for (int i = 0; i < buttons.length; i++)
{
    if ((ae.getSource().equals(buttons[i]))
            && (buttons[i].getText().equals("+")))
    {
        this.currentOperation = new OperationsFactory().GetOperation("+");
        this.currentOperation.setFirstOperand(Float.parseFloat(this.tf.getText()));
        this.firstPart = 0;
        this.secondPart = 0;
        this.decimal = true;
    }
    if ((ae.getSource().equals(buttons[i]))
            && (buttons[i].getText().equals("-")))
    {
        this.currentOperation = new OperationsFactory().GetOperation("-");
        this.currentOperation.setFirstOperand(Float.parseFloat(this.tf.getText()));
        this.firstPart = 0;
        this.secondPart = 0;
        this.decimal = true;
    }
...
}