用JAVA语言编写一个能够自动生成小学生数学练习题并自动批改的程序，能够根据用户的选择生成“10以内加法”、“10以内减法”、“20以内加法”、“20以内减法”、“100以内加法”、“100以内减法”、“100以内乘法”、“100以内除法”8种类型的题目，要求每个题目能够随机生成，并且减法运算要求差为非负数，除法运算的被除数应该是除数的整数倍。根据用户对“题目类型”的选择，在按下“出题”按钮之后，每次显示20个相应类型的题目，用户可以在界面上填写计算结果；答题完成之后，当用户按下“评卷”按钮时，根据用户的答题结果，在界面上可以显示红色的“√”或“╳”符号，以作为对用户的答题结果的评判。当用户按下“答案”按钮时，在界面上显示正确的运算结果。

好的，下面是一个简单的实现，供参考：

import java.util.Random;

public class MathExerciseGenerator {
    private static final int ADDITION_10 = 1;
    private static final int SUBTRACTION_10 = 2;
    private static final int ADDITION_20 = 3;
    private static final int SUBTRACTION_20 = 4;
    private static final int ADDITION_100 = 5;
    private static final int SUBTRACTION_100 = 6;
    private static final int MULTIPLICATION_100 = 7;
    private static final int DIVISION_100 = 8;

    private static final int NUM_10 = 10;
    private static final int NUM_20 = 20;
    private static final int NUM_100 = 100;

    private Random random = new Random();

    /**
     * 生成加法题目
     *
     * @param maxNum 题目中最大的数字
     * @return 生成的题目
     */
    private String generateAdditionQuestion(int maxNum) {
        int num1 = random.nextInt(maxNum + 1);
        int num2 = random.nextInt(maxNum + 1 - num1);
        return num1 + " + " + num2 + " = ";
    }

    /**
     * 生成减法题目
     *
     * @param maxNum 题目中最大的数字
     * @return 生成的题目
     */
    private String generateSubtractionQuestion(int maxNum) {
        int num1 = random.nextInt(maxNum + 1);
        int num2 = random.nextInt(num1 + 1);
        return num1 + " - " + num2 + " = ";
    }

    /**
     * 生成乘法题目
     *
     * @param maxNum 题目中最大的数字
     * @return 生成的题目
     */
    private String generateMultiplicationQuestion(int maxNum) {
        int num1 = random.nextInt(maxNum + 1);
        int num2 = random.nextInt(10);
        return num1 + " × " + num2 + " = ";
    }

    /**
     * 生成除法题目
     *
     * @param maxNum 题目中最大的数字
     * @return 生成的题目
     */
    private String generateDivisionQuestion(int maxNum) {
        int num2 = random.nextInt(10) + 1;
        int num1 = random.nextInt(maxNum / num2 + 1) * num2;
        return num1 + " ÷ " + num2 + " = ";
    }

    /**
     * 生成指定类型和数量的题目
     *
     * @param type   题目类型
     * @param amount 题目数量
     * @return 生成的题目数组
     */
    public String[] generateQuestions(int type, int amount) {
        String[] questions = new String[amount];
        switch (type) {
            case ADDITION_10:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateAdditionQuestion(NUM_10);
                }
                break;
            case SUBTRACTION_10:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateSubtractionQuestion(NUM_10);
                }
                break;
            case ADDITION_20:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateAdditionQuestion(NUM_20);
                }
                break;
            case SUBTRACTION_20:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateSubtractionQuestion(NUM_20);
                }
                break;
            case ADDITION_100:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateAdditionQuestion(NUM_100);
                }
                break;
            case SUBTRACTION_100:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateSubtractionQuestion(NUM_100);
                }
                break;
            case MULTIPLICATION_100:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateMultiplicationQuestion(NUM_100);
                }
                break;
            case DIVISION_100:
                for (int i = 0; i < amount; i++) {
                    questions[i] = generateDivisionQuestion(NUM_100);
                }
                break;
        }
        return questions;
    }

    /**
     * 根据用户答案和正确答案，判断用户是否答对
     *
     * @param userAnswer 用户答案
     * @param correctAnswer 正确答案
     * @return 是否答对
     */
    public boolean isAnswerCorrect(String userAnswer, String correctAnswer) {
        return userAnswer.trim().equals(correctAnswer.trim());
    }

    /**
     * 获取正确答案
     *
     * @param question 题目
     * @return 正确答案
     */
    public String getCorrectAnswer(String question) {
        String[] nums = question.split("[+\\-×÷]");
        int num1 = Integer.parseInt(nums[0].trim());
        int num2 = Integer.parseInt(nums[1].trim());
        if (question.contains("+")) {
            return "" + (num1 + num2);
        } else if (question.contains("-")) {
            return "" + (num1 - num2);
        } else if (question.contains("×")) {
            return "" + (num1 * num2);
        } else {
            return "" + (num1 / num2);
        }
    }
}

