How do I efficiently iterate over each entry in a Java Map?
JavaDictionaryCollectionsIterationJava Problem Overview
If I have an object implementing the Map
interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map?
Will the ordering of elements depend on the specific map implementation that I have for the interface?
Java Solutions
Solution 1 - Java
Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println(entry.getKey() + "/" + entry.getValue());
}
On Java 10+:
for (var entry : map.entrySet()) {
System.out.println(entry.getKey() + "/" + entry.getValue());
}
Solution 2 - Java
To summarize the other answers and combine them with what I know, I found 10 main ways to do this (see below). Also, I wrote some performance tests (see results below). For example, if we want to find the sum of all of the keys and values of a map, we can write:
-
Using iterator and Map.Entry
long i = 0; Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator(); while (it.hasNext()) { Map.Entry<Integer, Integer> pair = it.next(); i += pair.getKey() + pair.getValue(); }
-
Using foreach and Map.Entry
long i = 0; for (Map.Entry<Integer, Integer> pair : map.entrySet()) { i += pair.getKey() + pair.getValue(); }
-
Using forEach from Java 8
final long[] i = {0}; map.forEach((k, v) -> i[0] += k + v);
-
Using keySet and foreach
long i = 0; for (Integer key : map.keySet()) { i += key + map.get(key); }
-
Using keySet and iterator
long i = 0; Iterator<Integer> itr2 = map.keySet().iterator(); while (itr2.hasNext()) { Integer key = itr2.next(); i += key + map.get(key); }
-
Using for and Map.Entry
long i = 0; for (Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator(); entries.hasNext(); ) { Map.Entry<Integer, Integer> entry = entries.next(); i += entry.getKey() + entry.getValue(); }
-
Using the Java 8 Stream API
final long[] i = {0}; map.entrySet().stream().forEach(e -> i[0] += e.getKey() + e.getValue());
-
Using the Java 8 Stream API parallel
final long[] i = {0}; map.entrySet().stream().parallel().forEach(e -> i[0] += e.getKey() + e.getValue());
-
Using IterableMap of
Apache Collections
long i = 0; MapIterator<Integer, Integer> it = iterableMap.mapIterator(); while (it.hasNext()) { i += it.next() + it.getValue(); }
-
Using MutableMap of Eclipse (CS) collections
final long[] i = {0}; mutableMap.forEachKeyValue((key, value) -> { i[0] += key + value; });
Perfomance tests (mode = AverageTime, system = Windows 8.1 64-bit, Intel i7-4790 3.60 GHz, 16 GB)
-
For a small map (100 elements), score 0.308 is the best
Benchmark Mode Cnt Score Error Units test3_UsingForEachAndJava8 avgt 10 0.308 ± 0.021 µs/op test10_UsingEclipseMap avgt 10 0.309 ± 0.009 µs/op test1_UsingWhileAndMapEntry avgt 10 0.380 ± 0.014 µs/op test6_UsingForAndIterator avgt 10 0.387 ± 0.016 µs/op test2_UsingForEachAndMapEntry avgt 10 0.391 ± 0.023 µs/op test7_UsingJava8StreamApi avgt 10 0.510 ± 0.014 µs/op test9_UsingApacheIterableMap avgt 10 0.524 ± 0.008 µs/op test4_UsingKeySetAndForEach avgt 10 0.816 ± 0.026 µs/op test5_UsingKeySetAndIterator avgt 10 0.863 ± 0.025 µs/op test8_UsingJava8StreamApiParallel avgt 10 5.552 ± 0.185 µs/op
-
For a map with 10000 elements, score 37.606 is the best
Benchmark Mode Cnt Score Error Units test10_UsingEclipseMap avgt 10 37.606 ± 0.790 µs/op test3_UsingForEachAndJava8 avgt 10 50.368 ± 0.887 µs/op test6_UsingForAndIterator avgt 10 50.332 ± 0.507 µs/op test2_UsingForEachAndMapEntry avgt 10 51.406 ± 1.032 µs/op test1_UsingWhileAndMapEntry avgt 10 52.538 ± 2.431 µs/op test7_UsingJava8StreamApi avgt 10 54.464 ± 0.712 µs/op test4_UsingKeySetAndForEach avgt 10 79.016 ± 25.345 µs/op test5_UsingKeySetAndIterator avgt 10 91.105 ± 10.220 µs/op test8_UsingJava8StreamApiParallel avgt 10 112.511 ± 0.365 µs/op test9_UsingApacheIterableMap avgt 10 125.714 ± 1.935 µs/op
-
For a map with 100000 elements, score 1184.767 is the best
