1. Collection和Set的区别？

Set是内容不重复的Collection，继承了Collection接口

2. Map是不是Collection？

Map是一种映射关系（两个集合间的对应关系），不是Collection

3. TreeMap和HashMap的区别？

TreeMap是通过树来实现的映射，是有序的，不允许null；HashMap是通过hash表来实现的映射，无序，允许null

4. HashMap vs HashTable

HashTable是线程安全的，通过synchronized关键字实现同步，不允许为null；HashMap是线程不安全的，允许为null

5. 实现Key-Value的LRU缓存

import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.stream.Collectors;
import java.util.stream.StreamSupport;
public class LRUCache<K, V> implements Iterable<K>{
    private final int max = 3;
    private final LinkedHashMap<K, V> cache = new LinkedHashMap<>(max);
    public void cache(K key, V value) {
        if (cache.containsKey(key)) {
            cache.remove(key);
        } else if (cache.size() >= max) {
            Iterator<K> iterator = cache.keySet().iterator();
            iterator.next();
            iterator.remove();
        }
        cache.put(key, value);
    }
    @Override
    public Iterator<K> iterator() {
        return cache.keySet().iterator();
    }
    public static void main(String[] args) {
        LRUCache<String, Integer> cache = new LRUCache<>();
        cache.cache("A", 1);
        System.out.println(
                "leave <-" + StreamSupport.stream(cache.spliterator(), false)
                        .collect(Collectors.joining("<-"))
        );
        cache.cache("B", 2);
        System.out.println(
                "leave <-" + StreamSupport.stream(cache.spliterator(), false)
                        .collect(Collectors.joining("<-"))
        );
        cache.cache("C", 3);
        System.out.println(
                "leave <-" + StreamSupport.stream(cache.spliterator(), false)
                        .collect(Collectors.joining("<-"))
        );
        cache.cache("D", 4);
        System.out.println(
                "leave <-" + StreamSupport.stream(cache.spliterator(), false)
                        .collect(Collectors.joining("<-"))
        );
        cache.cache("C", 10);
        System.out.println(
                "leave <-" + StreamSupport.stream(cache.spliterator(), false)
                        .collect(Collectors.joining("<-"))
        );
    }
}