HashMap.containsKey() - 如何搜索 class？

Question

Hello
if you search in an HashMap<String,String> for a specific value of a key-value-pair, you can write the following:

myHashMap.containsKey(myString);

But how can I manage it if the key is not a string? I have a class which looks like this:

public class Kategorie implements Comparable {
    private String name;

    public Kategorie()  {
        super();
    }

    public Kategorie(String name)  {
        setName(name);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public int compareTo(Object o) {
        if (!(o instanceof Kategorie))  
           throw new ClassCastException();

        Kategorie k = (Kategorie)o;
        String name = k.getName();
        return this.getName().compareTo(name);

    }
}

In a map I saved keys and values of this type "Kategorie".

mapKategorieDEundEN.put(new Kategorie(strName_de), new Kategorie(strName_en));

Later in the code, I want to check if there is a key with a specific string.

if (mapKategorieDEundEN.containsKey(searchString))  {

...doesn't work, because the key is not a string but a "Kategorie", that's clear.

Then I tried something like this:

if (mapKategorieDEundEN.containsKey(new Kategorie(searchString)))  {

...doesn't work too. I assume that it doesn't find anything because the object is not the "original" object but a new one.

In this case, can I use containsKey at all or do I have to use a loop over the HashMap?

Answer 1

You class should override equals and hashCode , it will work after that.

The HashMap/Hashtable puts the items in "buckets" by using the hashCode of the key, so a new object that represents the same value as another object, and which should be considered as the same object must return the same hashCode. All keys that return the same hashCode will then be considered as candidates, and equals will be invoked on them. It's considered a match if equals returns true.

Answer 2

HashMap uses hashCode() and equals() . You have to implement them. If you don't know how. Check your IDE (eclipse) usually can generate them for you.

Answer 3

If you want to access your objects using your compareTo method, you should not use a hashCode/equals based Map, but a SortedMap, like TreeMap (or ConcurrentSkipListMap).

This has the added benefit that it enables range-based queries (eg "give me all categories larger than this one"), but is a bit slower (O(log n) instead of O(1)) for simple get accesses compared to hash-based access (with a good hash code, not a constant one).

For a general use class, defining both hashCode/equals and compareTo would be sensible, then the user of the class can decide which type of map to use. (If there are different ways to sort your objects, better provide different Comparator objects.)

As a side remark, you should not implement Comparable , but Comparable<Kategorie> . Then your compareTo method would look like this:

public int compareTo(Kategorie k) {
    String name = k.getName();
    return this.getName().compareTo(name);
}