Benchmark Mode Cnt Score Error Units test1_UsingWhileAndMapEntry avgt 10 1184.767 ± 332.968 µs/op test10_UsingEclipseMap avgt 10 1191.735 ± 304.273 µs/op test2_UsingForEachAndMapEntry avgt 10 1205.815 ± 366.043 µs/op test6_UsingForAndIterator avgt 10 1206.873 ± 367.272 µs/op test8_UsingJava8StreamApiParallel avgt 10 1485.895 ± 233.143 µs/op test5_UsingKeySetAndIterator avgt 10 1540.281 ± 357.497 µs/op test4_UsingKeySetAndForEach avgt 10 1593.342 ± 294.417 µs/op test3_UsingForEachAndJava8 avgt 10 1666.296 ± 126.443 µs/op test7_UsingJava8StreamApi avgt 10 1706.676 ± 436.867 µs/op test9_UsingApacheIterableMap avgt 10 3289.866 ± 1445.564 µs/op
Graphs (performance tests depending on map size)
Table (perfomance tests depending on map size)
100 600 1100 1600 2100
test10 0.333 1.631 2.752 5.937 8.024
test3 0.309 1.971 4.147 8.147 10.473
test6 0.372 2.190 4.470 8.322 10.531
test1 0.405 2.237 4.616 8.645 10.707
test2 0.376 2.267 4.809 8.403 10.910
test7 0.473 2.448 5.668 9.790 12.125
test9 0.565 2.830 5.952 13.220 16.965
test4 0.808 5.012 8.813 13.939 17.407
test5 0.810 5.104 8.533 14.064 17.422
test8 5.173 12.499 17.351 24.671 30.403
All tests are on GitHub.
Solution 3 - Java
In Java 8 you can do it clean and fast using the new lambdas features:
Map<String,String> map = new HashMap<>();
map.put("SomeKey", "SomeValue");
map.forEach( (k,v) -> [do something with key and value] );
// such as
map.forEach( (k,v) -> System.out.println("Key: " + k + ": Value: " + v));
The type of k
and v
will be inferred by the compiler and there is no need to use Map.Entry
anymore.
Easy-peasy!
Solution 4 - Java
Yes, the order depends on the specific Map implementation.
@ScArcher2 has the more elegant Java 1.5 syntax. In 1.4, I would do something like this:
Iterator entries = myMap.entrySet().iterator();
while (entries.hasNext()) {
Entry thisEntry = (Entry) entries.next();
Object key = thisEntry.getKey();
Object value = thisEntry.getValue();
// ...
}
Solution 5 - Java
Typical code for iterating over a map is:
Map<String,Thing> map = ...;
for (Map.Entry<String,Thing> entry : map.entrySet()) {
String key = entry.getKey();
Thing thing = entry.getValue();
...
}
HashMap
is the canonical map implementation and doesn't make guarantees (or though it should not change the order if no mutating operations are performed on it). SortedMap
will return entries based on the natural ordering of the keys, or a Comparator
, if provided. LinkedHashMap
will either return entries in insertion-order or access-order depending upon how it has been constructed. EnumMap
returns entries in the natural order of keys.
(Update: I think this is no longer true.) Note, IdentityHashMap
entrySet
iterator currently has a peculiar implementation which returns the same Map.Entry
instance for every item in the entrySet
! However, every time a new iterator advances the Map.Entry
is updated.
Solution 6 - Java
Example of using iterator and generics:
Iterator<Map.Entry<String, String>> entries = myMap.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry<String, String> entry = entries.next();
String key = entry.getKey();
String value = entry.getValue();
// ...
}
Solution 7 - Java
This is a two part question:
How to iterate over the entries of a Map - @ScArcher2 has answered that perfectly.
What is the order of iteration - if you are just using Map
, then strictly speaking, there are no ordering guarantees. So you shouldn't really rely on the ordering given by any implementation. However, the SortedMap
interface extends Map
and provides exactly what you are looking for - implementations will aways give a consistent sort order.
NavigableMap
is another useful extension - this is a SortedMap
with additional methods for finding entries by their ordered position in the key set. So potentially this can remove the need for iterating in the first place - you might be able to find the specific entry
you are after using the higherEntry
, lowerEntry
, ceilingEntry
, or floorEntry
methods. The descendingMap
method even gives you an explicit method of reversing the traversal order.
Solution 8 - Java
There are several ways to iterate over map.
