Hello Friends,
In this tutorial ,we will see how to iterate over very commonly used collections viz. ArrayList, HashMap and HashSet.
1. How to iterate over an ArrayList
a) Normal for loop
List list = new ArrayList();
list.add("a");
list.add("b");
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}
b) Using Iterator - before Java 5
List list = new ArrayList();
list.add("a");
list.add("b");
for (Iterator itrList= list.iterator(); itrList.hasNext();) {
System.out.println(itrList.next());
}
c) Using Iterator with For loop -After Java 5
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
for (Iterator<String> itrList= list.iterator(); itrList.hasNext();) {
System.out.println(itrList.next());
}
d) Using Iterator with while loop -After Java 5
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
Iterator<String> itrList = list.iterator();
while(itrList.hasNext()){
System.out.println(itrList.next());
}
e) Using ListIterator
To traverse in forward direction
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
ListIterator< String> lstItr = list.listIterator();
while(lstItr.hasNext()){
String str = (String)lstItr.next();
System.out.println(str);
}
To traverse in backward direction
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
ListIterator< String> lstItr = list.listIterator();
while(lstItr.hasPrevious()){
String str = (String)lstItr.previous();
System.out.println(str);
}
f) Using enhanced For Loop : Java 5 onwards
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
for(String str : list){
System.out.println(str);
}
g) using Lamda expression : Java 8 onwards
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
list.forEach(listVal-> System.out.println(listVal));
2.How to iterate over HashMap
a) Using iterator -Before Java 5
Map map = new HashMap();
map.put("a","a1");
map.put("b","b1");
Iterator itrMap = map.entrySet().iterator();
while (itrMap .hasNext()) {
Map.Entry entry = (Map.Entry) itrMap.next();
String key = (String)entry.getKey();
String value = (String)entry.getValue();
System.out.println("Key = " + key + ", Value = " + value);
}
b) Using iterator - After Java 5
Map<String, String> map = new HashMap<String, String>();
map.put("a","a1");
map.put("b","b1");
Set<Entry<String,String>> entrySet = map.entrySet();
Iterator<Entry<String,String>> itrEntrySet =entrySet .iterator();
while (itrEntrySet.hasNext()) {
Entry<String, String> entry = (Entry<String,String>)itrEntrySet.next();
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}
c) Using enhanced for loop - After Java 5 :Iterating over keys as well as
values
Map<String, String> map = new HashMap<String, String>();
map.put("a","a1");
map.put("b","b1");
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}
d) Using enhanced for loop - After Java 5 :Iterating only over keys or
values
Map<String, String> map = new HashMap<String, String>();
map.put("a","a1");
map.put("b","b1");
for (String key : map.keySet()) {
System.out.println("Key = " + key);
}
//iterating over values only
for (String value : map.values()) {
System.out.println("Value = " + value);
}
e) Iterating over keys and searching for values
Map<String, String> map = new HashMap<String, String>();
map.put("a","a1");
map.put("b","b1");
Map<String, String> map = new HashMap<String, String>();
for (String key : map.keySet()) {
String value = map.get(key);
System.out.println("Key = " + key + ", Value = " + value);
}
f) Using Lamda expression - Java 8 onwards
Map<String, String> map = new HashMap<String, String>();
map.put("a","a1");
map.put("b","b1");
map.forEach( (k,v) -> System.out.println("Key: " + k + ": Value: " + v));
3.How to iterate over HashSet
a) Using Iterator - Before Java 4
Set set = new HashSet();
set.add("x");
set.add("y");
Iterator itrSet = set.iterator();
while (itrSet .hasNext()) {
System.out.println(itrSet.next());
}
b) Using Iterator - After Java 5
Set<String> set = new HashSet<String>();
set.add("x");
set.add("y");
Iterator<String> itrSet = set.iterator();
while (itrSet .hasNext()) {
System.out.println(itrSet .next());
}
c)Using Enhanced For loop - After Java 5
Set<String> set = new HashSet<String>();
set.add("x");
set.add("y");
for (String s : set) {
System.out.println(s);
}
d) Using Lamda expression - After Java 8
Set<String> set = new HashSet<String>();
set.add("x");
set.add("y");
set.forEach(val ->System.out.println(val));
Hope this was helpful.Any feedback ,suggestion,questions on the post are most
welcome.
