Java hashcode() string conflict

I know little about hash codes I found that this code can print out collisions

Can you tell me what collision is and how to reduce it? Why do we use hash codes?

public static int getHash(String str,int limit)
{
    int hashCode = Math.abs(str.hashCode()%(limit));
    return hashCode;
}

/**
 * @param args
 */
public static void main(String[] args)
{
    int hashLimit = 10000;
    int stringsLimit = 10000;
    String[] arr = new String[hashLimit];
    List<String> test = new ArrayList<String>();
    Random r = new Random(2);
    for ( int i = 0 ; i < stringsLimit ; i++ )
    {
        StringBuffer buf = new StringBuffer("");
        for ( int j = 0 ; j < 10 ; j++ )
        {
            char c = (char)(35+60*r.nextDouble());
            buf.append(c);
        }
        test.add(buf.toString());
        //System.out.println(buf.toString());
    }
    int collisions = 0;
    for ( String curStr : test )
    {
        int hashCode = getHash(curStr,hashLimit);
        if ( arr[hashCode] != null && !arr[hashCode].equals(curStr) )
        {
            System.out.println("collision of ["+arr[hashCode]+"] ("+arr[hashCode].hashCode()+" = "+hashCode+") with ["+curStr+"] ("+curStr.hashCode()+" = "+hashCode+")");
            collisions++;
        }
        else
        {
            arr[hashCode] = curStr;
        }
    }
    System.out.println("Collisions: "+collisions);
}

Solution

The conflict is that two unequal objects have the same hash code They are facts in life - you need to deal with it

Because they can basically press the key quickly to find the value The hash table can use the hash code to quickly download the possible key matching set to a very small set (usually only one). At this time, you need to check the actual key equality

You should never assume that two hash codes are equal, which means that their derived objects are equal Only the opposite: assuming a correct implementation, if two objects give different hash codes, they are not equal