Here is comparison of their performances for a common data set stored in map by storing a million key value pairs in map and will iterate over map.
1) Using entrySet()
in for each loop
for (Map.Entry<String,Integer> entry : testMap.entrySet()) {
entry.getKey();
entry.getValue();
}
50 milliseconds
2) Using keySet()
in for each loop
for (String key : testMap.keySet()) {
testMap.get(key);
}
76 milliseconds
3) Using entrySet()
and iterator
Iterator<Map.Entry<String,Integer>> itr1 = testMap.entrySet().iterator();
while(itr1.hasNext()) {
Map.Entry<String,Integer> entry = itr1.next();
entry.getKey();
entry.getValue();
}
50 milliseconds
4) Using keySet()
and iterator
Iterator itr2 = testMap.keySet().iterator();
while(itr2.hasNext()) {
String key = itr2.next();
testMap.get(key);
}
75 milliseconds
I have referred this link
.
Solution 9 - Java
FYI, you can also use map.keySet()
and map.values()
if you're only interested in keys/values of the map and not the other.
Solution 10 - Java
The correct way to do this is to use the accepted answer as it is the most efficient. I find the following code looks a bit cleaner.
for (String key: map.keySet()) {
System.out.println(key + "/" + map.get(key));
}
Solution 11 - Java
With Java 8, you can iterate Map using forEach and lambda expression,
map.forEach((k, v) -> System.out.println((k + ":" + v)));
Solution 12 - Java
With Eclipse Collections, you would use the forEachKeyValue
method on the MapIterable
interface, which is inherited by the MutableMap
and ImmutableMap
interfaces and their implementations.
MutableBag<String> result = Bags.mutable.empty();
MutableMap<Integer, String> map = Maps.mutable.of(1, "One", 2, "Two", 3, "Three");
map.forEachKeyValue((key, value) -> result.add(key + value));
Assert.assertEquals(Bags.mutable.of("1One", "2Two", "3Three"), result);
Using an anonymous inner class, you can write the code as follows:
final MutableBag<String> result = Bags.mutable.empty();
MutableMap<Integer, String> map = Maps.mutable.of(1, "One", 2, "Two", 3, "Three");
map.forEachKeyValue(new Procedure2<Integer, String>()
{
public void value(Integer key, String value)
{
result.add(key + value);
}
});
Assert.assertEquals(Bags.mutable.of("1One", "2Two", "3Three"), result);
Note: I am a committer for Eclipse Collections.
Solution 13 - Java
In theory, the most efficient way will depend on which implementation of Map. The official way to do this is to call map.entrySet()
, which returns a set of Map.Entry
, each of which contains a key and a value (entry.getKey()
and entry.getValue()
).
In an idiosyncratic implementation, it might make some difference whether you use map.keySet()
, map.entrySet()
or something else. But I can't think of a reason why anyone would write it like that. Most likely it makes no difference to performance what you do.
And yes, the order will depend on the implementation - as well as (possibly) the order of insertion and other hard-to-control factors.
[edit] I wrote valueSet()
originally but of course entrySet()
is actually the answer.
Solution 14 - Java
Java 8
We have got forEach
method that accepts a lambda expression. We have also got stream APIs. Consider a map:
Map<String,String> sample = new HashMap<>();
sample.put("A","Apple");
sample.put("B", "Ball");
Iterate over keys:
sample.keySet().forEach((k) -> System.out.println(k));
Iterate over values:
sample.values().forEach((v) -> System.out.println(v));
Iterate over entries (Using forEach and Streams):
sample.forEach((k,v) -> System.out.println(k + ":" + v));
sample.entrySet().stream().forEach((entry) -> {
Object currentKey = entry.getKey();
Object currentValue = entry.getValue();
System.out.println(currentKey + ":" + currentValue);
});
The advantage with streams is they can be parallelized easily in case we want to. We simply need to use parallelStream()
in place of stream()
above.
forEachOrdered
vs forEach
with streams ?
The forEach
does not follow encounter order (if defined) and is inherently non-deterministic in nature where as the forEachOrdered
does. So forEach
does not guarantee that the order would be kept. Also check this for more.
Solution 15 - Java
> Lambda Expression Java 8
In Java 1.8 (Java 8) this has become lot easier by using forEach method from Aggregate operations(Stream operations) that looks similar to iterators from Iterable Interface.
Just copy paste below statement to your code and rename the HashMap variable from hm to your HashMap variable to print out key-value pair.
