Java图形界面 简易BMI计算器怎样设计算法

老师出差去了 布置作业做一个简易BMI计算器
写了半天才写出一个图形界面
不知道怎样实现键盘录入身高、体重，点击评估按钮得到BMI和结果（结果输出的是胖，瘦，正常三种情况）
代码、界面如下:

你的核心问题是如何对按钮进行响应，需要看下ActionListener的相关方法。

    public class BMI extends JFrame{

    private JLabel lblHeight;
    private JLabel lblWeight;
    private JLabel lblBMI;
    private JLabel lblResult;

    private JButton btnRun;
    private JPanel pnlMain;
    private JTextField txtHeight;
    private JTextField txtWeight;
    private JTextField txtBMI;
    private JTextField txtResult;
    DecimalFormat dformat = new DecimalFormat("#.00");
    public BMI(){
        lblHeight = new JLabel("身高(米/m)");
        txtHeight = new JTextField(10);
        lblWeight = new JLabel("体重(千克/kg)");
        txtWeight = new JTextField(10);
        lblBMI = new JLabel("健康值(BMI)");
        txtBMI = new JTextField(10);
        lblResult = new JLabel("结果");
        txtResult = new JTextField(10);
        btnRun = new JButton("评估");
        pnlMain = new JPanel();
        pnlMain.setLayout(null);
        lblHeight.setBounds(100, 50, 80, 25);
        txtHeight.setBounds(200, 50, 100, 25);
        lblWeight.setBounds(100, 80, 80, 25);
        txtWeight.setBounds(200, 80, 100, 25);
        lblBMI.setBounds(100, 110, 80, 25);
        txtBMI.setBounds(200, 110, 100, 25);
        lblResult.setBounds(100, 170, 80, 25);
        txtResult.setBounds(200, 170, 100, 25);
        btnRun.setBounds(150, 140, 80, 25);

        pnlMain.add(lblHeight);
        pnlMain.add(txtHeight);
        pnlMain.add(lblWeight);
        pnlMain.add(txtWeight);
        pnlMain.add(lblBMI);
        pnlMain.add(txtBMI);
        pnlMain.add(lblResult);
        pnlMain.add(txtResult);
        pnlMain.add(btnRun);
        this.setContentPane(pnlMain);
        setSize(400,300);
        setTitle("健康评估");
        setVisible(true);
        setResizable(false);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLocationRelativeTo(null);

        btnRun.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                // TODO Auto-generated method stub
                if( txtWeight.getText() == null){
                    JOptionPane.showMessageDialog(null, "身高为空，请输入一个正值身高！");
                    return;
                }
                if( txtHeight.getText() == null){
                    JOptionPane.showMessageDialog(null, "体重为空，请输入一个正值体重！");
                    return; 
                }
                double weight = Double.valueOf(txtWeight.getText());
                if(weight <= 0 ){
                    JOptionPane.showMessageDialog(null, "体重为0，请输入一个正值体重。");
                    return;
                }
                double height = Double.valueOf(txtHeight.getText());
                if(height <= 0 ){
                    JOptionPane.showMessageDialog(null, "身高为0，请输入一个正值身高。");
                    return;
                }
                double bmi = weight/height/height;
                txtBMI.setText(dformat.format(bmi));
                txtResult.setText(getResult(bmi));
            }

        });
    }
    private String getResult(double bmi) {
        // TODO Auto-generated method stub
        if(bmi < 18.5){
            return "瘦";
        }else if(bmi>=18.5 && bmi < 24){
            return "正常";
        }else{
            return "胖";
        }
    }

    public static void main(String[] args) {
        new BMI();
    }
}

同问，有知道的吗？谢了！

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class BMI extends JFrame {

/**
 * 简易BMI计算机
 */
private static final long serialVersionUID = -2317974201224415240L;
private JLabel lblHeight;
private JLabel lblWeight;
private JLabel lblBMI;
private JLabel lblResult;

private JButton btnRun;
private JPanel pnlMain;
private JTextField txtHeight;
private JTextField txtWeight;
private JTextField txtBMI;
private JTextField txtResult;
DecimalFormat dformat = new DecimalFormat("#.00");

public BMI() {
    lblHeight = new JLabel("身高(米/m)");
    txtHeight = new JTextField(10);
    lblWeight = new JLabel("体重(千克/kg)");
    txtWeight = new JTextField(10);
    lblBMI = new JLabel("健康值(BMI)");
    txtBMI = new JTextField(10);
    lblResult = new JLabel("结果");
    txtResult = new JTextField(10);
    btnRun = new JButton("评估");
    pnlMain = new JPanel();
    pnlMain.setLayout(null);
    lblHeight.setBounds(100, 50, 80, 25);
    txtHeight.setBounds(200, 50, 100, 25);
    lblWeight.setBounds(100, 80, 80, 25);
    txtWeight.setBounds(200, 80, 100, 25);
    lblBMI.setBounds(100, 110, 80, 25);
    txtBMI.setBounds(200, 110, 100, 25);
    lblResult.setBounds(100, 170, 80, 25);
    txtResult.setBounds(200, 170, 100, 25);
    btnRun.setBounds(150, 140, 80, 25);

    pnlMain.add(lblHeight);
    pnlMain.add(txtHeight);
    pnlMain.add(lblWeight);
    pnlMain.add(txtWeight);
    pnlMain.add(lblBMI);
    pnlMain.add(txtBMI);
    pnlMain.add(lblResult);
    pnlMain.add(txtResult);
    pnlMain.add(btnRun);
    this.setContentPane(pnlMain);
    setSize(400, 300);
    setTitle("健康评估");
    setVisible(true);
    setResizable(false);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setLocationRelativeTo(null);

    btnRun.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            if ("".equals(txtHeight.getText())) {
                JOptionPane.showMessageDialog(null, "身高为空，请输入一个正值身高！");
                return;
            }
            if ("".equals(txtWeight.getText())) {
                JOptionPane.showMessageDialog(null, "体重为空，请输入一个正值体重！");
                return;
            }
            double height = Double.valueOf(txtHeight.getText());
            if (height <= 0) {
                JOptionPane.showMessageDialog(null, "身高低于o.1cm，请输入一个正值身高。");
                return;
            }
            double weight = Double.valueOf(txtWeight.getText());
            if (weight <= 0) {
                JOptionPane.showMessageDialog(null, "体重小于0.1kg，请输入一个正值体重。");
                return;
            }
            // BMl＝[体重(千克)／身高(米)]/2
            double bmi = weight / height / 2;
            txtBMI.setText(dformat.format(bmi));
            txtResult.setText(getResult(bmi));
        }

    });
}

private String getResult(double bmi) {
    // BMl＜18.5为消瘦；BMl 在18.5～24.9之间为正常；BMl≥25为超重；
    if (bmi < 18.5) {
        return "瘦";
    } else if (bmi >= 18.5 && bmi < 25) {
        return "正常";
    } else {
        return "胖";
    }
}

public static void main(String[] args) {
    new BMI();
}

}

百度了一下BMI的计算公式，好像有几种，不过都差异不大，楼主问的算法应该不是问题了，
引用二楼的回答，判断输入是否空值，用null会出异常，所以我改成了"".equals()，应该差不多了。