Java calculator add numbers to textfield

Question

I am making a calculator to test my skills in java. How can make the numbers to show up in the jTextfield until i pressed one button to calculate the numbers; i want every numbers to show up in the textfield. for example if i pressed 1 and zero i want the textfield to have 10.

int num;
JTextField in = new JTextField(20); // input field where numbers will up;

public void actionPerformed(ActionEvent e) {
    if (e.getSource() == bouttons.get(0)) {
        num = 0;
        in.setText("" + num);
    }
    if (e.getSource() == bouttons.get(1)) {
        int num = 1;
        in.setText("" + num);
    }
}

The screenshot

Answer 1

To save yourself the hassle of a lot of if-else you can create an array of JButtons and go over them in a loop.
So button 0 will be at index 0.

Then, you can append the text to the JTextField as:

String alreadyDisplayed = in.getText(); //get the existing text
String toDisplay = alreadyDisplayed + Integer.toString(loopCounter);// append the position to the text
in.setText(toDisplay);// display the text  

You can loop as follows:

for(int i=0;i<jbuttonArray.length;i++){
    if(e.getSource()==jbuttonArray[i]){
        //insert above code here;
    }
}

Here is the tutorial by Oracle on this subject: http://docs.oracle.com/javase/tutorial/uiswing/components/textfield.html