HashMap<Integer,Integer> hm = new HashMap<Integer, Integer>();
/*
* Logic to put the Key,Value pair in your HashMap hm
*/
// Print the key value pair in one line.
hm.forEach((k, v) -> System.out.println("key: " + k + " value:" + v));
// Just copy and paste above line to your code.
Below is the sample code that I tried using Lambda Expression. This stuff is so cool. Must try.
HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
Random rand = new Random(47);
int i = 0;
while(i < 5) {
i++;
int key = rand.nextInt(20);
int value = rand.nextInt(50);
System.out.println("Inserting key: " + key + " Value: " + value);
Integer imap = hm.put(key, value);
if( imap == null) {
System.out.println("Inserted");
} else {
System.out.println("Replaced with " + imap);
}
}
hm.forEach((k, v) -> System.out.println("key: " + k + " value:" + v));
Output:
Inserting key: 18 Value: 5
Inserted
Inserting key: 13 Value: 11
Inserted
Inserting key: 1 Value: 29
Inserted
Inserting key: 8 Value: 0
Inserted
Inserting key: 2 Value: 7
Inserted
key: 1 value:29
key: 18 value:5
key: 2 value:7
key: 8 value:0
key: 13 value:11
Also one can use Spliterator for the same.
Spliterator sit = hm.entrySet().spliterator();
UPDATE
Including documentation links to Oracle Docs. For more on Lambda go to this link and must read Aggregate Operations and for Spliterator go to this link.
Solution 16 - Java
Java 8:
You can use lambda expressions:
myMap.entrySet().stream().forEach((entry) -> {
Object currentKey = entry.getKey();
Object currentValue = entry.getValue();
});
For more information, follow this.
Solution 17 - Java
In Map one can Iteration over keys
and/or values
and/or both (e.g., entrySet)
depends on one's interested in_ Like:
-
Iterate through the
keys -> keySet()
of the map:Map<String, Object> map = ...; for (String key : map.keySet()) { //your Business logic... }
-
Iterate through the
values -> values()
of the map:for (Object value : map.values()) { //your Business logic... }
-
Iterate through the
both -> entrySet()
of the map:for (Map.Entry<String, Object> entry : map.entrySet()) { String key = entry.getKey(); Object value = entry.getValue(); //your Business logic... }
Moreover, there are 3 different ways to iterate through a HashMap. They are as below:
//1.
for (Map.Entry entry : hm.entrySet()) {
System.out.print("key,val: ");
System.out.println(entry.getKey() + "," + entry.getValue());
}
//2.
Iterator iter = hm.keySet().iterator();
while(iter.hasNext()) {
Integer key = (Integer)iter.next();
String val = (String)hm.get(key);
System.out.println("key,val: " + key + "," + val);
}
//3.
Iterator it = hm.entrySet().iterator();
while (it.hasNext()) {
Map.Entry entry = (Map.Entry) it.next();
Integer key = (Integer)entry.getKey();
String val = (String)entry.getValue();
System.out.println("key,val: " + key + "," + val);
}
Solution 18 - Java
Try this with Java 1.4:
for( Iterator entries = myMap.entrySet().iterator(); entries.hasNext();){
Entry entry = (Entry) entries.next();
System.out.println(entry.getKey() + "/" + entry.getValue());
//...
