java Math.sqrt（数字和函数

Question

我是Java的新手，我正在制作一个计算器，我似乎无法让Math.sqrt处理任何不仅仅是数字的数字（即Math.sqrt(6 + 3））。请有人能告诉我我做错了什么。

等于按钮：

    //Equals button
    JButton equals = new JButton();
    equals.setText("=");
    panel.add(equals);
    frame.add(panel);

    equals.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent eq) {

        if(calculation.getText().contains("Math.sqrt")){
            calculation.setText(calculation.getText().replace("Math.sqrt(", ""));
            calculation.setText(Double.toString(Math.sqrt(Double.valueOf(calculation.getText()).longValue())));
        }
        else{
        double answer = (Equals.eval(calculation.getText()));
        Double answer1 = Double.valueOf(answer);
        calculation.setText(Double.toString(answer));
        }
    }
    });;

Square root 按钮：

    //root
    JButton root = new JButton();
    root.setText("Sqrt");
    panel.add(root);
    frame.add(panel);
    root.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent arg0)   {
            calculation.setText(calculation.getText() + "Math.sqrt(");

        }
    });;

感谢。