How can i iterate over a Hashmap<String, ArrayList<String>> which looks like those?
Name:Sam,Peter,Andrea,Sandra
Age:20,17,24,40
City:London,London,Munich,London
The result should look like this four arrays.
Sam,20,London
Peter,17,London
Andrea,24,Munich
Sandra,40,London
I've tried with two for(...) loops again and again, but it doesn't work.
Best regards :D
If this is something that you've written, then I'd like to suggest an alternate data structure. Instead of Hashmap<String, ArrayList<String>>, use ArrayList<HashMap<String, String>>
so that you have
{Name:Sam, Age:20, City:London},
{Name:Peter, Age:17, City:London},
{Name:Andrea, Age:24, City:Munich},
{Name:Sandra, Age:40, City:London}
If the HashMap of Lists is not something that you've written, and you still need help to figure out how to iterate that, please show us what you have tried.
Though I absolutely agree with #GreyBeardedGeek in his answer, and you should probably change your data structure. I did however have a look at the code needed to change Map<String, List<String>> into List<Map<String, String>>
public static <S, T> List<Map<S, T>> unravel(Map<S, List<T>> org) {
List<Map<S, T>> out = new LinkedList<>();
for (Entry<S, List<T>> entry : org.entrySet()) {
if (entry.getValue() == null || entry.getValue().isEmpty()) {
continue;
}
while (out.size() < entry.getValue().size()) {
out.add(new HashMap<S, T>());
}
for (int i = 0, size = entry.getValue().size(); i < size; i++) {
T value = entry.getValue().get(i);
if (value == null) {
continue;
}
out.get(i).put(entry.getKey(), value);
}
}
return out;
}
I find myself often writing such code:
Map < String, Set < String > > map = new TreeMap < String, Set < String > >();
String key;
String element;
/* ... */
Set < String > value = map.get(key);
if (value == null) {
value = new TreeSet < String >();
map.put(key, value);
}
value.add(element);
I hate the if statement above -- how can I get rid of it in standard Java? If you can confirm that there is no standard Java solution, it would then be nice if you could suggest a non-standard library that addresses this need.
Apache Commons Collections has a MultiMap:
MultiMap mhm = new MultiHashMap();
mhm.put(key, "A");
mhm.put(key, "B");
mhm.put(key, "C");
Collection coll = (Collection) mhm.get(key);
Guava (used to be Google Collections) has one too, which supports generics, and has both a tree and hash version:
Multimap<String,String> mhm = new TreeMultimap<String,String>();
// etc.
To clarify, Multimap<T, S> is basically the same as a Map<T, Collection<S>>, and put automatically creates the collection if it needs to.
EDIT: Updated to link to Guava, since apparently Google Collections is deprecated.
Hmm... I like Brendan's multimap answer, but you could also use a static utility method and keep to standard java.util classes/interfaces:
public static <K,V> Set<V> getTreeSet(Map<K,Set<V>> map, K key) {
Set<V> set = map.get(key);
if (set == null) {
set = new TreeSet<V>();
map.put(key, set);
}
return set;
}
Then your code with the null checks could be written:
Map < String, Set < String > > map = new TreeMap < String, Set < String > >();
String key;
String element;
/* ... */
MapUtils.getTreeSet(map, key).add(element);
You'd need static method for each collection type, but that seems better than writing the same code over and over again.
ArrayList<Object> list = new ArrayList<Object>();
list.add(1);
list.add("Java");
list.add(3.14);
System.out.println(list.toString());
I tried:
ArrayList<String> list2 = (String)list;
But it gave me a compile error.
Since this is actually not a list of strings, the easiest way is to loop over it and convert each item into a new list of strings yourself:
List<String> strings = list.stream()
.map(object -> Objects.toString(object, null))
.toList();
Or when you're not on Java 16 yet:
List<String> strings = list.stream()
.map(object -> Objects.toString(object, null))
.collect(Collectors.toList());
Or when you're not on Java 8 yet:
List<String> strings = new ArrayList<>(list.size());
for (Object object : list) {
strings.add(Objects.toString(object, null));
}
Or when you're not on Java 7 yet:
List<String> strings = new ArrayList<String>(list.size());
for (Object object : list) {
strings.add(object != null ? object.toString() : null);
}
Note that you should be declaring against the interface (java.util.List in this case), not the implementation.
It's not safe to do that!
Imagine if you had:
ArrayList<Object> list = new ArrayList<Object>();
list.add(new Employee("Jonh"));
list.add(new Car("BMW","M3"));
list.add(new Chocolate("Twix"));
It wouldn't make sense to convert the list of those Objects to any type.