}
Solution 19 - Java
The ordering will always depend on the specific map implementation. Using Java 8 you can use either of these:
map.forEach((k,v) -> { System.out.println(k + ":" + v); });
Or:
map.entrySet().forEach((e) -> {
System.out.println(e.getKey() + " : " + e.getValue());
});
The result will be the same (same order). The entrySet backed by the map so you are getting the same order. The second one is handy as it allows you to use lambdas, e.g. if you want only to print only Integer objects that are greater than 5:
map.entrySet()
.stream()
.filter(e-> e.getValue() > 5)
.forEach(System.out::println);
The code below shows iteration through LinkedHashMap and normal HashMap (example). You will see difference in the order:
public class HMIteration {
public static void main(String[] args) {
Map<Object, Object> linkedHashMap = new LinkedHashMap<>();
Map<Object, Object> hashMap = new HashMap<>();
for (int i=10; i>=0; i--) {
linkedHashMap.put(i, i);
hashMap.put(i, i);
}
System.out.println("LinkedHashMap (1): ");
linkedHashMap.forEach((k,v) -> { System.out.print(k + " (#="+k.hashCode() + "):" + v + ", "); });
System.out.println("\nLinkedHashMap (2): ");
linkedHashMap.entrySet().forEach((e) -> {
System.out.print(e.getKey() + " : " + e.getValue() + ", ");
});
System.out.println("\n\nHashMap (1): ");
hashMap.forEach((k,v) -> { System.out.print(k + " (#:"+k.hashCode() + "):" + v + ", "); });
System.out.println("\nHashMap (2): ");
hashMap.entrySet().forEach((e) -> {
System.out.print(e.getKey() + " : " + e.getValue() + ", ");
});
}
}
Output:
LinkedHashMap (1):
10 (#=10):10, 9 (#=9):9, 8 (#=8):8, 7 (#=7):7, 6 (#=6):6, 5 (#=5):5, 4 (#=4):4, 3 (#=3):3, 2 (#=2):2, 1 (#=1):1, 0 (#=0):0,
LinkedHashMap (2):
10 : 10, 9 : 9, 8 : 8, 7 : 7, 6 : 6, 5 : 5, 4 : 4, 3 : 3, 2 : 2, 1 : 1, 0 : 0,
HashMap (1):
0 (#:0):0, 1 (#:1):1, 2 (#:2):2, 3 (#:3):3, 4 (#:4):4, 5 (#:5):5, 6 (#:6):6, 7 (#:7):7, 8 (#:8):8, 9 (#:9):9, 10 (#:10):10,
HashMap (2):
0 : 0, 1 : 1, 2 : 2, 3 : 3, 4 : 4, 5 : 5, 6 : 6, 7 : 7, 8 : 8, 9 : 9, 10 : 10,
Solution 20 - Java
Most compact with Java 8:
map.entrySet().forEach(System.out::println);
Solution 21 - Java
>If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map?
If efficiency of looping the keys is a priority for your app, then choose a Map
implementation that maintains the keys in your desired order.
>Will the ordering of elements depend on the specific map implementation that I have for the interface?
Yes, absolutely.
- Some
Map
implementations promise a certain iteration order, others do not. - Different implementations of
Map
maintain different ordering of the key-value pairs.
See this table I created summarizing the various Map
implementations bundled with Java 11. Specifically, notice the iteration order column. Click/tap to zoom.
You can see there are four Map
implementations maintaining an order:
TreeMap
ConcurrentSkipListMap
LinkedHashMap
EnumMap
NavigableMap
interface
Two of those implement the NavigableMap
interface: TreeMap
& ConcurrentSkipListMap
.
The older SortedMap
interface is effectively supplanted by the newer NavigableMap
interface. But you may find 3rd-party implementations implementing the older interface only.
Natural order
If you want a Map
that keeps its pairs arranged by the “natural order” of the key, use TreeMap
or ConcurrentSkipListMap
. The term “natural order” means the class of the keys implements Comparable
. The value returned by the compareTo
method is used for comparison in sorting.
Custom order
If you want to specify a custom sorting routine for your keys to be used in maintaining a sorted order, pass a Comparator
implementation appropriate to the class of your keys. Use either TreeMap
or ConcurrentSkipListMap
, passing your Comparator
.
Original insertion order
If you want the pairs of your map to be kept in their original order in which you inserted them into the map, use LinkedHashMap
.
Enum-definition order
If you are using an enum such as DayOfWeek
or Month
as your keys, use the EnumMap
class. Not only is this class highly optimized to use very little memory and run very fast, it maintains your pairs in the order defined by the enum. For DayOfWeek
, for example, the key of DayOfWeek.MONDAY
will be first found when iterated, and the key of DayOfWeek.SUNDAY
will be last.
Other considerations
In choosing a Map
implementation, also consider:
- NULLs. Some implementations forbid/accept a NULL as key and/or value.
- Concurrency. If you are manipulating the map across threads, you must use an implementation that supports concurrency. Or wrap the map with
Collections::synchronizedMap
(less preferable).
Both of these considerations are covered in the graphic table above.
