Why i'm getting such null in my input

I need to code simple calculator with UI in one class and model in another, but how can I right access data from another class? for example? if i press 123 i'll get in text such output null123 ? Help me please? also how can i optimise my code, where I have mistakes?

View class:

package calc;

import org.eclipse.swt.SWT;
import org.eclipse.swt.events.KeyEvent;
import org.eclipse.swt.events.KeyListener;
import org.eclipse.swt.events.SelectionEvent;
import org.eclipse.swt.events.SelectionListener;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Text;
import org.eclipse.ui.part.ViewPart;
import org.eclipse.wb.swt.SWTResourceManager;

public class View extends ViewPart {
    public Text inputText;
    public Text inputText2;
    public Calculation calcul;

    public View()
    {
        calcul = new Calculation();
    }

    public void createPartControl(Composite parent) {
        Composite mainComposite = new Composite(parent, SWT.NONE);
        GridLayout gltop = new GridLayout(1, false);        
        gltop.numColumns = 1;       
        mainComposite.setLayout(gltop);
        //mainComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLUE));
        //ВЫзов верхнего и нижнего композита
        createTop(mainComposite);
        createBott(mainComposite);      
    }

    private void createTop(Composite parent)
    {
        GridLayout gltop = new GridLayout(1, false);        
        gltop.numColumns = 1;
        GridData data = new GridData(GridData.FILL_HORIZONTAL);     
        Composite topComposite = new Composite(parent, SWT.NONE);       
        topComposite.setLayout(gltop);
        topComposite.setLayoutData(data);
        //topComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_RED));

        inputText = new Text(topComposite, SWT.NONE);
        inputText.setLayoutData(data);
        inputText.setFont(SWTResourceManager.getFont("", 18, SWT.BOLD));
    }

    private void createBott(Composite parent)
    {
        GridLayout glbot = new GridLayout(1, false);        
        glbot.numColumns = 2;
        GridData data = new GridData(GridData.FILL_BOTH);       
        Composite botComposite = new Composite(parent, SWT.NONE);       
        botComposite.setLayout(glbot);
        botComposite.setLayoutData(data);
        //botComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_YELLOW));    


        createLeft(botComposite);       
        createRight(botComposite);      
    }

    private void createLeft(Composite parent)
    {           
        Composite leftComposite = new Composite(parent, SWT.NONE);  
        GridLayout glleft = new GridLayout(1, false);       
        GridData data = new GridData(GridData.FILL_HORIZONTAL); 
        glleft.marginTop = 26;
        leftComposite.setLayout(glleft);
        leftComposite.setLayoutData(data);
        //leftComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_CYAN)); 

        inputText2 = new Text(leftComposite, SWT.BORDER_SOLID);
        inputText2.setText("Hi");
        Button buttonMC = createFuncDigButtons(leftComposite, data, "MC", 'M');
        Button buttonMR = createFuncDigButtons(leftComposite, data, "MR", 'R');
        Button buttonMS = createFuncDigButtons(leftComposite, data, "MS", 'S');
        Button buttonMpl = createFuncDigButtons(leftComposite, data, "MC", 'P');
    }

    private void createRight(Composite parent)
    {
        GridLayout glright = new GridLayout(1, false);      
        GridData data = new GridData(GridData.FILL_BOTH);       
        Composite rightComposite = new Composite(parent, SWT.NONE);     
        glright.numColumns = 1;
        rightComposite.setLayout(glright);
        rightComposite.setLayoutData(data);
        //rightComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_DARK_GREEN));  

        createRightTop(rightComposite);
        createRightBot(rightComposite);
    }

    private void createRightTop(Composite parent)
    {
        Composite rightTopComposite = new Composite(parent, SWT.NONE);
        GridLayout glltop = new GridLayout(1, false);       
        GridData data = new GridData(GridData.FILL_BOTH);   
        glltop.numColumns = 3;
        glltop.marginBottom = 0;
        glltop.marginTop = 0;
        rightTopComposite.setLayout(glltop);
        rightTopComposite.setLayoutData(data);
        //rightTopComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_DARK_MAGENTA)); 

