以下是一个用 Java 编写的随机生成不重复用户名称的示例代码：

import java.util.HashSet;
import java.util.Random;
import java.util.Set;

public class RandomUserNameGenerator {
    private static final String CHARACTERS = "abcdefghijklmnopqrstuvwxyz0123456789";
    private static final int NAME_LENGTH = 8;
    private static final int MAX_ATTEMPTS = 1000;
    private Set<String> existingNames = new HashSet<>();

    public String generateUserName() {
        Random random = new Random();
        String userName;
        int attempts = 0;

        do {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < NAME_LENGTH; i++) {
                int index = random.nextInt(CHARACTERS.length());
                sb.append(CHARACTERS.charAt(index));
            }
            userName = sb.toString();
            attempts++;
        } while (existingNames.contains(userName) &amp;&amp; attempts < MAX_ATTEMPTS);

        if (attempts == MAX_ATTEMPTS) {
            throw new RuntimeException("Failed to generate a unique username after " + attempts + " attempts.");
        }

        existingNames.add(userName);
        return userName;
    }
}