Java Set实现类面试题

HashSet

Q1: HashSet的实现原理是什么？

HashSet是基于HashMap实现的，使用HashMap的key来存储元素，value统一使用一个Object对象。

public class HashSetPrincipleExample {// 模拟HashSet的基本实现public class SimpleHashSet<E> {private static final Object PRESENT = new Object();private HashMap<E, Object> map;public SimpleHashSet() {map = new HashMap<>();}public boolean add(E e) {return map.put(e, PRESENT) == null;}public boolean remove(Object o) {return map.remove(o) == PRESENT;}public boolean contains(Object o) {return map.containsKey(o);}public int size() {return map.size();}}
}

Q2: HashSet如何保证元素不重复？

public class HashSetDuplicationExample {public static class Person {private String name;private int age;public Person(String name, int age) {this.name = name;this.age = age;}// 错误的hashCode实现public int badHashCode() {return 1; // 所有对象都返回相同的hash值}// 正确的hashCode实现@Overridepublic int hashCode() {return Objects.hash(name, age);}// 正确的equals实现@Overridepublic boolean equals(Object obj) {if (this == obj) return true;if (!(obj instanceof Person)) return false;Person other = (Person) obj;return age == other.age && Objects.equals(name, other.name);}}public void demonstrateDuplication() {Set<Person> set = new HashSet<>();set.add(new Person("Tom", 20));set.add(new Person("Tom", 20)); // 不会被添加，因为equals返回trueSystem.out.println("Set size: " + set.size()); // 输出1}
}

TreeSet

Q3: TreeSet的实现原理是什么？

TreeSet是基于TreeMap实现的，它是一个有序集合。

public class TreeSetPrincipleExample {// 1. 使用自然排序public void naturalOrdering() {TreeSet<String> set = new TreeSet<>();set.add("C");set.add("A");set.add("B");System.out.println(set); // 输出 [A, B, C]}// 2. 使用自定义排序public void customOrdering() {TreeSet<Person> set = new TreeSet<>((p1, p2) -> {int ageCompare = Integer.compare(p1.getAge(), p2.getAge());if (ageCompare != 0) return ageCompare;return p1.getName().compareTo(p2.getName());});set.add(new Person("Tom", 20));set.add(new Person("Jerry", 18));set.add(new Person("Bob", 20));}// 3. 实现Comparable接口public static class ComparablePerson implements Comparable<ComparablePerson> {private String name;private int age;@Overridepublic int compareTo(ComparablePerson other) {int ageCompare = Integer.compare(this.age, other.age);if (ageCompare != 0) return ageCompare;return this.name.compareTo(other.name);}}
}

LinkedHashSet

Q4: LinkedHashSet的特点是什么？

LinkedHashSet是HashSet的子类，它使用LinkedHashMap来存储元素，维护了元素的插入顺序。

public class LinkedHashSetExample {public void demonstrateOrdering() {// 1. HashSet不保证顺序Set<String> hashSet = new HashSet<>();hashSet.add("C");hashSet.add("A");hashSet.add("B");System.out.println("HashSet: " + hashSet); // 顺序不确定// 2. LinkedHashSet保持插入顺序Set<String> linkedHashSet = new LinkedHashSet<>();linkedHashSet.add("C");linkedHashSet.add("A");linkedHashSet.add("B");System.out.println("LinkedHashSet: " + linkedHashSet); // 输出 [C, A, B]// 3. TreeSet按照自然顺序Set<String> treeSet = new TreeSet<>();treeSet.add("C");treeSet.add("A");treeSet.add("B");System.out.println("TreeSet: " + treeSet); // 输出 [A, B, C]}
}

性能比较

Q5: 不同Set实现类的性能对比？

public class SetPerformanceComparison {public void comparePerformance() {int n = 100000;// 1. 添加性能long start = System.currentTimeMillis();Set<Integer> hashSet = new HashSet<>();for (int i = 0; i < n; i++) {hashSet.add(i);}System.out.println("HashSet添加耗时：" + (System.currentTimeMillis() - start));start = System.currentTimeMillis();Set<Integer> treeSet = new TreeSet<>();for (int i = 0; i < n; i++) {treeSet.add(i);}System.out.println("TreeSet添加耗时：" + (System.currentTimeMillis() - start));start = System.currentTimeMillis();Set<Integer> linkedHashSet = new LinkedHashSet<>();for (int i = 0; i < n; i++) {linkedHashSet.add(i);}System.out.println("LinkedHashSet添加耗时：" + (System.currentTimeMillis() - start));// 2. 查找性能start = System.currentTimeMillis();for (int i = 0; i < n; i++) {hashSet.contains(i);}System.out.println("HashSet查找耗时：" + (System.currentTimeMillis() - start));start = System.currentTimeMillis();for (int i = 0; i < n; i++) {treeSet.contains(i);}System.out.println("TreeSet查找耗时：" + (System.currentTimeMillis() - start));}
}

Q6: 如何选择合适的Set实现类？

public class SetSelectionExample {public void demonstrateUsage() {// 1. 需要最高性能，不关心顺序Set<String> hashSet = new HashSet<>();// 2. 需要排序Set<String> treeSet = new TreeSet<>();// 3. 需要记住插入顺序Set<String> linkedHashSet = new LinkedHashSet<>();// 4. 自定义排序示例Set<Person> customSortedSet = new TreeSet<>((p1, p2) -> {// 按年龄排序，年龄相同按姓名排序int ageCompare = Integer.compare(p1.getAge(), p2.getAge());if (ageCompare != 0) return ageCompare;return p1.getName().compareTo(p2.getName());});// 5. 需要线程安全Set<String> synchronizedSet = Collections.synchronizedSet(new HashSet<>());// 6. 并发环境下的有序SetSet<String> concurrentSkipListSet = new ConcurrentSkipListSet<>();}// 使用场景示例public void usageScenarios() {// 1. 去重场景Set<String> uniqueEmails = new HashSet<>();uniqueEmails.add("user@example.com");uniqueEmails.add("user@example.com"); // 重复邮箱不会被添加// 2. 排序场景Set<Integer> sortedNumbers = new TreeSet<>();sortedNumbers.add(3);sortedNumbers.add(1);sortedNumbers.add(2);// 遍历时自动排序// 3. 记录访问顺序场景Set<String> pageViews = new LinkedHashSet<>();pageViews.add("/home");pageViews.add("/products");pageViews.add("/about");// 保持用户访问页面的顺序}
}