How to measure the time of a process

To measure the time of a process just call the System.nanoTime method when process starts and again when process ends. The time of the process will be the diference betwen the two calls.

package time;

public class Time {

    public static void main(String[] args) {

        long start_time, end_time, elapsedTime;

        start_time = System.nanoTime();

        // ... the code being measured ...

        end_time = System.nanoTime();
        elapsedTime = end_time - start_time;
        
        System.out.println("Start: " + start_time);
        System.out.println("End  : " + end_time);
    ...

The System.nanoTime method returns values in nanoseconds, to convert nanoseconds to miliseconds or seconds you just need to make a few divisions:

     System.out.println("The process took approximately:\n "
                        +elapsedTime+" nano seconds\n " 
                        +(elapsedTime/1000000.0)+" miliseconds\n " 
                        +(elapsedTime/1000000000.0)+" seconds");

This tip can be found at: http://download.oracle.com/javase/7/docs/api/

Removing entries from a HashMap

There are a few solutions for looping a HashMap, but one of the most elegant looks like below:

HashMap<String, Integer> map = new HashMap<String, Integer>();
  map.put("One", 1);
  map.put("Two", 2);
  map.put("Three", 3);
  map.put("Four", 4);
  map.put("Five", 5);
  map.put("Six", 6);
  map.put("Seven", 7);
  map.put("Eight", 8);
  map.put("Nine", 9);
  map.put("Ten", 10);
        
  for (Map.Entry<String, Integer> entry : map.entrySet()) {
       System.out.println(map.getKey() + "    " + map.getValue());      
  }

Now, that is great, but if you try to remove an entry while looping

  for (Map.Entry<String, Integer> entry : map.entrySet()) {
     if(entry.getValue() > 5){
        map.remove(entry.getKey());
      }
}

an java.util.ConcurrentModificationException will occur!
Solving this issue involves adding an Iterator which conforming to documentation “Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.” will allows us to call remove(). Therefore, here it is:

for(Iterator<Map.Entry<String,Integer>>it=map.entrySet().iterator();it.hasNext();){
     Map.Entry<String, Integer> entry = it.next();
     if (entry.getValue() > 5) {
          it.remove();
     }
 }

Done !