I am making a method, which takes a provided TreeMap, removes entries where the key is a multiple of keyFilter and the value contains the valueFilter character, and then returns the resulting TreeMap.
This is what I have so far:
public static TreeMap<Integer, String> filterTreeMap(
TreeMap<Integer, String> map, int keyFilter, char valueFilter) {
for (Map.Entry<Integer, String> entry : map.entrySet()) {
int mapKey = entry.getKey();
String mapValue = entry.getValue();
if (mapKey%keyFilter == 0 && mapValue.indexOf(valueFilter) != -1) {
map.remove(mapKey);
}
}
return map;
}
However, under the if condition where I want to delete the entries, I don't know how to delete entries in tree map. As far as I know, there is no existing method that I can use?
Use an Iterator. As the Iterator.remove() Javadoc notes
The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.
Something like
public static TreeMap<Integer, String> filterTreeMap(TreeMap<Integer, String> map,
int keyFilter, char valueFilter) {
Iterator<Map.Entry<Integer, String>> iter = map.entrySet().iterator();
while (iter.hasNext()) {
Map.Entry<Integer, String> entry = iter.next();
int mapKey = entry.getKey();
String mapValue = entry.getValue();
if (mapKey % keyFilter == 0 && mapValue.indexOf(valueFilter) != -1) {
iter.remove();
}
}
return map;
}
It is possible to apply method removeIf to the entry set.
default boolean removeIf(Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
Implementation Requirements:
The default implementation traverses all elements of the collection using its iterator(). Each matching element is removed using Iterator.remove(). If the collection's iterator does not support removal then an UnsupportedOperationException will be thrown on the first matching element.
Then the method filterTreeMap may have void return type because the input map is modified and this change will be "visible" outside this method.
public static void filterTreeMap(
TreeMap<Integer, String> map, int keyFilter, char valueFilter) {
map.entrySet().removeIf(e ->
e.getKey() % keyFilter == 0
&& e.getValue().indexOf(valueFilter) != -1
);
}
Keys on the map are unique. So, find that keys, and then remove them form the map.
public static TreeMap<Integer, String> filterTreeMap(TreeMap<Integer, String> map,
int keyFilter, char valueFilter) {
Set<Integer> keysToRemove = map.entrySet().stream()
.filter(kv -> kv.getKey() % keyFilter == 0 && kv.getValue().indexOf(valueFilter) != -1) // can be Predicate parameter
.map(Map.Entry::getKey)
.collect(Collectors.toSet());
keysToRemove.forEach(map::remove);
return map; // keep in mind, map is modified here. You might want to return a new map instead
}
Iterate over a copy and you can add/remove just fine:
for (Map.Entry<Integer, String> entry : new LinkedHashMap<Integer,String> (map).entrySet()) {
int mapKey = entry.getKey();
String mapValue = entry.getValue();
if (mapKey%keyFilter == 0 && mapValue.indexOf(valueFilter) != -1) {
map.remove(mapKey);
}
}
It's not even any more lines of code, because the copy is made in-line via the copy constructor. LinkedHashMap was chosen to preserve iteration order (if that matters).
Related
Let's say I have the LinkedHashMap with some unknown data inside.
//==================
Map< Integer, String > map = new LinkedHashMap<>();
map.put(10, "C");
map.put(20, "C++");
map.put(50, "JAVA");
map.put(40, "PHP");
map.put(30, "Kotlin");
//=============
And I know just the key = 50;
I am wondering what is the best way to get the next element to the element that I have by this key (50)? This is not a multi-threaded application. I don't worry about thread-safety.
I don't like the way to iterate all keys through entrySet from the beginning.
It would be great to somehow get access to the next() of LinkedHashMaps Entry.
This is LinkedHashMap so it remembers the order of elements insertion.
public static Map.Entry<Integer, String> getNextEntry(LinkedHashMap<Integer, String> map, Integer key) {
List<Integer> keys = new ArrayList<>(map.keySet());
int index = keys.indexOf(key);
if (index < 0 || index >= keys.size() - 1)
return null;
int k = keys.get(index + 1);
return Map.entry(k, map.get(k));
}
Or you can use Iterator:
public static Map.Entry<Integer, String> getNextEntry(LinkedHashMap<Integer, String> map, Integer key) {
boolean found = false;
for (Map.Entry<Integer, String> entry : map.entrySet()) {
if (found)
return Map.entry(entry.getKey(), entry.getValue());
if (entry.getKey().intValue() == key)
found = true;
}
return null;
}
LinkedHashMap doesn't offer a functionality which would allow finding the next key or entry.
In case if you simply don't want to bother with managing iteration yourself manually, then sure you can alternate this process, but keep in mind that the iteration should happen somewhere anyway.
