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MrRage

[java] Simple HashMap example please...

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I'm sorry but I've been struggling with this all day and for the life of me can't make it work. After a little narrowing it down the null is coming from myHash.get(key).toString(), so... I'm confused. java.lang.NullPointerException at hash.hash.main(hash.java:22)
package hash;

import java.util.Iterator;
import java.util.HashMap;
import java.util.Map;
import java.lang.Integer;

public class hash {
    public hash() {
    }

    public static void main(String[] args) {
        hash hash = new hash();
        Map myHash = new HashMap();
        myHash.put(new Integer(1), "One");
        myHash.put(new Integer(23), "Twenty Three");
        myHash.put(new Integer(45), "Fourty Five");
        
        Iterator it = myHash.entrySet().iterator();
        while (it.hasNext()) {
            Object key = it.next();
            System.out.println(key.toString() + " " + myHash.get(key).toString()); // This line 22
        }
    }
}



Found it... If one were to replace entrySet with keySet... it works. That was supper lame. [Edited by - MrRage on June 12, 2007 1:26:04 AM]

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You should make use of generics to increase type safety, they have been around since Java 5. Boxing makes the code more readable. You don't need to create an instance of your class. And there's nothing wrong with entrySet. Take a look at this:


public static void main(String[] args)
{
Map<Integer, String> myHash = new HashMap<Integer, String>();
myHash.put(1, "One");
myHash.put(23, "Twenty Three");
myHash.put(45, "Fourty Five");
for (Map.Entry<Integer, String> entry: myHash.entrySet())
System.out.println(entry.getKey() + " " + entry.getValue());
}


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