Solution 22 - Java
If you have a generic untyped Map you can use:
Map map = new HashMap();
for (Map.Entry entry : ((Set<Map.Entry>) map.entrySet())) {
System.out.println(entry.getKey() + "/" + entry.getValue());
}
Solution 23 - Java
public class abcd{
public static void main(String[] args)
{
Map<Integer, String> testMap = new HashMap<Integer, String>();
testMap.put(10, "a");
testMap.put(20, "b");
testMap.put(30, "c");
testMap.put(40, "d");
for (Integer key:testMap.keySet()) {
String value=testMap.get(key);
System.out.println(value);
}
}
}
OR
public class abcd {
public static void main(String[] args)
{
Map<Integer, String> testMap = new HashMap<Integer, String>();
testMap.put(10, "a");
testMap.put(20, "b");
testMap.put(30, "c");
testMap.put(40, "d");
for (Entry<Integer, String> entry : testMap.entrySet()) {
Integer key=entry.getKey();
String value=entry.getValue();
}
}
}
Solution 24 - Java
Iterator iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry element = (Map.Entry)it.next();
LOGGER.debug("Key: " + element.getKey());
LOGGER.debug("value: " + element.getValue());
}
Solution 25 - Java
You can do it using generics:
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry<Integer, Integer> entry = entries.next();
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}
Solution 26 - Java
Use Java 8:
map.entrySet().forEach(entry -> System.out.println(entry.getValue()));
Solution 27 - Java
An effective iterative solution over a Map is a for
loop from Java 5 through Java 7. Here it is:
for (String key : phnMap.keySet()) {
System.out.println("Key: " + key + " Value: " + phnMap.get(key));
}
From Java 8 you can use a lambda expression to iterate over a Map. It is an enhanced forEach
phnMap.forEach((k,v) -> System.out.println("Key: " + k + " Value: " + v));
If you want to write a conditional for lambda you can write it like this:
phnMap.forEach((k,v)->{
System.out.println("Key: " + k + " Value: " + v);
if("abc".equals(k)){
System.out.println("Hello abc");
}
});
Solution 28 - Java
//Functional Oprations
Map<String, String> mapString = new HashMap<>();
mapString.entrySet().stream().map((entry) -> {
String mapKey = entry.getKey();
return entry;
}).forEach((entry) -> {
String mapValue = entry.getValue();
});
//Intrator
Map<String, String> mapString = new HashMap<>();
for (Iterator<Map.Entry<String, String>> it = mapString.entrySet().iterator(); it.hasNext();) {
Map.Entry<String, String> entry = it.next();
String mapKey = entry.getKey();
String mapValue = entry.getValue();
}
//Simple for loop
Map<String, String> mapString = new HashMap<>();
for (Map.Entry<String, String> entry : mapString.entrySet()) {
String mapKey = entry.getKey();
String mapValue = entry.getValue();
}
Solution 29 - Java
Iterating a Map is very easy.
for(Object key: map.keySet()){
Object value= map.get(key);
//Do your stuff
}
For instance, you have a Map<String, int> data;
for(Object key: data.keySet()){
int value= data.get(key);
}
Solution 30 - Java
There are a lot of ways to do this. Below is a few simple steps:
Suppose you have one Map like:
Map<String, Integer> m = new HashMap<String, Integer>();
Then you can do something like the below to iterate over map elements.
// ********** Using an iterator ****************
Iterator<Entry<String, Integer>> me = m.entrySet().iterator();
while(me.hasNext()){
Entry<String, Integer> pair = me.next();
System.out.println(pair.getKey() + ":" + pair.getValue());
}
// *********** Using foreach ************************
for(Entry<String, Integer> me : m.entrySet()){
System.out.println(me.getKey() + " : " + me.getValue());
}
// *********** Using keySet *****************************
for(String s : m.keySet()){
System.out.println(s + " : " + m.get(s));
}
// *********** Using keySet and iterator *****************
Iterator<String> me = m.keySet().iterator();
while(me.hasNext()){
String key = me.next();
System.out.println(key + " : " + m.get(key));
}
Solution 31 - Java
Yes, as many people agreed this is the best way to iterate over a Map
.
But there are chances to throw nullpointerexception
if the map is null
. Don't forget to put null
.check in.
|
|
- - - -
|
|
for (Map.Entry<String, Object> entry : map.entrySet()) {
String key = entry.getKey();
Object value = entry.getValue();
}
Solution 32 - Java
package com.test;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
public class Test {
public static void main(String[] args) {
Map<String, String> map = new HashMap<String, String>();
map.put("ram", "ayodhya");
map.put("krishan", "mathura");
map.put("shiv", "kailash");
System.out.println("********* Keys *********");
Set<String> keys = map.keySet();
for (String key : keys) {
System.out.println(key);
}
System.out.println("********* Values *********");
Collection<String> values = map.values();
for (String value : values) {
System.out.println(value);
}
System.out.println("***** Keys and Values (Using for each loop) *****");
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println("Key: " + entry.getKey() + "\t Value: "
+ entry.getValue());
}
System.out.println("***** Keys and Values (Using while loop) *****");
Iterator<Entry<String, String>> entries = map.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry<String, String> entry = (Map.Entry<String, String>) entries
.next();
System.out.println("Key: " + entry.getKey() + "\t Value: "
+ entry.getValue());
}
System.out
.println("** Keys and Values (Using java 8 using lambdas )***");
map.forEach((k, v) -> System.out
.println("Key: " + k + "\t value: " + v));
}
}
Solution 33 - Java
These are all the possible ways of iterating HashMap.
