOException 
{
    w = width;
    h = height;
    img = image;
    img2 = image2;

}

public void check() throws IOException
    {
        getPixelRGB1 pixel = new getPixelRGB1();
        getPixelData1 newPD = new getPixelData1();

        int[] rgb;
        int count = 0;

        int[][] pixelData = new int[w * h][3];
        check_hex2 = new String[w][h];
        check_hex4 = new String[w][h];

        for(int i = 0; i < w; i++) 
        {
            for(int j = 0; j < h; j++)
            {
                rgb = newPD.getPixelData(img, i, j);

                for(int k = 0; k < rgb.length; k++)
                {
                    pixelData[count][k] = rgb[k];
                }

                    if(pixel.display_imgHex2()[i][j].equals(pixel.display_img2Hex2()[i][j]))
                    {
                        System.out.println("\nPixel values at position 2 are the same." + "\n" + pixel.display_imgHex2()[i][j] + "  " + pixel.display_img2Hex2()[i][j]);
                        not_stega2[i][j] = pixel.display_img2Hex2()[i][j]; // i've done the same as check_hex2 and check_hex4 method but why the error still occur?
                    }
                    if(pixel.display_imgHex4()[i][j].equals(pixel.display_img2Hex4()[i][j]))
                    {
                        System.out.println("\nPixel values at position 4 are the same." + "\n" + pixel.display_imgHex4()[i][j] + "  " + pixel.display_img2Hex4()[i][j]);
                        not_stega4[i][j] = pixel.display_img2Hex4()[i][j];
                    }
                    if(!pixel.display_imgHex2()[i][j].equals(pixel.display_img2Hex2()[i][j]))
                    {
                        System.out.println("\nPixel values at position 2 are not the same." + "\n" + pixel.display_imgHex2()[i][j] + "  " + pixel.display_img2Hex2()[i][j]);
                        check_hex2[i][j] = pixel.display_img2Hex2()[i][j];
                        System.out.println("\nOutput Hex 2: " + check_hex2[i][j]);
                    }
                    if(!pixel.display_imgHex4()[i][j].equals(pixel.display_img2Hex4()[i][j]))
                    {
                        System.out.println("\nPixel values at position 4 are not the same." + "\n" + pixel.display_imgHex4()[i][j] + "  " + pixel.display_img2Hex4()[i][j]);
                        check_hex4[i][j] = pixel.display_img2Hex4()[i][j];
                        System.out.println("\nOutput Hex 4: " + check_hex4[i][j]);
                    }
                    if(!pixel.display_imgHex2()[i][j].equals(pixel.display_img2Hex2()[i][j]) || (!pixel.display_imgHex4()[i][j].equals(pixel.display_img2Hex4()[i][j])))
                    {
                        System.out.println("\nOne of the pixel values at position 2 and 4 are not the same." + "\n" + pixel.display_imgHex2()[i][j] + "  " + pixel.display_img2Hex2()[i][j] + "\n" + pixel.display_imgHex4()[i][j] + "  " + pixel.display_img2Hex4()[i][j]);

                        if(!pixel.display_imgHex2()[i][j].equals(pixel.display_img2Hex2()[i][j]) || (pixel.display_imgHex2()[i][j].equals(pixel.display_img2Hex2()[i][j])))
                        {
                            check_hex2[i][j] = pixel.display_img2Hex2()[i][j];
                            System.out.println("\nOutput Hex 2: " + check_hex2[i][j]);
                        }

                        if(!pixel.display_imgHex4()[i][j].equals(pixel.display_img2Hex4()[i][j]) || (pixel.display_imgHex4()[i][j].equals(pixel.display_img2Hex4()[i][j])))
                        {
                            check_hex4[i][j] = pixel.display_img2Hex4()[i][j];
                            System.out.println("\nOutput Hex 4: " + check_hex4[i][j]);
                        }

                    }
                count++;
                System.out.println("\nOutput Count: " + count);
            }

        }

    }

public String[][] getCheck_hex2()
{
    return check_hex2;
}

public String[][] getCheck_hex4()
{
    return check_hex4;
}

public String[][] getCheck_notStega2()
{
    return not_stega2;
}

public String[][] getCheck_notStega4()
{
    return not_stega4;
}
 }

Hoping to eliminate these problems fast. Appreciate any help!

Answer 1

public static void charExtract()
{ ...
}

and

String out = em.charExtract(); //error stated incompatible types

obviously don't match. Besides, why are you trying to call the static method charExtract() upon the instance em ?

What do you store in those String[][] arrays?

char1 = hexRGB.getCheck_hex2();
char2 = hexRGB.getCheck_hex4();

Without knowing what they are supposed to contain, we cannot help you convert their content to anything.

At the moment I assume that you don't really want to deal with "hex" numbers; calculations are much easier performed on int s than on any String representation of numbers.

Answer 2

As i see there are 2 flaws in your code::

1. If charExtract() method is static then you need to access it in a static way as::

String out = extractMessage.charExtract(); 

2. Secondly as you are storing the value from charExtract() to a String variable "out" so you need to return a string from the charExtract() method ,Declare it as

public static String charExtract()
{ ...
}

and return some value from charExtract like:

return combine[o][1];