Stream API
Alternatively you can make use of the Stream API if you don't want to bother with loops.
public static Optional<Map.Entry<Integer, String>> getNextEntry(Map<Integer, String> map,
int previous) {
return map.entrySet().stream()
.dropWhile(entry -> entry.getKey() != previous) // discard the entries, until the target key has been encountered
.skip(1) // skip the entry with the target key
.findFirst(); // grab the next entry and return it as an Optional (because the next entry might not exist)
}
TreeMap
However, you would be able to navigate through the keys of the map if you were using a TreeMap.
TreeMap maintains a red-black tree under the hood, and it keep the entries in sorted order based on keys. And it offers various method like higherEntry(), higherKey().
NavigableMap<Integer, String> map = new TreeMap<>();
// populating the map
int key = 50;
Map.Entry<Integer, String> next = map.higherEntry(key);
I am using a treemap but created my own comparator so that the treemap is ordered by the values rather than the keys. This works fine but whenever I come to overwrite a <key, value> mapping, instead of being overwritten, a new mapping is added with the same key (which shouldn't happen because maps in Java are meant to have unique keys). I have even tried to remove the mapping first before adding another one but nothing gets deleted from the treemap. When I remove the comparator, there are no unique values and the treemap works as expected. Why does this happen?
Here is my code:
public Map<String, List<String>> mapQtToNonSampledCase(List<Entry> cases, Map<String, Integer> populationDistribution) {
Map<String. Integer> distribution = new HashMap<>(populationDistribution);
Map<String. List<String>> qtToCases = new HashMap<>();
Comparator<String> valueComparator = new Comparator<String>() {
public int compare(String k1, String k2) {
int compare = distribution.get(k1).compareTo(distribution.get(k2));
if (compare == 0)
return 1;
else
return compare;
}
};
TreeMap<String, Integer> sortedByValues = new TreeMap<>(valueComparator);
sortedByValues.putAll(distribution);
for(Entry entry: cases) {
List<Map.Entry<String, Integer>> listEntries = sortedByValues.entrySet().stream().collect(Collectors.tolist());
Map.Entry<String, Integer> qt = sortedByValues.firstEntry().getKey().equals(entry.get(UtilsClass.ID).toString()) ? (listEntries.get(1) != null ? listEntries.get(1) : null) : sortedByValues.firstEntry();
if(qt != null) {
if(!qtToCases.containsKey(qt.getKey()) {
qtToCases.put(qt.getKey(), new ArrayList<>());
);
}
qtToCases.get(qt.getKey()).add(entry.get(UtilsClass.ID).toString());
sortedByValues.put(qt.getKey(), qt.getValue() - 1);
}
}
// Printing keys
for(Map.Entry<String, Integer> entry : sortedByValues.entrySet()) {
System.out.println(entry.getKey());
}
}
And here is the console output (apologies for the quality, it's a picture from another device):
Your custom comparator is not consistent with equals: When you try to update a key with a different value, your comparator will return a value != 0, but the keys are the same.
See this comment in TreeMap API doc:
Note that the ordering maintained by a tree map, like any sorted map,
and whether or not an explicit comparator is provided, must be
consistent with equals if this sorted map is to correctly implement
the Map interface.
The term 'consistent with equals' is defined here: [Comparable API doc]:2
The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C.
I understand that the Set returned from a Map's keySet() method does not guarantee any particular order.
My question is, does it guarantee the same order over multiple iterations. For example
Map<K,V> map = getMap();
for( K k : map.keySet() )
{
}
...
for( K k : map.keySet() )
{
}
In the above code, assuming that the map is not modified, will the iteration over the keySets be in the same order. Using Sun's jdk15 it does iterate in the same order, but before I depend on this behavior, I'd like to know if all JDKs will do the same.
EDIT
I see from the answers that I cannot depend on it. Too bad. I was hoping to get away with not having to build some new Collection to guarantee my ordering. My code needed to iterate through, do some logic, and then iterate through again with the same ordering. I'll just create a new ArrayList from the keySet which will guarantee order.
You can use a LinkedHashMap if you want a HashMap whose iteration order does not change.
Moreover you should always use it if you iterate through the collection. Iterating over HashMap's entrySet or keySet is much slower than over LinkedHashMap's.
If it is not stated to be guaranteed in the API documentation, then you shouldn't depend on it. The behavior might even change from one release of the JDK to the next, even from the same vendor's JDK.
You could easily get the set and then just sort it yourself, right?
Map is only an interface (rather than a class), which means that the underlying class that implements it (and there are many) could behave differently, and the contract for keySet() in the API does not indicate that consistent iteration is required.
