First time here so I hope this makes sense!
I have a Map which contains a String as it's Key, and a List of Strings as it's Value. I need to iterate over all vlaues contained within each List within the Map.
So, first I want to get the Keys, which works:
Set<String> keys = theMap.keySet();
This returns me a Set containing all my Keys. Great :)
This is where I've got stuck - most of the info on the web seems to assume that the values I'd want returned from the Key would be a simple String or Integer, not another Set, or in this case a List. I tried theMap.values() but that didn't work, and I tried a forloop / for:eachloop, and neither of those did the trick.
Thanks y'all!
for(List<String> valueList : map.values()) {
for(String value : valueList) {
...
}
}
That's really the "normal" way to do it. Or, if you need the key as well...
for(Map.Entry<String, List<String>> entry : map.entrySet()) {
String key = entry.getKey();
for (String value : entry.getValue()) {
...
}
}
That said, if you have the option, you might be interested in Guava's ListMultimap, which is a lot like a Map<K, List<V>>, but has a lot more features -- including a Collection<V> values() that acts exactly like what you're asking for, "flattening" all the values in the multimap into one collection. (Disclosure: I contribute to Guava.)
I recommend iterating over Map.entrySet() as it is faster (you have both, the key and the value, found in one step).
Map<String, List<String>> m = Collections.singletonMap(
"list1", Arrays.asList("s1", "s2", "s3"));
for (Map.Entry<String, List<String>> me : m.entrySet()) {
String key = me.getKey();
List<String> valueList = me.getValue();
System.out.println("Key: " + key);
System.out.print("Values: ");
for (String s : valueList) {
System.out.print(s + " ");
}
}
Or the same using the Java 8 API (Lambda functions):
m.entrySet().forEach(me -> {
System.out.println("Key: " + me.getKey());
System.out.print("Values: ");
me.getValue().forEach(s -> System.out.print(s + " "));
});
Or with a little bit of Java Stream API mapping hardcore and method reference :-)
m.entrySet().stream().map(me -> {
return "Key: " + me.getKey() + "\n"
+ "Values: " + me.getValue().stream()
.collect(Collectors.joining(" "));
})
.forEach(System.out::print);
And the output is, as expected:
Key: list1
Values: s1 s2 s3
You need a Map<String, List<String>>
The left hand side String is the key, the right hand side List<String> is the value, which in this case is a List of Strings
Another example with the Java 8 API (lambda function).
When you want to iterate over:
Map<String, List<String>> theMap = new HashMap<>();
theMap.forEach((key, value) -> {
System.out.println("KEY: " + key);
System.out.print("VALUES: ");
value.forEach(System.out::println);
});
Related
I want to build up a map based on 2 arrays where 1 key has many objects inside it.
Key: "Letter A" Value: "Albatross"
Value: "Alligator"
Key: "Letter B" Value: "Badger"
Value: "Bandicoot"
The structure must show the key 1 time, without repetitions
Hope the code is self explanatory.
Java 7:
public Map<String, List<String>> group(String[] input) {
Map<String, List<String>> result = new HashMap<>();
for (String str : input) {
String key = "Letter " + str.charAt(0);
if (result.containsKey(key)) {
result.get(key).add(str);//if Key already exists, just add this word to existing list.
} else {
List<String> list = new ArrayList<>();
list.add(str);
result.put(key, list); //Otherwise, create a new list and add the new word into the list
}
}
return result;
}
Java 8:
public static Map<String, List<String>> group(String[] input) {
return Arrays.stream(input)
.collect(Collectors.groupingBy(k -> "Letter " + k.charAt(0)));
//Provide the key for how you want to group. In your case it is first character of string.
}
You can use Guava's Mutlimap implementation, however that may not be Java 7 compatible. https://guava.dev/releases/23.0/api/docs/com/google/common/collect/Multimap.html
You can get the same effect by using a List for the values in your map like so:
Map<String, List<String>> map = new HashMap<>();
Then, let's say for each entry you want to add to the map you have the key in key and value in val, add it like so:
List<String> list = map.get(key);
if (list == null) {
list = new ArrayList<>();
map.put(key, list);
}
list.add(val);
Quick question and its probably the most simple answer but i need to print a textual representation of my HashMaps contents.
My code so far is:
public void printAll() {
Set< String> Names = customersDetails.keySet();
Collection< CustomerDetails> eachCustomersNames = customersDetails.values();
for (String eachName : Names) {
System.out.println(eachName)
}
for (CustomerDetails eachCustomer : eachCustomersNames) {
System.out.println(eachCustomer);
}
}
But this results in the list of keys and then a list of values but i need each line of text to read something like
Bob [example]
Where Bob is the key and example is the value.