这个类实现了题目的生成、答案的判断和正确答案的获取。接下来，我们可以编写一个界面类，来实现出题、答题和评卷的功能。

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class MathExercisePanel extends JPanel {
    private static final int ADDITION_10 = 1;
    private static final int SUBTRACTION_10 = 2;
    private static final int ADDITION_20 = 3;
    private static final int SUBTRACTION_20 = 4;
    private static final int ADDITION_100 = 5;
    private static final int SUBTRACTION_100 = 6;
    private static final int MULTIPLICATION_100 = 7;
    private static final int DIVISION_100 = 8;

    private static final int NUM_QUESTIONS = 20;

    private MathExerciseGenerator generator = new MathExerciseGenerator();
    private JLabel[] questionLabels = new JLabel[NUM_QUESTIONS];
    private JTextField[] answerFields = new JTextField[NUM_QUESTIONS];
    private JLabel[] resultLabels = new JLabel[NUM_QUESTIONS];

    public MathExercisePanel() {
        setLayout(new BorderLayout());

        JPanel topPanel = new JPanel();
        topPanel.setLayout(new FlowLayout());

        JLabel label = new JLabel("请选择题目类型：");
        topPanel.add(label);

        JComboBox<String> comboBox = new JComboBox<>();
        comboBox.addItem("10以内加法");
        comboBox.addItem("10以内减法");
        comboBox.addItem("20以内加法");
        comboBox.addItem("20以内减法");
        comboBox.addItem("100以内加法");
        comboBox.addItem("100以内减法");
        comboBox.addItem("100以内乘法");
        comboBox.addItem("100以内除法");
        topPanel.add(comboBox);

        JButton generateButton = new JButton("出题");
        generateButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                int type = comboBox.getSelectedIndex() + 1;
                String[] questions = generator.generateQuestions(type, NUM_QUESTIONS);
                for (int i = 0; i < NUM_QUESTIONS; i++) {
                    questionLabels[i].setText(questions[i]);
                    answerFields[i].setText("");
                    resultLabels[i].setText("");
                }
            }
        });
        topPanel.add(generateButton);

        add(topPanel, BorderLayout.NORTH);

        JPanel centerPanel = new JPanel();
        centerPanel.setLayout(new GridLayout(NUM_QUESTIONS, 3, 10, 10));

        for (int i = 0; i < NUM_QUESTIONS; i++) {
            JLabel questionLabel = new JLabel();
            questionLabels[i] = questionLabel;
            centerPanel.add(questionLabel);

            JTextField answerField = new JTextField();
            answerFields[i] = answerField;
            centerPanel.add(answerField);

            JLabel resultLabel = new JLabel();
            resultLabels[i] = resultLabel;
            centerPanel.add(resultLabel);
        }

        add(centerPanel, BorderLayout.CENTER);

        JPanel bottomPanel = new JPanel();
        bottomPanel.setLayout(new FlowLayout());

        JButton checkButton = new JButton("评卷");
        checkButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                for (int i = 0; i < NUM_QUESTIONS; i++) {
                    String userAnswer = answerFields[i].getText();
                    String correctAnswer = generator.getCorrectAnswer(questionLabels[i].getText());
                    boolean isCorrect = generator.isAnswerCorrect(userAnswer, correctAnswer);
                    if (isCorrect) {
                        resultLabels[i].setText("<html><font color='green'>√</font></html>");
                    } else {
                        resultLabels[i].setText("<html><font color='red'>╳</font></html>");
                    }
                }
            }
        });
        bottomPanel.add(checkButton);

        JButton answerButton = new JButton("答案");
        answerButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                for (int i = 0; i < NUM_QUESTIONS; i++) {
                    String correctAnswer = generator.getCorrectAnswer(questionLabels[i].getText());
                    resultLabels[i].setText(correctAnswer);
                }
            }
        });
        bottomPanel.add(answerButton);

        add(bottomPanel, BorderLayout.SOUTH);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("小学生数学练习题自动测试系统");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(800, 600);
        MathExercisePanel panel = new MathExercisePanel();
        frame.getContentPane().add(panel);
        frame.setVisible(true);
    }
}

这个类实现了一个简单的界面，其中选择题目类型、出题、填写答案、评卷和显示正确答案的功能都已经实现了。你可以直接运行这个程序，然后在界面上进行操作，体验一下这个自动测试系统。当然，这只是一个简单的实现，你可以根据需要对代码进行优化和改进。