Which approach to use to iterate a collection in Java

Table of Contents

• Ways to iterate
• Use foreach or stream api when possible and make sense
• Iterator
• For loop

Ways to iterate

There are different ways to iterate a collection in Java: - Foreach loop - Iterator - For Loop - While Loop - Stream API

Use foreach or stream api when possible and make sense

If possible, use foreach loop, or stream api, as they are simplest and less error-prone: no extra state to track. - But don’t use stream.foreach

All other approaches need track more states: - when use Iterator, need call iterator.hasNext(), iterator.next() - when use for loop, need track the index, same for the while loop

Iterator

• Iterator works for all collections
  • compared with for loop, list.get(index) would have performance issue for linkedlist.
• if(iterator.hasNext()) is less error prone, and easier to read and write compared with if(index == list.size()-1 )

For loop

int index = 0;
while (valuesListIt.hasNext()) {
  
  values = valuesIt.next();
  // ...
  index++;
}

In the last example, we use iterator to iterate the collection, and also track the index. It would be simpler to use for loop.

for (int index = 0; index < valuesList.size(); index++ ) {
  // 
}

int index = 0;
while (valuesListIt.hasNext()) {
  
  values = valuesIt.next();
  //....
  index++;
  // is last element?
  if(valuesListIt.hasNext()){
  
  }
}

Similarly, the above code can be simplified to as below:

for (int index = 0; index < valuesList.size(); index++ ) {
  
  if( index != valuesList.size() -1 ){
    
  }
}