CODEWITHSUNDEEP
hashmapcontainskey
Marnix
Marnix

Reputation: 3

Java util hashmap containsKey()

I have been having some trouble using the function containsKey. I wrote a small program to show where I am expecting containsKey to give me a different result: 

HashMap<IdentifierInterface, Set<NaturalNumberInterface>> hashMap;
HashMap<StringBuffer, Integer> works;

TryHashmap(){
    hashMap = new HashMap<IdentifierInterface, Set<NaturalNumberInterface>>();
    works = new HashMap<StringBuffer, Integer>();
}
private void start() {      
    Identifier iden = new Identifier('a');
    NaturalNumber nn = new NaturalNumber('8');
    Set<NaturalNumberInterface> set = new Set<NaturalNumberInterface>();
    set.insert(nn);

    hashMap.put(iden, set);
    System.out.println(hashMap.containsKey(iden));

    Identifier newIden = new Identifier('a');
    System.out.println(hashMap.containsKey(newIden)); //TODO why is this not true?

    iden.init('g');
    System.out.println(hashMap.containsKey(iden));
}

public static void main(String[] argv) {
    new TryHashmap().start();
}

The constructor of the Identifier class is as follows, the init() is similar but it will remove anything that was in the identifier before.

Identifier(char c){
    iden = new StringBuffer();
    iden.append(c);
}

I put something into the hashmap using an Identifier as key, but when I try to use an Identifier with a different name but with the same content the containsKey function returns false where I am expecting a true. (the output prints true false true)

Thanks in advance!

Upvotes: 0

Views: 1974

Answers (2)

stackoverflow
stackoverflow

Reputation: 78

method containsKey in HashMap.class

/**
 * Returns <tt>true</tt> if this map contains a mapping for the
 * specified key.
 *
 * @param   key   The key whose presence in this map is to be tested
 * @return <tt>true</tt> if this map contains a mapping for the specified
 * key.
 */
public boolean containsKey(Object key) {
    return getEntry(key) != null;
}

method getEntry in HashMap.class

   /** 
     * Returns the entry associated with the specified key in the 
     * HashMap.  Returns null if the HashMap contains no mapping 
     * for the key. 
     */  
    final Entry<K,V> getEntry(Object key) {  
        int hash = (key == null) ? 0 : hash(key.hashCode());  
        for (Entry<K,V> e = table[indexFor(hash, table.length)];  
             e != null;  
             e = e.next) {  
            Object k;  
            if (e.hash == hash &&  
                ((k = e.key) == key || (key != null && key.equals(k))))  
                return e;  
        }  
        return null;  
    }

the method getEntry told us that the result will be true, only if the Object a has the same hashCode() as the Object b and a.equals(b)

Upvotes: 0

Rubbal
Rubbal

Reputation: 809

Implement equals() and hashCode() for the identifier object. hashCode is needed to find the relevant bucket and equals is required to handle collisions while hashing.

Further Reading

Upvotes: 1

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