Java Stream API：面试问题每个开发人员都应练习

准备Java开发人员面试？Stream API是面试中常见的考点，它以优雅的方式处理数据集合而闻名。本文将带您了解15道真实的Stream API面试题，助您掌握Java Stream。

问题1：在数组中查找最大元素

<code class="java">int arr[] = {5,1,2,8};
int max = Arrays.stream(arr).max().getAsInt();</code>

问题2：打印字符串中每个字符的计数

<code class="java">String str = "now is the winter";
Map<String, Long> charFreq = Arrays.stream(str.split(""))
    .collect(Collectors.groupingBy(
        Function.identity(), 
        Collectors.counting()
    ));</code>

问题3：合并两个Person对象数组，按年龄升序排序，年龄相同则按姓名升序排序

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<code class="java">class Person {
    String name;
    int age;
    // constructors and getters
}

Person[] plist1 = {new Person("alice", 25), new Person("bob", 30), new Person("charlie", 25)};
Person[] plist2 = {new Person("david", 30), new Person("eve", 25), new Person("alice", 25)};

Stream.concat(Arrays.stream(plist1), Arrays.stream(plist2))
    .sorted(Comparator.comparingInt(Person::getAge)
        .thenComparing(Person::getName))
    .forEach(System.out::println);</code>

问题4：查找字符串列表中最长字符串的长度

<code class="java">List<String> names = Arrays.asList("alice", "bob", "charlie", "david", "eva");
int maxLength = names.stream()
    .mapToInt(String::length)
    .max()
    .orElse(0);</code>

问题5：检查整数列表是否包含素数

<code class="java">List<Integer> numbers = Arrays.asList(4, 6, 8, 11, 12, 13, 14, 15);
boolean hasPrime = numbers.stream()
    .anyMatch(num -> isPrime(num));

private static boolean isPrime(int num) {
    if (num <= 1) return false;
    for (int i = 2; i <= Math.sqrt(num); i++)
        if (num % i == 0) return false;
    return true;
}</code>

问题6：计算多个句子中不同单词（不区分大小写）的总数

<code class="java">List<String> sentences = Arrays.asList(
    "Java Stream API provides a fluent interface",
    "It supports functional-style operations on streams",
    "In this exercise, you need to count words"
);

long uniqueWords = sentences.stream()
    .map(x -> x.toLowerCase().split(" "))
    .flatMap(Arrays::stream)
    .distinct()
    .count();</code>

问题7：查找并连接前两个长度为偶数的单词

<code class="java">List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "elderberry");
String result = words.stream()
    .filter(x -> x.length() % 2 == 0)
    .limit(2)
    .collect(Collectors.joining(""));</code>

问题8：给定交易列表，查找每天的交易总额，并按日期排序

<code class="java">class Transaction {
    String date;
    long amount;
    // constructors and getters
}

List<Transaction> transactions = Arrays.asList(
    new Transaction("2022-01-01", 100),
    new Transaction("2022-01-01", 200),
    new Transaction("2022-01-02", 300)
);

Map<String, Long> dailyTotals = transactions.stream()
    .collect(Collectors.groupingBy(
        Transaction::getDate,
        TreeMap::new, // Use TreeMap for sorted order
        Collectors.summingLong(Transaction::getAmount)
    ));</code>

问题9：合并两个整数数组，排序，并过滤掉大于指定阈值的数字

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<code class="java">int[] array1 = {1, 5, 3, 9, 7};
int[] array2 = {2, 4, 6, 8, 10};
int threshold = 7;

IntStream.concat(Arrays.stream(array1), Arrays.stream(array2))
    .boxed()
    .sorted(Comparator.naturalOrder())
    .filter(x -> x <= threshold) // Corrected filter condition
    .forEach(System.out::println);</code>

问题10：将员工记录列表转换为部门到平均工资的地图

<code class="java">class Employee {
    String department;
    double salary;
    // constructor and getters
}

Map<String, Double> deptAvgSalary = employees.stream()
    .collect(Collectors.groupingBy(
        Employee::getDepartment,
        Collectors.averagingDouble(Employee::getSalary)
    ));</code>

问题11：将数字列表分成两组：素数和非素数

<code class="java">List<Integer> numbers = Arrays.asList(2, 3, 4, 5, 6, 7, 8, 9, 10);
Map<Boolean, List<Integer>> partitioned = numbers.stream()
    .collect(Collectors.partitioningBy(num -> isPrime(num)));</code>

问题12：使用Stream生成斐波那契数列，最多n项

<code class="java">Stream.iterate(new int[]{0, 1}, 
    arr -> new int[]{arr[1], arr[0] + arr[1]})
    .limit(10)
    .map(arr -> arr[0])
    .forEach(System.out::println);</code>

问题13：按首字母分组字符串，并计算每个组的出现次数

<code class="java">List<String> words = Arrays.asList("apple", "banana", "bear", "cat", "apple");
Map<Character, Long> frequency = words.stream()
    .collect(Collectors.groupingBy(
        str -> str.charAt(0),
        Collectors.counting()
    ));
// output: {a=2, b=2, c=1}</code>

问题14：使用Java Stream查找两个列表的交集

<code class="java">List<Integer> list3 = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> list4 = Arrays.asList(3, 4, 5, 6, 7);

List<Integer> intersection =
    list3.stream().filter(list4::contains).toList();
System.out.println(intersection);</code>

问题15：在Java中处理重复键时，如何将对象列表转换为排序的地图？

<code class="java">class Employee {
    int id;
    String name;
    // constructor and getters
}

List<Employee> employees = Arrays.asList(
    new Employee(101, "Alice"),
    new Employee(102, "Bob"),
    new Employee(101, "Charlie"),
    new Employee(103, "David"),
    new Employee(102, "Eve")
);

Map<Integer, List<Employee>> employeeMap = employees.stream()
    .collect(Collectors.groupingBy(
        Employee::getId,
        TreeMap::new,
        Collectors.toList()
    ));</code>

解释：

Collectors.groupingBy(Employee::getId, TreeMap::new, Collectors.toList())：

1. 按id分组（键）。
2. 使用TreeMap确保按键排序。
3. 使用Collectors.toList()将多个员工存储在同一键下。
4. 处理重复：如果多个员工具有相同的id，则将其存储在该键下的列表中。

希望这些题目和解答能帮助您在Java Stream API面试中取得好成绩！ 记住，理解背后的逻辑比仅仅记住代码更重要。 多练习，多思考，才能真正掌握Stream API的精髓。