If you're using Java 8, you can take advantage of lambda syntax and .forEach() like so:
customersDetails.forEach((k,v) -> {
System.out.println(k + "[" + v + "]");
});
Where k is your key and v is the value tied to key k.
Every key maps to just one value, so you can just do this:
Set < String> Names = customersDetails.keySet();
for (String eachName: Names) {
System.out.println(eachName + " [" + customersDetails.get(eachName).toString() + "]")
}
If you're not using Java 8, simply print both key and value for each key:
for (String eachName : Names) {
System.out.println(eachName + " [" + customersDetails.get(eachName) + "]");
}
You can print your Map like so :
Map<String, String> customersDetails = new HashMap<>();
for (Map.Entry<String, String> entry : customersDetails.entrySet()) {
System.out.println(entry.getKey() + '[' + entry.getValue() + ']');
}
If you are using java 8 you can use :
customersDetails.entrySet().forEach((entry) -> {
System.out.println(entry.getKey() + '[' + entry.getValue() + ']');
});
If you start dealing with maps with more complicated types consider using ReflectionToStringBuilder. Internally it uses reflection to build a string of an object and its fieldd. It recurses through the object graph too.
It may not be efficient, but it helps a lot with debugging and printing operations.
You don't need to iterate over your keys/values in order to print your map, as the HashMap.toString() method already does this for you very efficiently (actually, it's the AbstractMap.toString() method).
If you have your CustomerDetails class implement the toString() method, then you only need to do:
System.out.println(customerDetails);
And this will print your map in the format you require.
I have a map with Integer keys and values. I need to transform it into a String with this specific format: key1 - val1, key2 - val2, key3 - val3. Now, I'm using forEach to format each element, collect them into a List, and then do String.join();
List<String> ships = new ArrayList<>(4);
for (Map.Entry<Integer, Integer> entry : damagedMap.entrySet())
{
ships.add(entry.getKey() + " - " + entry.getValue());
}
result = String.join(",", ships);
Is there any shorter way to do it? And it would be good to do it with lambda, because I need some practice using lambdas.
I think you're looking for something like this:
import java.util.*;
import java.util.stream.*;
public class Test {
public static void main(String[] args) throws Exception {
Map<Integer, String> map = new HashMap<>();
map.put(1, "foo");
map.put(2, "bar");
map.put(3, "baz");
String result = map.entrySet()
.stream()
.map(entry -> entry.getKey() + " - " + entry.getValue())
.collect(Collectors.joining(", "));
System.out.println(result);
}
}
To go through the bits in turn:
entrySet() gets an iterable sequence of entries
stream() creates a stream for that iterable
map() converts that stream of entries into a stream of strings of the form "key - value"
collect(Collectors.joining(", ")) joins all the entries in the stream into a single string, using ", " as the separator. Collectors.joining is a method which returns a Collector which can work on an input sequence of strings, giving a result of a single string.
Note that the order is not guaranteed here, because HashMap isn't ordered. You might want to use TreeMap to get the values in key order.
This question already has answers here:
Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop
(31 answers)
Closed 5 years ago.
I was doing:
for (Object key : map.keySet())
if (something)
map.remove(key);
which threw a ConcurrentModificationException, so i changed it to:
for (Object key : new ArrayList<Object>(map.keySet()))
if (something)
map.remove(key);
this, and any other procedures that modify the map are in synchronized blocks.
is there a better solution?
Here is a code sample to use the iterator in a for loop to remove the entry.
Map<String, String> map = new HashMap<String, String>() {
{
put("test", "test123");
put("test2", "test456");
}
};
for(Iterator<Map.Entry<String, String>> it = map.entrySet().iterator(); it.hasNext(); ) {
Map.Entry<String, String> entry = it.next();
if(entry.getKey().equals("test")) {
it.remove();
}
}
As of Java 8 you could do this as follows:
map.entrySet().removeIf(e -> <boolean expression>);
Oracle Docs: entrySet()
The set is backed by the map, so changes to the map are reflected in the set, and vice-versa
Use a real iterator.
Iterator<Object> it = map.keySet().iterator();
while (it.hasNext())
{
it.next();
if (something)
it.remove();
}
Actually, you might need to iterate over the entrySet() instead of the keySet() to make that work.
is there a better solution?
Well, there is, definitely, a better way to do so in a single statement, but that depends on the condition based on which elements are removed.
For eg: remove all those elements where value is test, then use below:
map.values().removeAll(Collections.singleton("test"));
UPDATE
It can be done in a single line using Lambda expression in Java 8.
map.entrySet().removeIf(e-> <boolean expression> );
I know this question is way too old, but there isn't any harm in updating the better way to do the things :)
ConcurrentHashMap
You can use java.util.concurrent.ConcurrentHashMap.
It implements ConcurrentMap (which extends the Map interface).
