Java hashcode()字符串冲突

你能告诉我什么是碰撞以及如何减少它？
我们为什么要使用哈希码？

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);
}