我想从一个数组中随机选择20个问题并检查答案，但系统不会显示问题

Question

我的代码是这样的......所有问题都不一样......但是当我尝试它时，它会一次又一次地重复同样的问题。或者，它不会显示所有问题......

import java.util.Random;
import javax.swing.JOptionPane;
public class Questions extends JFrame {
    public static void main(String[] args) {
        int[] questionsdone = {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 };
        Random rnd = new Random();
        for (int i = 0; i < 40; i++) {
            int ramNum = rnd.nextInt(30);
            //randomly choose a number between 0 to 30
            int checkingSize = 0;
            for (int z = 0; z < questionsdone.length; z++) {
                if (questionsdone[z] != ramNum) {
                    checkingSize++;

                    if (checkingSize == 20) {
                        questionsdone[i] = ramNum;
                        String answer = JOptionPane.showInputDialog(null, questions[z], "Grade 11 Computer Science Review Exercise", JOptionPane.INFORMATION_MESSAGE);

                        //show the questions
                        if (answer.equals(answers[z])) {
                            JOptionPane.showMessageDialog(null, "Correct! The answer is " + answers[z], "Grade 11 Computer Science Review Exercise", JOptionPane.INFORMATION_MESSAGE);
                            mark = mark + 1;
                        } else {
                            JOptionPane.showMessageDialog(null, "Wrong! The answer shuold be " + answers[z], "Grade 11 Computer Science Review Exercise", JOptionPane.INFORMATION_MESSAGE);
                        }
                    }
                    //check the answers
                    if (questionsdone.length == 20) {
                        break;
                    }

                } //end if
            } //end for
        } //end for
    }
}