用java编写一则程序在可视化窗口上随机生成30道4则运算，要求题目避免重复，减法不允许出现负数，乘法结果不允许出现四位数，除法结果必须为整数。并在答题结束后显示错题数，正确率。并且增加倒计时功能，要求在指定时间内完成测试，超时未做完视为错题。

在Java中，你可以利用Swing库来创建一个简单的可视化窗口，并结合Math类、Random类以及ArrayList来满足这个需求。以下是一个简化的示例代码：

import javax.swing.*;
import java.awt.*;
import java.util.ArrayList;
import java.util.Random;

public class MathQuiz extends JFrame {
    private ArrayList<String> questions = new ArrayList<>();
    private JLabel questionLabel;
    private JTextField answerField;
    private JButton checkButton;
    private int correctAnswers = 0;
    private int totalQuestions = 30;
    private Random random;
    private Timer timer;
    private long startTime;

    public MathQuiz() {
        // 初始化窗口和组件
        setTitle("数学小测验");
        setSize(500, 300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new FlowLayout());

        // 创建倒计时部分
        JPanel countdownPanel = new JPanel();
        JLabel countdownText = new JLabel("剩余时间: ");
        countdownText.setForeground(Color.BLUE);
        countdownPanel.add(countdownText);
        add(countdownPanel);

        // 生成题目
        random = new Random();
        generateQuestions();

        // 显示题目
        questionLabel = new JLabel();
        add(questionLabel);

        // 答案输入框
        answerField = new JTextField(4);
        add(answerField);

        // 检查按钮
        checkButton = new JButton("检查答案");
        checkButton.addActionListener(e -> checkAnswer());
        add(checkButton);

        // 开始倒计时
        startTimer(60 * 1000); // 60秒倒计时
    }

    private void generateQuestions() {
        while (questions.size() < totalQuestions) {
            int op1 = random.nextInt(10) + 1;
            int op2 = random.nextInt(10) + 1;
            int limit = (op1 * op2) / 1000; // 防止乘积超过四位数

            if (!isSubtractionNegative(op1, op2)) {
                String operation = random.nextBoolean() ? "+" : "-";
                String q = "计算 " + op1 + operation + " " + op2 + ": ";
                if (!questions.contains(q)) {
                    questions.add(q);
                }
            } else if (!isMultiplicationFourDigits(op1, op2)) {
                operation = random.nextBoolean() ? "*" : "/";
                q = "计算 " + op1 + operation + " " + op2 + ": ";
                if (!questions.contains(q)) {
                    questions.add(q);
                }
            }
        }

        // 更新题目
        updateQuestion();
    }

    private boolean isSubtractionNegative(int a, int b) {
        return b > a && b - a < 0;
    }

    private boolean isMultiplicationFourDigits(int a, int b) {
        return a * b >= 1000;
    }

    private void updateQuestion() {
        questionLabel.setText(questions.remove(random.nextInt(questions.size())));
        answerField.setText("");
    }

    private void checkAnswer() {
        // ... (在这里添加检查用户输入并更新统计的代码)
        // 如果时间耗尽，视为剩余问题为错题
        if (System.currentTimeMillis() - startTime > 60 * 1000) {
            JOptionPane.showMessageDialog(this, "时间已到，未完成的问题视为错误！");
        }
    }

    private void startTimer(long delay) {
        timer = new Timer(delay, e -> {
            long remainingTime = (delay - System.currentTimeMillis()) / 1000;
            countdownText.setText("剩余时间: " + remainingTime + " 秒");
            if (remainingTime <= 0) {
                timer.stop();
            }
        });
        timer.start();
        startTime = System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            MathQuiz quiz = new MathQuiz();
            quiz.setVisible(true);
        });
    }
}