Looping through a HashMap using iterator does not stop for an if condition

Question

A HashMap<Integer, Integer> has 10 entries but I only want to print 3 entries.

Code:

HashMap<Integer,Integer> hm = new HashMap<Integer,Integer>();
hm.put(2, 1);
hm.put(5, 3);
hm.put(7, 2);
hm.put(4, 1);
hm.put(6, 3);
hm.put(8, 2);
hm.put(9, 1);
hm.put(3, 3);
hm.put(1, 2);       
hm.put(0, 2);
Iterator <Map.Entry<Integer, Integer>> itr = hm.entrySet().iterator();
int n=4;
int i=0;
while(itr.hasNext()){
    if(i<n){
        Map.Entry<Integer, Integer> entry = itr.next();
        System.out.println(entry.getKey()+" repeated "+entry.getValue());
    }
    i++;
}

Output

0 repeated 2
1 repeated 2
2 repeated 1
3 repeated 3 //program will wait for 2 or 3 seconds here
4 repeated 1
5 repeated 3
6 repeated 3
7 repeated 2
8 repeated 2
9 repeated 1

Expecting output:

0 repeated 2
1 repeated 2
2 repeated 1
3 repeated 3

Why is the if-condition inside while-loop is not working?

Hint: Print i inside of your loop. What do you think is happening? — maloomeister
– maloomeister, Commented Nov 15, 2023 at 12:42

Thiyagu · Accepted Answer · 2023-11-15 12:46:12Z

5

You don't exit out of the loop once i becomes 4. Also, you don't consume/read the subsequent elements from the iterator as itr.next() is inside the if block. So, it keeps incrementing i until it overflows to Integer.MIN_VALUE (-2147483648) and this takes a few seconds (which you have observed).

At this point the condition of the if block becomes true (i < n) and hence it starts printing the other elements from the iterator.

You can print the value of i to see this.

System.out.println(entry.getKey()+" repeated "+entry.getValue() +". The value of i is: " + i);

Prints,

0 repeated 2. The value of i is: 0
1 repeated 2. The value of i is: 1
2 repeated 1. The value of i is: 2
3 repeated 3. The value of i is: 3
4 repeated 1. The value of i is: -2147483648
5 repeated 3. The value of i is: -2147483647
6 repeated 3. The value of i is: -2147483646
7 repeated 2. The value of i is: -2147483645
8 repeated 2. The value of i is: -2147483644
9 repeated 1. The value of i is: -2147483643

answered Nov 15, 2023 at 12:46

Thiyagu

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2 Comments

user2817235 Over a year ago

Please help me understand why it keeps incrementing until it overflows to Integer.MIN_VALUE (-2147483648)? instead of Integer.MAX_VALUE(2147483647) as we are adding(++) , not subtracting(--)

Thomas Kläger Over a year ago

@user2817235 Integer.MAX_VALUE + 1 == Integer.MIN_VALUE

Lajos Arpad · Accepted Answer · 2023-11-15 13:14:39Z

1

When i reaches n the if condition will become false and i will be incremented indefinitely whereas the iterator will not step to the next. At some point i starts to be smaller than n due to number overflow (but a lot of while iterations are passed without going to the next in the meantime) and then your loop ends in an undesired manner. You will need

} else {
    itr.next();
}

or change somehow the condition of your loop so it will not continue its work when it no longer makes sense.

answered Nov 15, 2023 at 13:14

Lajos Arpad

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Comments

Ahmed Mera · Accepted Answer · 2023-11-17 08:55:29Z

1

You must move out of if the iter.next(), so the solution is:

import java.util.*;

public class Main {
  public static void main(String[] args) {
HashMap<Integer,Integer> hm = new HashMap<Integer,Integer>();
hm.put(2, 1);
hm.put(5, 3);
hm.put(7, 2);
hm.put(4, 1);
hm.put(6, 3);
hm.put(8, 2);
hm.put(9, 1);
hm.put(3, 3);
hm.put(1, 2);       
hm.put(0, 2);
Iterator <Map.Entry<Integer, Integer>> itr = hm.entrySet().iterator();
int n=4;
int i=0;
while(itr.hasNext()){
       Map.Entry<Integer, Integer> entry = itr.next();
    if(i<n){
        System.out.println(entry.getKey()+" repeated "+entry.getValue());
    }
 
    i++;
}
  }
}

OUTOUT is:

0 repeated 2
1 repeated 2
2 repeated 1
3 repeated 3

answered Nov 17, 2023 at 8:55

Ahmed Mera

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