我可以使用字符的 HashMap 代替字符串吗？

Question

I want to have a function to build a String from inputted characters and stop building if it gets an input character that it contains.

I know I can use String.contains() for this but I am learning about HashMaps and am wondering if a faster way to do this could be storing the inputted characters in a HashMap and using the HashMap.contains() method.

Answer 1

HashMap::containsKey is O(1) , String::contains is not . The implementation may change dependending of the JVM version but it's more something like O(n).

So yes, using an HashMap to look for a value should be faster (on small data you'll probably don't notice a difference) than calling String::contains . But a Map stores a key and a value, if you don't care about the value, you can use a Set (be careful, all the values are unique in this type of collection) because Set::contains is O(1) .

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As @n247s mentionned in the comment. Except if you really have a performance issue, String::contains should works fine and make the code simpler to read.

Answer 2

HashMap<> is just a class that extends the Map<> interface, you can use containsKey() or containsValue() . If you want to loop through the values in the HashMap , you can use the HashMaps.values() method and concat/add the value to the String.

UNTESTED:

int count = -1;
String new = "";
for (char c : map.values()) {
    count++;
    if (string.charAt(count).equals(c))
      break;
    new.concat(c);
}

Answer 3

A Set would be a good data structure to use here.

Just take note of 1 thing though,

If you need case-sensitive search then you can use a HashSet . Example

Set<String> set = new HashSet<>();

Else, if you need case-insensitive search then a TreeSet . Example

Set<String> set = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);

Answer 4

The best Approach would be to use a Set if you do not care about insertion order, but if you do, there is a LinkedHashSet you should use, that will keep your characters in order of insertion.

Here is a link to the LinkedHashSet doc. https://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashSet.html

But if you actually want to use the key / value pair of the HashMap , you can indeed use a HashMap<Character, Character> , then you can check with map.containsKey() O(1) , and map.containsValue() O(n) keep in mind since containsValue() will use O(n) , it's better to just check String::contain in that scenario. And if you care about insertion order, there is also a LinkedHashMap that will do that for you. Link to the doc bellow https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html