HashMap<Integer,String> map=new HashMap<Integer,String>();
map.put(1,"David"); //Adding elements in Map
map.put(2,"John");
map.put(4,"Samyuktha");
map.put(3,"jasmin");
System.out.println("Iterating Hashmap...");
//way 1 (java 8 Method)
map.forEach((key, value) -> {
System.out.println(key+" : "+ value);
});
//way 2 (java 7 Method)
for(Map.Entry me : map.entrySet()){
System.out.println(me.getKey()+" "+me.getValue());
}
//way 3 (Legacy way to iterate HashMap)
Iterator iterator = map.entrySet().iterator();//map.keySet().iterator()
while (iterator.hasNext())
{
Map.Entry me =(Map.Entry)iterator.next();
System.out.println(me.getKey()+" : "+ me.getValue());
}
}
Solution 34 - Java
Map.forEach
What about simply using Map::forEach
where both the key and the value are passed to your BiConsumer
?
map.forEach((k,v)->{
System.out.println(k+"->"+v);
});
Solution 35 - Java
Here is a generic type-safe method which can be called to dump any given Map
.
import java.util.Iterator;
import java.util.Map;
public class MapUtils {
static interface ItemCallback<K, V> {
void handler(K key, V value, Map<K, V> map);
}
public static <K, V> void forEach(Map<K, V> map, ItemCallback<K, V> callback) {
Iterator<Map.Entry<K, V>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<K, V> entry = it.next();
callback.handler(entry.getKey(), entry.getValue(), map);
}
}
public static <K, V> void printMap(Map<K, V> map) {
forEach(map, new ItemCallback<K, V>() {
@Override
public void handler(K key, V value, Map<K, V> map) {
System.out.println(key + " = " + value);
}
});
}
}
Example
Here is an example of its use. Notice that the type of the Map
is inferred by the method.
import java.util.*;
public class MapPrinter {
public static void main(String[] args) {
List<Map<?, ?>> maps = new ArrayList<Map<?, ?>>() {
private static final long serialVersionUID = 1L;
{
add(new LinkedHashMap<String, Integer>() {
private static final long serialVersionUID = 1L;
{
put("One", 0);
put("Two", 1);
put("Three", 3);
}
});
add(new LinkedHashMap<String, Object>() {
private static final long serialVersionUID = 1L;
{
put("Object", new Object());
put("Integer", new Integer(0));
put("Double", new Double(0.0));
}
});
}
};
for (Map<?, ?> map : maps) {
MapUtils.printMap(map);
System.out.println();
}
}
}
Output
One = 0
Two = 1
Three = 3
Object = java.lang.Object@15db9742
Integer = 0
Double = 0.0
Solution 36 - Java
Since Java 10, you can use local variable inference (a.k.a. "var") to make a lot of the already available answers less bloated. For example:
for (var entry : map.entrySet()) {
System.out.println(entry.getKey() + " : " + entry.getValue());
}
Solution 37 - Java
There are several ways to iterate a map. Please refer to the following code.
When you iterate a map using iterator Interface you must go with Entry<K,V>
or entrySet()
.
It looks like this:
import java.util.*;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
public class IteratMapDemo{
public static void main(String arg[]){
Map<String, String> mapOne = new HashMap<String, String>();
mapOne.put("1", "January");
mapOne.put("2", "February");
mapOne.put("3", "March");
mapOne.put("4", "April");
mapOne.put("5", "May");
mapOne.put("6", "June");
mapOne.put("7", "July");
mapOne.put("8", "August");
mapOne.put("9", "September");
mapOne.put("10", "Octomber");
mapOne.put("11", "November");
mapOne.put("12", "December");
Iterator it = mapOne.entrySet().iterator();
while(it.hasNext())
{
Map.Entry me = (Map.Entry) it.next();
//System.out.println("Get Key through While loop = " + me.getKey());
}
for(Map.Entry<String, String> entry:mapOne.entrySet()){
//System.out.println(entry.getKey() + "=" + entry.getValue());
}
for (Object key : mapOne.keySet()) {
System.out.println("Key: " + key.toString() + " Value: " +
mapOne.get(key));
}
}
}
Solution 38 - Java
If your reason for iterating trough the Map
, is to do an operation on the value and write to a resulting Map
. I recommend using the transform
-methods in the Google Guava Maps
class.
