Collections Looping Example

Concept of the Iterator

An iterator is an object that enables us to traverse a collection. There is an iterator (java.util.Iterator) in all the top level interfaces of the Java Collections Framework that inherits java.util.Collection interface. These interfaces are java.util.List,java.util.Queue, java.util.Deque, and java.util.Set. Furthermore, there is the java.util.Map interface that does not inheritjava.util.Collection.

Lists also have a special iterator called a list iterator (java.util.ListIterator). What’s the difference? The java.util.Iterator is forward looking only while the java.util.ListIterator is bidirectional (forward and backward). Furthermore, thejava.util.ListIterator inherits java.util.Iterator. The result of using either iterator to loop through a list will be the same as we will see later.

Collections Looping Approaches

Can you guess how many ways we can loop through a collection given there are 2 forms of loop constructs and all interfaces have an iterator.

Iterate through List Collections

We declare a list of strings and populate it.

List<String> strList = new ArrayList<String>();

strList.add("list item 1");

strList.add("list item 2");

strList.add("list item 3");

strList.add("list item 4");

strList.add("list item 5");

The enhanced for loop is the first way and most direct.

for (String s : strList) {

  System.out.println(s);

}

Next is the classic for loop, which uses an iterator.

for (Iterator<String> it = strList.iterator(); it.hasNext(); ) {

  System.out.println(it.next());

}

Notice that there is no increment/decrement in this classic for loop.

We mentioned earlier that Lists have a special iterator called ListIterator. We can easily replace for loop’s initialization to either of the following and it would work as well.

ListIterator<String> it = strList.listIterator();

Iterator<String> it = strList.listIterator();

Next is the while loop, which is basically the classic for loop represented in a different format.

Iterator<String> itA = strList.iterator();

while (itA.hasNext()) {

  System.out.println(itA.next());

}

Similarly the iterator declaration (variable itA) can be replaced to either of the following and it would work as well.

ListIterator<String> itA = strList.listIterator();

Iterator<String> itA = strList.listIterator();

Finally the last approach is the do-while loop. We can easily refactor the while loop to become a do-while loop as shown below.

Iterator<String> itB = strList.iterator();

do {

  System.out.println(itB.next());

} while (itB.hasNext());

Again the iterator declaration (variable itB) can be replaced with the list iterator form.

ListIterator<String> itB = strList.listIterator();

Iterator<String> itB = strList.listIterator();

In addition to using the iterator, we can use the List’s position or index. The classic for loop:

for (int i=0; i<strList.size(); i++) {

  System.out.println(strList.get(i));

}

The variable i inside the above for loop indicates the current position or index of the list.

The while loop:

int i=0;

while (i<strList.size()) {

  System.out.println(strList.get(i));

  i++;

}

The do-while loop:

int j=0;

do {

  System.out.println(strList.get(j));

  j++;

} while (j<strList.size());

Iterate through Set Collections

We declare a set of strings and populate it.

Set<String> strSet = new HashSet<String>();

strSet.add("set item 1");

strSet.add("set item 2");

strSet.add("set item 3");

strSet.add("set item 4");

strSet.add("set item 5");

The enhanced for loop:

for (String s : strSet) {

  System.out.println(s);

}

The classic for loop:

for (Iterator<String> it = strSet.iterator(); it.hasNext(); ) {

  System.out.println(it.next());

}




The while loop:

Iterator<String> itA = strSet.iterator();

while (itA.hasNext()) {

  System.out.println(itA.next());

}

The do-while loop:

Iterator<String> itB = strSet.iterator();

do {

  System.out.println(itB.next());

} while (itB.hasNext());

Iterate through Map collections

We declare a map of string keys with string values and populate it.

Map<String, String> strMap = new HashMap<String, String>();

strMap.put("key 1", "value 1");

strMap.put("key 2", "value 2");

strMap.put("key 3", "value 3");

strMap.put("key 4", "value 4");

strMap.put("key 5", "value 5");

The enhanced for loop:

for (Map.Entry<String, String> entry : strMap.entrySet()) {

  System.out.println(entry.getKey() + "=" + entry.getValue());

}

The classic for loop:

for (Iterator<String> it = strMap.keySet().iterator(); it.hasNext(); ) {

  String key = it.next();

  System.out.println(key + "=" + strMap.get(key));

}

The while loop:

Iterator<String> itA = strMap.keySet().iterator();

while (itA.hasNext()) {

  String key = itA.next();

  System.out.println(key + "=" + strMap.get(key));

}

The do-while loop:

Iterator<String> itB = strMap.keySet().iterator();

do {

  String key = itB.next();

  System.out.println(key + "=" + strMap.get(key));

} while (itB.hasNext());