E.g.:
Map<Object, Content> map = new ConcurrentHashMap<Object, Content>();
for (Object key : map.keySet()) {
if (something) {
map.remove(key);
}
}
This approach leaves your code untouched. Only the map type differs.
Java 8 support a more declarative approach to iteration, in that we specify the result we want rather than how to compute it. Benefits of the new approach are that it can be more readable, less error prone.
public static void mapRemove() {
Map<Integer, String> map = new HashMap<Integer, String>() {
{
put(1, "one");
put(2, "two");
put(3, "three");
}
};
map.forEach( (key, value) -> {
System.out.println( "Key: " + key + "\t" + " Value: " + value );
});
map.keySet().removeIf(e->(e>2)); // <-- remove here
System.out.println("After removing element");
map.forEach( (key, value) -> {
System.out.println( "Key: " + key + "\t" + " Value: " + value );
});
}
And result is as follows:
Key: 1 Value: one
Key: 2 Value: two
Key: 3 Value: three
After removing element
Key: 1 Value: one
Key: 2 Value: two
You have to use Iterator to safely remove element while traversing a map.
I agree with Paul Tomblin. I usually use the keyset's iterator, and then base my condition off the value for that key:
Iterator<Integer> it = map.keySet().iterator();
while(it.hasNext()) {
Integer key = it.next();
Object val = map.get(key);
if (val.shouldBeRemoved()) {
it.remove();
}
}
An alternative, more verbose way
List<SomeObject> toRemove = new ArrayList<SomeObject>();
for (SomeObject key: map.keySet()) {
if (something) {
toRemove.add(key);
}
}
for (SomeObject key: toRemove) {
map.remove(key);
}
And this should work as well..
ConcurrentMap<Integer, String> running = ... create and populate map
Set<Entry<Integer, String>> set = running.entrySet();
for (Entry<Integer, String> entry : set)
{
if (entry.getKey()>600000)
{
set.remove(entry.getKey());
}
}
Maybe you can iterate over the map looking for the keys to remove and storing them in a separate collection. Then remove the collection of keys from the map. Modifying the map while iterating is usually frowned upon. This idea may be suspect if the map is very large.
Set s=map.entrySet();
Iterator iter = s.iterator();
while (iter.hasNext()) {
Map.Entry entry =(Map.Entry)iter.next();
if("value you need to remove".equals(entry.getKey())) {
map.remove();
}
}
In Python, you can have key,value pairs in a dictionary where you can loop through them, as shown below:
for k,v in d.iteritems():
print k,v
Is there a way to do this with Java HashMaps?
Yes - for example:
Map<String, String> map = new HashMap<String, String>();
// add entries to the map here
for (Map.Entry<String, String> entry : map.entrySet()) {
String k = entry.getKey();
String v = entry.getValue();
System.out.printf("%s %s\n", k, v);
}
The HashMap.entrySet() will return beans of key value pairs similar to the dictionary.iteritems(). You can then loop through them.
I think is the closest thing to the Python version.
As shown in the answers, there are basically two ways to iterate over a Map (let's assume Map<String, String> in those examples).
Iterate over Map#entrySet():
for (Entry<String, String> entry : map.entrySet()) {
System.out.println(entry.getKey() + "=" + entry.getValue());
}
Iterate over Map#keySet() and then use Map#get() to get the value for every key:
for (String key : map.keySet()) {
System.out.println(key + "=" + map.get(key));
}
The second one is maybe more readable, but it has a performance cost of unnecessarily calling get() on every iteration. One may argument that creating the keyset iterator is less expensive because it doesn't need to take values into account. But believe it or not, the keySet().iterator() creates and uses the same iterator as entrySet().iterator(). The only difference is that in case of the keySet() the next() call of the iterator returns it.next().getKey() instead of it.next().
The AbstractMap#keySet()'s javadoc proves this:
The subclass's iterator method returns a "wrapper object" over this map's entrySet() iterator.
The AbstractMap source code also proves this. Here's an extract of keySet() method (somewhere around line 300 in Java 1.6):
public Iterator<K> iterator() {
return new Iterator<K>() {
private Iterator<Entry<K,V>> i = entrySet().iterator(); // <-----
public boolean hasNext() {
return i.hasNext();
}
public K next() {
return i.next().getKey(); // <-----
}
public void remove() {
i.remove();
}
};
}
Note that readability should be preferred over premature optimization, but it's important to have this in mind.
Set<Map.Entry> set = d.entrySet();
for(Map.Entry i : set){
System.out.println(i.getKey().toString() + i.getValue().toString);
}
Something like that...
In Java, you can do the same like the following.
HashMap<String, String> h = new HashMap<String, String>();
h.put("1","one");
h.put("2","two");
h.put("3","three");
for(String key:h.keySet()){
System.out.println("Key: "+ key + " Value: " + h.get(key));
}