用java设计和编写一个简易的带小数点的计算器：实现计算器的界面；实现计算按钮不同的计算功能；显示计算器按钮运算后的结果。

好的，我会尽力回答你的问题。

首先，为了实现计算器的界面，我们可以使用Java的Swing库。我们可以使用JFrame作为主窗口，并在其中添加各种按钮和文本框。以下是一个示例代码：

import javax.swing.*;

public class CalculatorGUI {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Calculator");
        frame.setSize(300, 400); // 设置窗口大小
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // 设置关闭窗口时的操作

        // 添加文本框
        JTextField textField = new JTextField();
        textField.setBounds(10, 10, 260, 40);
        frame.add(textField);

        // 添加数字按钮
        int x = 10, y = 60, width = 60, height = 60;
        for (int i = 0; i < 10; i++) {
            JButton button = new JButton(String.valueOf(i));
            button.setBounds(x, y, width, height);
            frame.add(button);
            x += 70;
            if (i % 3 == 2) {
                x = 10;
                y += 70;
            }
        }

        // 添加小数点
        JButton dotButton = new JButton(".");
        dotButton.setBounds(80, 290, width, height);
        frame.add(dotButton);

        // 添加计算按钮
        JButton addButton = new JButton("+");
        addButton.setBounds(220, 60, width, height);
        frame.add(addButton);

        JButton minusButton = new JButton("-");
        minusButton.setBounds(220, 130, width, height);
        frame.add(minusButton);

        JButton multiplyButton = new JButton("*");
        multiplyButton.setBounds(220, 200, width, height);
        frame.add(multiplyButton);

        JButton divideButton = new JButton("/");
        divideButton.setBounds(220, 270, width, height);
        frame.add(divideButton);

        // 添加等号按钮
        JButton equalButton = new JButton("=");
        equalButton.setBounds(150, 290, width, height);
        frame.add(equalButton);

        frame.setLayout(null);
        frame.setVisible(true);
    }
}

接下来，我们需要实现计算按钮不同的计算功能。我们可以使用Java的计算表达式库来实现。以下是一个示例代码：

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.swing.*;

public class CalculatorGUI {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Calculator");
        frame.setSize(300, 400); // 设置窗口大小
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // 设置关闭窗口时的操作

        // 添加文本框
        JTextField textField = new JTextField();
        textField.setBounds(10, 10, 260, 40);
        frame.add(textField);

        // 添加数字按钮
        int x = 10, y = 60, width = 60, height = 60;
        for (int i = 0; i < 10; i++) {
            JButton button = new JButton(String.valueOf(i));
            button.setBounds(x, y, width, height);
            button.addActionListener(e -> {
                textField.setText(textField.getText() + i);
            });
            frame.add(button);
            x += 70;
            if (i % 3 == 2) {
                x = 10;
                y += 70;
            }
        }

        // 添加小数点
        JButton dotButton = new JButton(".");
        dotButton.setBounds(80, 290, width, height);
        dotButton.addActionListener(e -> {
            textField.setText(textField.getText() + ".");
        });
        frame.add(dotButton);

        // 添加计算按钮
        JButton addButton = new JButton("+");
        addButton.setBounds(220, 60, width, height);
        addButton.addActionListener(e -> {
            String expression = textField.getText();
            if (!expression.isEmpty()) {
                textField.setText("");
                try {
                    ScriptEngineManager manager = new ScriptEngineManager();
                    ScriptEngine engine = manager.getEngineByName("js");
                    Object result = engine.eval(expression);
                    textField.setText(result.toString());
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(frame, "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        frame.add(addButton);

        JButton minusButton = new JButton("-");
        minusButton.setBounds(220, 130, width, height);
        minusButton.addActionListener(e -> {
            String expression = textField.getText();
            if (!expression.isEmpty()) {
                textField.setText("");
                try {
                    ScriptEngineManager manager = new ScriptEngineManager();
                    ScriptEngine engine = manager.getEngineByName("js");
                    Object result = engine.eval(expression);
                    textField.setText(result.toString());
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(frame, "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        frame.add(minusButton);

        JButton multiplyButton = new JButton("*");
        multiplyButton.setBounds(220, 200, width, height);
        multiplyButton.addActionListener(e -> {
            String expression = textField.getText();
            if (!expression.isEmpty()) {
                textField.setText("");
                try {
                    ScriptEngineManager manager = new ScriptEngineManager();
                    ScriptEngine engine = manager.getEngineByName("js");
                    Object result = engine.eval(expression);
                    textField.setText(result.toString());
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(frame, "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        frame.add(multiplyButton);

        JButton divideButton = new JButton("/");
        divideButton.setBounds(220, 270, width, height);
        divideButton.addActionListener(e -> {
            String expression = textField.getText();
            if (!expression.isEmpty()) {
                textField.setText("");
                try {
                    ScriptEngineManager manager = new ScriptEngineManager();
                    ScriptEngine engine = manager.getEngineByName("js");
                    Object result = engine.eval(expression);
                    textField.setText(result.toString());
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(frame, "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        frame.add(divideButton);

        // 添加等号按钮
        JButton equalButton = new JButton("=");
        equalButton.setBounds(150, 290, width, height);
        equalButton.addActionListener(e -> {
            String expression = textField.getText();
            if (!expression.isEmpty()) {
                try {
                    ScriptEngineManager manager = new ScriptEngineManager();
                    ScriptEngine engine = manager.getEngineByName("js");
                    Object result = engine.eval(expression);
                    textField.setText(result.toString());
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(frame, "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        frame.add(equalButton);

        frame.setLayout(null);
        frame.setVisible(true);
    }
}

最后，我们需要显示计算器按钮运算后的结果。我们可以使用Java的Swing库中的JTextField来实现。在每个计算按钮的ActionListener中，我们使用Java的计算表达式库来计算表达式的结果，并将结果显示在JTextField中。

以上就是一个简易的带小数点的计算器的设计和编写过程。希望对你有所帮助！