Using Java 8 you can do:
List<Object> list = ...;
List<String> strList = list.stream()
.map( Object::toString )
.collect( Collectors.toList() );
You can use wildcard to do this as following
ArrayList<String> strList = (ArrayList<String>)(ArrayList<?>)(list);
If you want to do it the dirty way, try this.
#SuppressWarnings("unchecked")
public ArrayList<String> convert(ArrayList<Object> a) {
return (ArrayList) a;
}
Advantage: here you save time by not iterating over all objects.
Disadvantage: may produce a hole in your foot.
Using guava:
List<String> stringList=Lists.transform(list,new Function<Object,String>(){
#Override
public String apply(Object arg0) {
if(arg0!=null)
return arg0.toString();
else
return "null";
}
});
Here is another alternative using Guava
List<Object> lst ...
List<String> ls = Lists.transform(lst, Functions.toStringFunction());
Your code ArrayList<String> list2 = (String)list; does not compile because list2 is not of type String. But that is not the only problem.
Using Java 8 lambda:
ArrayList<Object> obj = new ArrayList<>();
obj.add(1);
obj.add("Java");
obj.add(3.14);
ArrayList<String> list = new ArrayList<>();
obj.forEach((xx) -> list.add(String.valueOf(xx)));
With Java Generics Takes a list of X and returns a list of T that extends or implements X, Sweet!
// the cast is is actually checked via the method API
#SuppressWarnings("unchecked")
public static <T extends X, X> ArrayList<T> convertToClazz(ArrayList<X> from, Class<X> inClazz, Class<T> outClazz) {
ArrayList<T> to = new ArrayList<T>();
for (X data : from) {
to.add((T) data);
}
return to;
}
A simple solution:
List<Object> lst =listOfTypeObject;
ArrayList<String> aryLst = new ArrayList<String>();
for (int i = 0; i < lst.size(); i++) {
aryLst.add(lst.get(i).toString());
}
Note: this works when the list contains all the elements of datatype String.
I have an ArrayList, a Collection class of Java, as follows:
ArrayList<String> animals = new ArrayList<String>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
As you can see, the animals ArrayList consists of 3 bat elements and one owl element. I was wondering if there is any API in the Collection framework that returns the number of bat occurrences or if there is another way to determine the number of occurrences.
I found that Google's Collection Multiset does have an API that returns the total number of occurrences of an element. But that is compatible only with JDK 1.5. Our product is currently in JDK 1.6, so I cannot use it.
I'm pretty sure the static frequency-method in Collections would come in handy here:
int occurrences = Collections.frequency(animals, "bat");
That's how I'd do it anyway. I'm pretty sure this is jdk 1.6 straight up.
In Java 8:
Map<String, Long> counts =
list.stream().collect(Collectors.groupingBy(e -> e, Collectors.counting()));
Alternative Java 8 solution using Streams:
long count = animals.stream().filter(animal -> "bat".equals(animal)).count();
This shows, why it is important to "Refer to objects by their interfaces" as described in Effective Java book.
If you code to the implementation and use ArrayList in let's say, 50 places in your code, when you find a good "List" implementation that count the items, you will have to change all those 50 places, and probably you'll have to break your code ( if it is only used by you there is not a big deal, but if it is used by someone else uses, you'll break their code too)
By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to "CountItemsList" (for instance ) or some other class.
Below is a very basic sample on how this could be written. This is only a sample, a production ready List would be much more complicated.
import java.util.*;
public class CountItemsList<E> extends ArrayList<E> {
// This is private. It is not visible from outside.
private Map<E,Integer> count = new HashMap<E,Integer>();
// There are several entry points to this class
// this is just to show one of them.
public boolean add( E element ) {
if( !count.containsKey( element ) ){
count.put( element, 1 );
} else {
count.put( element, count.get( element ) + 1 );
}
return super.add( element );
}
// This method belongs to CountItemList interface ( or class )
// to used you have to cast.
public int getCount( E element ) {
if( ! count.containsKey( element ) ) {
return 0;
}
return count.get( element );
}
public static void main( String [] args ) {
List<String> animals = new CountItemsList<String>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
System.out.println( (( CountItemsList<String> )animals).getCount( "bat" ));
}
}
OO principles applied here: inheritance, polymorphism, abstraction, encapsulation.
Sorry there's no simple method call that can do it. All you'd need to do though is create a map and count frequency with it.
HashMap<String,int> frequencymap = new HashMap<String,int>();
foreach(String a in animals) {
if(frequencymap.containsKey(a)) {
frequencymap.put(a, frequencymap.get(a)+1);
}
else{ frequencymap.put(a, 1); }
}
There is no native method in Java to do that for you. However, you can use IterableUtils#countMatches() from Apache Commons-Collections to do it for you.