        Button buttonBack = createFuncDigButtons(rightTopComposite, data, "Backspace", 'B');
        Button buttonC = createFuncDigButtons(rightTopComposite, data, "C", 'C');
        Button buttonCE = createFuncDigButtons(rightTopComposite, data, "CE", 'E');
    }

    private void createRightBot(Composite parent)
    {
        GridLayout glright = new GridLayout(1, false);      
        GridData data = new GridData(GridData.FILL_BOTH);       
        Composite rightBotComposite = new Composite(parent, SWT.NONE);      
        glright.numColumns = 5;
        rightBotComposite.setLayout(glright);
        rightBotComposite.setLayoutData(data);
        //rightBotComposite.setBackground(SWTResourceManager.getColor(SWT.COLOR_DARK_GREEN));       
        Button button7 = createFuncDigButtons(rightBotComposite, data, "7", '7');
        Button button8 = createFuncDigButtons(rightBotComposite, data, "8", '8');
        Button button9 = createFuncDigButtons(rightBotComposite, data, "9", '9');
        Button buttonDev = createFuncDigButtons(rightBotComposite, data, "/", '/');
        Button buttonSQRT = createFuncDigButtons(rightBotComposite, data, "sqrt", 'R');
        Button button4 = createFuncDigButtons(rightBotComposite, data, "4", '4');
        Button button5 = createFuncDigButtons(rightBotComposite, data, "5", '5');
        Button button6 = createFuncDigButtons(rightBotComposite, data, "6", '6');
        Button buttonMult = createFuncDigButtons(rightBotComposite, data, "*", '*');
        Button buttonPer = createFuncDigButtons(rightBotComposite, data, "%", '%');
        Button button1 = createFuncDigButtons(rightBotComposite, data, "1", '1');
        Button button2 = createFuncDigButtons(rightBotComposite, data, "2", '2');
        Button button3 = createFuncDigButtons(rightBotComposite, data, "3", '3');
        Button buttonMinus = createFuncDigButtons(rightBotComposite, data, "-", '-');
        Button buttonDev1 = createFuncDigButtons(rightBotComposite, data, "1/x", '1');
        Button button0 = createFuncDigButtons(rightBotComposite, data, "0", '0');
        Button buttonPM = createFuncDigButtons(rightBotComposite, data, "+/-", 'p');
        Button buttonD = createFuncDigButtons(rightBotComposite, data, ".", '.');       
        Button buttonPlus = createFuncDigButtons(rightBotComposite, data, "+", '+');        
        Button buttonR = createFuncDigButtons(rightBotComposite, data, "=", '=');
    }

    public Button createFuncDigButtons(Composite parent, GridData gridData, final String digit, final char formethod)
    {
        Button button = new Button(parent, SWT.PUSH);
          button.setLayoutData(gridData);
          button.setText(String.valueOf(digit));
          button.addSelectionListener(new SelectionListener() {
            @Override
            public void widgetSelected(SelectionEvent arg0) {
             // update(formethod);
                calcul.getDigit(formethod);
                inputText.setText(calcul.num1);
                System.out.println("f " + calcul.num1);
            }

            @Override
            public void widgetDefaultSelected(SelectionEvent arg0) {
              /* do nothing */
            }
          });
          button.addKeyListener(new KeyListener() {

            @Override
            public void keyReleased(KeyEvent arg0) {
            }

            @Override
            public void keyPressed(KeyEvent arg0) {
                calcul.getDigit(arg0.character);
            }
        });
        return button;
    }

    public void setFocus() {
    }
}

And pseudomodel class:

package calc;

public class Calculation {

    public String num1;
    public String num2;

    public Calculation()
    {
    }

    public char getDigit(char a)
    {
        char value = a;
        switch (a) {
        case 'q':

            break;

        default:
            num1 += value;
            break;
        }
        System.out.开发者_开发问答println("getbyclass" + a + "bb" +num1);
        return value;       
    }
}

Help me please, becouse i need help to understand

initialize the num1 and num2 strings to ""

otherwise the strings are set to null and when you concatenate that with another string it gets parsed to "null"

This will help:

in your function createFuncDigButtons change the 3rd argument from final String digit to final char digit and pass in a character.

in your keyPressed handler change this line

calcul.getDigit(arg0.character);

to this

calcul.getDigit(digit);

You also need to initialize your string variables in your Calculation constructor.