如何使用HashMap模拟缓存

Question

I want to calculate unique characters in a given string and cache them using collections, so that if the string is already present it won't calculate again. For this I have used HashMap , I have made the String as key and it's count as value. I have written the following code, but it won't add those key-value pairs in the map. How to resolve this problem?

class UniqueCharacters {

    public int uniqueCharacters(String s)
    {
        List<Character> list=new ArrayList<Character>();
        for(int i=0; i<s.length();i++)
        {
            if(!(list.contains(s.charAt(i))))
            {
                list.add(s.charAt(i));
            }
        }
        for(Character c:list)
        {
            System.out.println(c);
        }
        int count=list.size();
        maintainCache(s, count);

        System.out.println(count);
        return count;
    }

    public void maintainCache(String s, int count)
    {
        Map<String,Integer> map=new HashMap<String,Integer>();
        for(Map.Entry<String, Integer> entry: map.entrySet())
        {
            if(entry.getKey().equals(s))
            {
                System.out.println(entry.getKey()+" "+entry.getValue());
                System.out.println("String was already there");
            }
            else
            {
                map.put(s, count);
                System.out.println("String added to the cache");
            }
        }
    }
}

public class UniqueCharactersTest {

    public static void main(String[] args) {
        UniqueCharacters u=new UniqueCharacters();
        u.uniqueCharacters("hello");
    }
}

Answer 1

The maintainCache creates a new local variable map each time its called. If you want that map to retain its values between calls, you should extract it out of the method and keep it as a member.

Answer 2

import java.util.HashMap;
import java.util.Map;

public class StringCacheTest {

    public static void main(String[] args) {
        StringCache cache = new StringCache();

        System.out.println("Unique characters : " + cache.getUniqueCharactersCount("test"));
        System.out.println("Unique characters : " + cache.getUniqueCharactersCount("test2"));
        System.out.println("Unique characters : " + cache.getUniqueCharactersCount("test"));
    }

}

class StringCache {

    private Map<String, Long> cache;

    public StringCache(){
        this.cache = new HashMap<>();
    }

    public Long getUniqueCharactersCount(String string){
        if(string == null){
            throw new RuntimeException("Null string");
        }
        if(cache.containsKey(string)){
            System.out.println("String " + string + " found in cache");
            return cache.get(string);
        } else {
            System.out.println("String " + string + " not found in cache");
            long uniqueCharactersCount = string
                    .chars()
                    .distinct()
                    .count();
            cache.put(string, uniqueCharactersCount);
            return uniqueCharactersCount;
        }
    }

}

Answer 3

The problem is that HashMap is created newly everytime maintainCache is called. Please have the map as an property of UniqueCharacters class instead of local variable under method maintainCache