Java - Force jTextField to accept integer

import javax.swing.*;
import javax.swing.text.*;

/**
 * Force a jTextField to accept only integer value.
 * In order to invoke this to a jTextField,
 * you must customize your jTextField's code, from default-code to custom-creation.
 * Then change the creation of jTextField's instance to the instance of this class (IntegerField).
 *
 * @author Erieze Lagera
 */
public class IntegerField extends JTextField {

    private int limit;
    private String field_name;

    /**
     * Create a simple instance of IntegerField.
     * @param name Name of jTextField who created this instance
     */
    public IntegerField(String name) {
        super();
        setFieldName(name);
    }

    /**
     * Create an instance with column length limitation.
     * This will limit user input length based on your set limit.
     * @param name Name of jTextField who created this instance
     * @param limit Preferred column length
     */
    public IntegerField(String name, int limit) {
        super(limit);
        setLimit(limit);
        setFieldName(name);
        if (limit <= 0) {
            System.out.println("[WARNING] Bad limit value from field " + getFieldName() + ", no limitation will be implemented...");
        }
    }

    @Override
    protected Document createDefaultModel() {
        return new UpperCaseDocument();
    }

    /**
     * Overrides the document of your jTextField, to perform the hack.
     */
    private class UpperCaseDocument extends PlainDocument {

        @Override
        public void insertString(int offset, String str, AttributeSet a) throws BadLocationException {
            if (str == null) {
                return;
            }

            char[] charArr = str.toCharArray();
            boolean ok = true;

            for (int i = 0; i < charArr.length; i++) {
                try {
                    Integer.parseInt(String.valueOf(charArr[i]));
                } catch (NumberFormatException e) {
                    ok = false;
                    break;
                }
            }
            if (ok) {
                if (limit > 0) {
                    if (offset < limit) {
                        super.insertString(offset, new String(charArr), a);
                    }
                }
                else {
                    super.insertString(offset, new String(charArr), a);
                }
            }
        }
    }

    /**
     * Get the current value for limit.
     * @return The value of <i> limit </i>.
     */
    public int getLimit() {
        return limit;
    }

    /**
     * Set the input limit for your jTextField.
     * @param newValue Limit value
     */
    private void setLimit(int newValue) {
        this.limit = newValue;
    }

    /**
     * Get the jTextField name who create this instance.
     * @return Name of your jTextField
     */
    public final String getFieldName() {
        return field_name;
    }

    /**
     * Set the name of jTextField who create this instance.
     * @param field_name Name of your jTextField
     */
    public void setFieldName(String field_name) {
        this.field_name = field_name;
    }

}