IconTextComponentHelper.java

class IconTextComponentHelper {
    private static final int ICON_SPACING = 4;

    private Border mBorder;
    private Icon mIcon;
    private Border mOrigBorder;
    private JTextComponent mTextComponent;

    IconTextComponentHelper(JTextComponent component) {
        mTextComponent = component;
        mOrigBorder = component.getBorder();
        mBorder = mOrigBorder;
    }

    Border getBorder() {
        return mBorder;
    }

    void onPaintComponent(Graphics g) {
        if (mIcon != null) {
            Insets iconInsets = mOrigBorder.getBorderInsets(mTextComponent);
            mIcon.paintIcon(mTextComponent, g, iconInsets.left, iconInsets.top);
        }
    }

    void onSetBorder(Border border) {
        mOrigBorder = border;

        if (mIcon == null) {
            mBorder = border;
        } else {
            Border margin = BorderFactory.createEmptyBorder(0, mIcon.getIconWidth() + ICON_SPACING, 0, 0);
            mBorder = BorderFactory.createCompoundBorder(border, margin);
        }
    }

    void onSetIcon(Icon icon) {
        mIcon = icon;
        resetBorder();
    }

    private void resetBorder() {
        mTextComponent.setBorder(mOrigBorder);
    }
}

IconTextField.java

public class IconTextField extends JTextField {
    private IconTextComponentHelper mHelper = new IconTextComponentHelper(this);

    public IconTextField() {
        super();
    }

    public IconTextField(int cols) {
        super(cols);
    }

    private IconTextComponentHelper getHelper() {
        if (mHelper == null)
            mHelper = new IconTextComponentHelper(this);

        return mHelper;
    }

    @Override
    protected void paintComponent(Graphics graphics) {
        super.paintComponent(graphics);
        getHelper().onPaintComponent(graphics);
    }

    public void setIcon(Icon icon) {
        getHelper().onSetIcon(icon);
    }

    public void setIconSpacing(int spacing) {
        getHelper().onSetIconSpacing(spacing);
    }

    @Override
    public void setBorder(Border border) {
        getHelper().onSetBorder(border);
        super.setBorder(getHelper().getBorder());
    }
}