Simple Way to find the occurrence of string value in an array using Java 8 features.
public void checkDuplicateOccurance() {
List<String> duplicateList = new ArrayList<String>();
duplicateList.add("Cat");
duplicateList.add("Dog");
duplicateList.add("Cat");
duplicateList.add("cow");
duplicateList.add("Cow");
duplicateList.add("Goat");
Map<String, Long> couterMap = duplicateList.stream().collect(Collectors.groupingBy(e -> e.toString(),Collectors.counting()));
System.out.println(couterMap);
}
Output : {Cat=2, Goat=1, Cow=1, cow=1, Dog=1}
You can notice "Cow" and cow are not considered as same string, in case you required it under same count, use .toLowerCase(). Please find the snippet below for the same.
Map<String, Long> couterMap = duplicateList.stream().collect(Collectors.groupingBy(e -> e.toString().toLowerCase(),Collectors.counting()));
Output : {cat=2, cow=2, goat=1, dog=1}
To achieve that one can do it in several ways, namely:
Methods that return the number of occurrence of a single element:
Collection Frequency
Collections.frequency(animals, "bat");
Java Stream:
Filter
animals.stream().filter("bat"::equals).count();
Just iteration thought the list
public static long manually(Collection<?> c, Object o){
int count = 0;
for(Object e : c)
if(e.equals(o))
count++;
return count;
}
Methods that create a map of frequencies:
Collectors.groupingBy
Map<String, Long> counts =
animals.stream()
.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
merge
Map<String, Long> map = new HashMap<>();
c.forEach(e -> map.merge(e, 1L, Long::sum));
Manually
Map<String, Integer> mp = new HashMap<>();
animals.forEach(animal -> mp.compute(animal, (k, v) -> (v == null) ? 1 : v + 1));
A running example with all the methods:
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;
public class Frequency {
public static int frequency(Collection<?> c, Object o){
return Collections.frequency(c, o);
}
public static long filter(Collection<?> c, Object o){
return c.stream().filter(o::equals).count();
}
public static long manually(Collection<?> c, Object o){
int count = 0;
for(Object e : c)
if(e.equals(o))
count++;
return count;
}
public static Map<?, Long> mapGroupBy(Collection<?> c){
return c.stream()
.collect(Collectors.groupingBy(Function.identity() , Collectors.counting()));
}
public static Map<Object, Long> mapMerge(Collection<?> c){
Map<Object, Long> map = new HashMap<>();
c.forEach(e -> map.merge(e, 1L, Long::sum));
return map;
}
public static Map<Object, Long> manualMap(Collection<?> c){
Map<Object, Long> map = new HashMap<>();
c.forEach(e -> map.compute(e, (k, v) -> (v == null) ? 1 : v + 1));
return map;
}
public static void main(String[] args){
List<String> animals = new ArrayList<>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
System.out.println(frequency(animals, "bat"));
System.out.println(filter(animals,"bat"));
System.out.println(manually(animals,"bat"));
mapGroupBy(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
mapMerge(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
manualMap(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
}
}
The methods name should have reflected what those methods are doing, however, I used the name to reflect the approach being used instead (given that in the current context it is okey).
I wonder, why you can't use that Google's Collection API with JDK 1.6. Does it say so? I think you can, there should not be any compatibility issues, as it is built for a lower version. The case would have been different if that were built for 1.6 and you are running 1.5.
Am I wrong somewhere?
Actually, Collections class has a static method called : frequency(Collection c, Object o) which returns the number of occurrences of the element you are searching for, by the way, this will work perfectly for you:
ArrayList<String> animals = new ArrayList<String>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
System.out.println("Freq of bat: "+Collections.frequency(animals, "bat"));
A slightly more efficient approach might be
Map<String, AtomicInteger> instances = new HashMap<String, AtomicInteger>();
void add(String name) {
AtomicInteger value = instances.get(name);
if (value == null)
instances.put(name, new AtomicInteger(1));
else
value.incrementAndGet();
}
To get the occurrences of the object from the list directly:
int noOfOccurs = Collections.frequency(animals, "bat");
To get the occurrence of the Object collection inside list, override the equals method in the Object class as:
#Override
public boolean equals(Object o){
Animals e;
if(!(o instanceof Animals)){
return false;
}else{
e=(Animals)o;
if(this.type==e.type()){
return true;
}
}
return false;
}
Animals(int type){
this.type = type;
}
Call the Collections.frequency as:
int noOfOccurs = Collections.frequency(animals, new Animals(1));
What you want is a Bag - which is like a set but also counts the number of occurances. Unfortunately the java Collections framework - great as they are dont have a Bag impl. For that one must use the Apache Common Collection link text
List<String> list = Arrays.asList("as", "asda", "asd", "urff", "dfkjds", "hfad", "asd", "qadasd", "as", "asda",
"asd", "urff", "dfkjds", "hfad", "asd", "qadasd" + "as", "asda", "asd", "urff", "dfkjds", "hfad", "asd",
"qadasd", "as", "asda", "asd", "urff", "dfkjds", "hfad", "asd", "qadasd");
Method 1:
Set<String> set = new LinkedHashSet<>();
set.addAll(list);
for (String s : set) {
System.out.println(s + " : " + Collections.frequency(list, s));
}
Method 2:
int count = 1;
Map<String, Integer> map = new HashMap<>();
Set<String> set1 = new LinkedHashSet<>();
for (String s : list) {
if (!set1.add(s)) {
count = map.get(s) + 1;
}
map.put(s, count);
count = 1;
}
System.out.println(map);
If you use Eclipse Collections, you can use a Bag. A MutableBag can be returned from any implementation of RichIterable by calling toBag().
