Java集合List快速实现重复判断的10种方案

public static boolean hasDuplicate(List<?> list) {
    for (int i = 0; i < list.size(); i++) {
        for (int j = i + 1; j < list.size(); j++) {
            if (list.get(i).equals(list.get(j))) {
                return true;
            }
        }
    }
    return false;
}

public static boolean hasDuplicateByHashSet(List<?> list) {
    Set<Object> set = new HashSet<>(list.size());
    for (Object item : list) {
        if (!set.add(item)) { // add返回false表示存在重复
            return true;
        }
    }
    return false;
}

public static boolean hasDuplicateByStream(List<?> list) {
    return list.stream().distinct().count() < list.size();
}

public static boolean hasDuplicateByTreeSet(List<?> list) {
    Set<Object> set = new TreeSet<>(list);
    return set.size() < list.size();
}

public static boolean hasDuplicateParallel(List<?> list) {
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return list.parallelStream().anyMatch(e -> !seen.add(e));
}

public static boolean hasDuplicateByBitSet(List<Integer> list) {
    BitSet bitSet = new BitSet();
    for (Integer num : list) {
        if (bitSet.get(num)) return true;
        bitSet.set(num);
    }
    return false;
}

import com.google.common.collect.Sets;

public static boolean hasDuplicateByGuava(List<?> list) {
    return Sets.newHashSet(list).size() < list.size();
}

import org.apache.commons.collections4.CollectionUtils;

public static boolean hasDuplicateByCommons(List<?> list) {
    return CollectionUtils.getCardinalityMap(list).values()
           .stream().anyMatch(count -> count > 1);
}

class User {
    private Long id;
    
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof User user)) return false;
        return Objects.equals(id, user.id);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

Set<Object> safeSet = Collections.synchronizedSet(new HashSet<>());

// 错误示例：并行流中可能导致漏判
list.parallelStream().forEach(e -> {
    if (set.contains(e)) flag = true;
    set.add(e);
});

public static boolean hasDuplicateByMultiField(List<User> users) {
    Set<String> seen = new HashSet<>();
    return users.stream()
        .map(u -> u.getName() + "|" + u.getEmail())
        .anyMatch(key -> !seen.add(key));
}

public static boolean hasDuplicateInChunks(List<?> list, int chunkSize) {
    for (int i = 0; i < list.size(); i += chunkSize) {
        List<?> subList = list.subList(i, Math.min(i + chunkSize, list.size()));
        if (hasDuplicateByHashSet(subList)) {
            return true;
        }
    }
    return false;
}