Iterating LinkedHashMap values from the key

Question

I'm struggling with a little bit of code, and I just cannot figure out what the correct syntax would be. Here is an example of the problem:

for (Integer key : map1.keySet()) {
 for (Integer value : map2.values()) {
 if (value == key) {
 // ++ the corresponding value of the looping map1 key.
 // tried statements like map1.values()++ 
 // but this is an invalid operation.
 }
 }
}

I am trying to loop map1 keys through map2 values and if within this process a map1 key matches a map2 value, I want to add one to the value of the corresponding map1 key.

Should be a simple problem but Eclipse keeps telling me that I have the syntax wrong, can anyone suggest what statement I may need to iterate the values?

• Guy Gavriely: please add Eclipse syntax error
• takendarkk: A stacktrace would be great.

Answer 1

Here is how you can do it with a very small modification to your code:

for (Map.Entry<Integer,Integer> entry : map1.entrySet()) {
 for (Integer value : map2.values()) {
 if (value.equals(entry.getKey())) {
 entry.setValue(entry.getValue()+1);
 }
 }
}

EDIT : Since map2 could contain the same value more than once, the other solution that uses a hash set to speed things up will not work as expected.

• user2941526: Thanks for the help! with this set up I am getting just 1 for the values of map1. I need plus 1 for each time a map1 key equals a map2 value. Do I just need to move the "+1" elsewhere?
• Sergey Kalinichenko: @user2941526 I see - map2 can have the same value multiple times, right? Then you should the version with two loops.
• Louis Wasserman: You should probably use .equals, not ==, since you're working with boxed Integer objects.
• Sergey Kalinichenko: @LouisWasserman Absolutely! Thank you very much - I copy/pasted the code, and missed this important issue. Great spotting! Thanks!

Answer 2

How you init yours maps? Like Map<Integer, Integer> map1=new LinkedHashMap<Integer, Integer>(); or Map map1=new LinkedHashMap(); If the second way the key will be Object not Integer. And in all cases compare two Integer using == bad practise. Use equals

Answer 3

I'm not sure why Eclipse report a syntax error, but in any case this loop does not make much sense, consider changing it to:

for (Integer value : map2.values())
 if (map1.containsKey(value))
 // bla bla

it will run in O(n) rather than the former O(n*m)

when n is the size of map2 and m is the size of map1 (as notified by @dasblinkenlight)

• Guy Gavriely: I believe a map should find in and average of O(1)
• Guy Gavriely: @dasblinkenlight pls see stackoverflow.com/questions/1055243/is-a-java-hashmap-really-o1
• Guy Gavriely: yes, as you can see I look for the key there, thats why I switched the order of the blocks
• Sergey Kalinichenko: Gee, you're right, I did not notice that you've switched the order of the loops. You should mention that n is the size of map2, because the sizes of the two maps may differ.

Answer 4

You can use containsKey method like this:

for (Integer value : map2.values()){
if (map1.containsKey(value))
 map1.put(key, value);
...
}