import com.google.common.collect.Maps;
After you have added the Maps
to your imports, you can use Maps.transformValues
and Maps.transformEntries
on your maps, like this:
public void transformMap(){
Map<String, Integer> map = new HashMap<>();
map.put("a", 2);
map.put("b", 4);
Map<String, Integer> result = Maps.transformValues(map, num -> num * 2);
result.forEach((key, val) -> print(key, Integer.toString(val)));
// key=a,value=4
// key=b,value=8
Map<String, String> result2 = Maps.transformEntries(map, (key, value) -> value + "[" + key + "]");
result2.forEach(this::print);
// key=a,value=2[a]
// key=b,value=4[b]
}
private void print(String key, String val){
System.out.println("key=" + key + ",value=" + val);
}
Solution 39 - Java
I like to concat a counter, then save the final value of the counter;
int counter = 0;
HashMap<String, String> m = new HashMap<String, String>();
for(int i = 0;i<items.length;i++)
{
m.put("firstname"+i, items.get(i).getFirstName());
counter = i;
}
m.put("recordCount",String.valueOf(counter));
Then when you want to retrieve:
int recordCount = Integer.parseInf(m.get("recordCount"));
for(int i =0 ;i<recordCount;i++)
{
System.out.println("First Name :" + m.get("firstname"+i));
}
Solution 40 - Java
> Using Java 7
Map<String,String> sampleMap = new HashMap<>();
for (sampleMap.Entry<String,String> entry : sampleMap.entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
/* your Code as per the Business Justification */
}
> Using Java 8
Map<String,String> sampleMap = new HashMap<>();
sampleMap.forEach((k, v) -> System.out.println("Key is : " + k + " Value is : " + v));
Solution 41 - Java
It doesn't quite answer the OP's question, but might be useful to others who find this page:
If you only need the values and not the keys, you can do this:
Map<Ktype, Vtype> myMap = [...];
for (Vtype v: myMap.values()) {
System.out.println("value: " + v);
}
Ktype
, Vtype
are pseudocode.
Solution 42 - Java
If you want to iterate through the map in the order that the elements were added, use LinkedHashMap
as opposed to just Map
.
This approach has worked for me in the past:
LinkedHashMap<String,Integer> test=new LinkedHashMap();
test.put("foo",69);
test.put("bar",1337);
for(int i=0;i<test.size();i++){
System.out.println(test.get(test.keySet().toArray()[i]));
}
Output:
69
1337
Solution 43 - Java
Map<String, String> map =
for (Map.Entry<String, String> entry : map.entrySet()) {
MapKey = entry.getKey()
MapValue = entry.getValue();
}
Solution 44 - Java
You can search for the key and with the help of the key you can find the associated value of the map as map has unique key, see what happens when key is duplicate here or here.
Demo map :
Map<String, String> map = new HashMap();
map.put("name", "Name");
map.put("age", "23");
map.put("address", "NP");
map.put("faculty", "BE");
map.put("major", "CS");
map.put("head", "MDK");
To get key only, you can use map.keySet();
like this :
for(String key : map.keySet()) {
System.out.println(key);
}
To get value only , you can use map.values();
like this:
for(String value : map.values()) {
System.out.println(value);
}
To get both key and its value you still can use map.keySet();
and get its corresponding value, like this :
//this prints the key value pair
for (String k : map.keySet()) {
System.out.println(k + " " + map.get(k) + " ");
}
map.get(key)
gives the value pointed by that key.
Solution 45 - Java
I copied the data of a map to another with this code:
HashMap product =(HashMap)shopping_truck.get(i);
HashMap tmp = new HashMap();
for (Iterator it = product.entrySet().iterator(); it.hasNext();) {
Map.Entry thisEntry = (Map.Entry) it.next();
tmp.put(thisEntry.getKey(), thisEntry.getValue());
}
Solution 46 - Java
This is the easiest way of doing it I believe...
/* For example, this could be a map object */
Map<String, Integer> MAP = new Map<>();
// Do something like put keys/value pairs into the map, etc...
MAP.put("Denver", 35);
MAP.put("Patriots", 14);
/* Then, simply use a for each loop like this to iterate */
for (Object o : MAP.entrySet()) {
Map.Entry pair = (Map.Entry) o;
// Do whatever with the pair here (i.e. pair.getKey(), or pair.getValue();
}