MutableList<String> animals = Lists.mutable.with("bat", "owl", "bat", "bat");
MutableBag<String> bag = animals.toBag();
Assert.assertEquals(3, bag.occurrencesOf("bat"));
Assert.assertEquals(1, bag.occurrencesOf("owl"));
The HashBag implementation in Eclipse Collections is backed by a MutableObjectIntMap.
Note: I am a committer for Eclipse Collections.
Put the elements of the arraylist in the hashMap to count the frequency.
So do it the old fashioned way and roll your own:
Map<String, Integer> instances = new HashMap<String, Integer>();
void add(String name) {
Integer value = instances.get(name);
if (value == null) {
value = new Integer(0);
instances.put(name, value);
}
instances.put(name, value++);
}
Java 8 - another method
String searched = "bat";
long n = IntStream.range(0, animals.size())
.filter(i -> searched.equals(animals.get(i)))
.count();
package traversal;
import java.util.ArrayList;
import java.util.List;
public class Occurrance {
static int count;
public static void main(String[] args) {
List<String> ls = new ArrayList<String>();
ls.add("aa");
ls.add("aa");
ls.add("bb");
ls.add("cc");
ls.add("dd");
ls.add("ee");
ls.add("ee");
ls.add("aa");
ls.add("aa");
for (int i = 0; i < ls.size(); i++) {
if (ls.get(i) == "aa") {
count = count + 1;
}
}
System.out.println(count);
}
}
Output: 4
Integer[] spam = new Integer[] {1,2,2,3,4};
List<Integer> list=Arrays.asList(spam);
System.out.println(list.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting())));
System.out.println(list.stream().collect(Collectors.groupingBy(Function.identity(),HashMap::new,Collectors.counting())));
output
{1=1, 2=2, 3=1, 4=1}
If you are a user of my ForEach DSL, it can be done with a Count query.
Count<String> query = Count.from(list);
for (Count<Foo> each: query) each.yield = "bat".equals(each.element);
int number = query.result();
I didn't want to make this case more difficult and made it with two iterators
I have a HashMap with LastName -> FirstName. And my method should delete items with dulicate FirstName.
public static void removeTheFirstNameDuplicates(HashMap<String, String> map)
{
Iterator<Map.Entry<String, String>> iter = map.entrySet().iterator();
Iterator<Map.Entry<String, String>> iter2 = map.entrySet().iterator();
while(iter.hasNext())
{
Map.Entry<String, String> pair = iter.next();
String name = pair.getValue();
int i = 0;
while(iter2.hasNext())
{
Map.Entry<String, String> nextPair = iter2.next();
if (nextPair.getValue().equals(name))
i++;
}
if (i > 1)
iter.remove();
}
}
List<String> lst = new ArrayList<String>();
lst.add("Ram");
lst.add("Ram");
lst.add("Shiv");
lst.add("Boss");
Map<String, Integer> mp = new HashMap<String, Integer>();
for (String string : lst) {
if(mp.keySet().contains(string))
{
mp.put(string, mp.get(string)+1);
}else
{
mp.put(string, 1);
}
}
System.out.println("=mp="+mp);
Output:
=mp= {Ram=2, Boss=1, Shiv=1}
Map<String,Integer> hm = new HashMap<String, Integer>();
for(String i : animals) {
Integer j = hm.get(i);
hm.put(i,(j==null ? 1 : j+1));
}
for(Map.Entry<String, Integer> val : hm.entrySet()) {
System.out.println(val.getKey()+" occurs : "+val.getValue()+" times");
}
You can use groupingBy feature of Java 8 for your use case.
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
public class Test {
public static void main(String[] args) {
List<String> animals = new ArrayList<>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
Map<String,Long> occurrenceMap =
animals.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
System.out.println("occurrenceMap:: " + occurrenceMap);
}
}
Output
occurrenceMap:: {bat=3, owl=1}