If you are looking at a specific class that implements Map (HashMap, LinkedHashMap, TreeMap, etc) then you could see how it implements the keySet() function to determine what the behaviour would be by checking out the source, you'd have to really take a close look at the algorithm to see if the property you are looking for is preserved (that is, consistent iteration order when the map has not had any insertions/removals between iterations). The source for HashMap, for example, is here (open JDK 6): http://www.docjar.com/html/api/java/util/HashMap.java.html
It could vary widely from one JDK to the next, so i definitely wouldn't rely on it.
That being said, if consistent iteration order is something you really need, you might want to try a LinkedHashMap.
The API for Map does not guarantee any ordering whatsoever, even between multiple invocations of the method on the same object.
In practice I would be very surprised if the iteration order changed for multiple subsequent invocations (assuming the map itself did not change in between) - but you should not (and according to the API cannot) rely on this.
EDIT - if you want to rely on the iteration order being consistent, then you want a SortedMap which provides exactly these guarantees.
Just for fun, I decided to write some code that you can use to guarantee a random order each time. This is useful so that you can catch cases where you are depending on the order but you should not be. If you want to depend on the order, than as others have said, you should use a SortedMap. If you just use a Map and happen to rely on the order then using the following RandomIterator will catch that. I'd only use it in testing code since it makes use of more memory then not doing it would.
You could also wrap the Map (or the Set) to have them return the RandomeIterator which would then let you use the for-each loop.
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
public class Main
{
private Main()
{
}
public static void main(final String[] args)
{
final Map<String, String> items;
items = new HashMap<String, String>();
items.put("A", "1");
items.put("B", "2");
items.put("C", "3");
items.put("D", "4");
items.put("E", "5");
items.put("F", "6");
items.put("G", "7");
display(items.keySet().iterator());
System.out.println("---");
display(items.keySet().iterator());
System.out.println("---");
display(new RandomIterator<String>(items.keySet().iterator()));
System.out.println("---");
display(new RandomIterator<String>(items.keySet().iterator()));
System.out.println("---");
}
private static <T> void display(final Iterator<T> iterator)
{
while(iterator.hasNext())
{
final T item;
item = iterator.next();
System.out.println(item);
}
}
}
class RandomIterator<T>
implements Iterator<T>
{
private final Iterator<T> iterator;
public RandomIterator(final Iterator<T> i)
{
final List<T> items;
items = new ArrayList<T>();
while(i.hasNext())
{
final T item;
item = i.next();
items.add(item);
}
Collections.shuffle(items);
iterator = items.iterator();
}
public boolean hasNext()
{
return (iterator.hasNext());
}
public T next()
{
return (iterator.next());
}
public void remove()
{
iterator.remove();
}
}
I agree with LinkedHashMap thing. Just putting my findings and experience while I was facing the problem when I was trying to sort HashMap by keys.
My code to create HashMap:
HashMap<Integer, String> map;
#Before
public void initData() {
map = new HashMap<>();
map.put(55, "John");
map.put(22, "Apple");
map.put(66, "Earl");
map.put(77, "Pearl");
map.put(12, "George");
map.put(6, "Rocky");
}
I have a function showMap which prints entries of map:
public void showMap (Map<Integer, String> map1) {
for (Map.Entry<Integer, String> entry: map1.entrySet()) {
System.out.println("[Key: "+entry.getKey()+ " , "+"Value: "+entry.getValue() +"] ");
}
}
Now when I print the map before sorting, it prints following sequence:
Map before sorting :
[Key: 66 , Value: Earl]
[Key: 22 , Value: Apple]
[Key: 6 , Value: Rocky]
[Key: 55 , Value: John]
[Key: 12 , Value: George]
[Key: 77 , Value: Pearl]
Which is basically different than the order in which map keys were put.
Now When I sort it with map keys:
List<Map.Entry<Integer, String>> entries = new ArrayList<>(map.entrySet());
Collections.sort(entries, new Comparator<Entry<Integer, String>>() {
#Override
public int compare(Entry<Integer, String> o1, Entry<Integer, String> o2) {
return o1.getKey().compareTo(o2.getKey());
}
});
HashMap<Integer, String> sortedMap = new LinkedHashMap<>();
for (Map.Entry<Integer, String> entry : entries) {
System.out.println("Putting key:"+entry.getKey());
sortedMap.put(entry.getKey(), entry.getValue());
}
System.out.println("Map after sorting:");
showMap(sortedMap);
the out put is:
Sorting by keys :
Putting key:6
Putting key:12
Putting key:22
Putting key:55
Putting key:66
Putting key:77
Map after sorting:
[Key: 66 , Value: Earl]
[Key: 6 , Value: Rocky]
[Key: 22 , Value: Apple]
[Key: 55 , Value: John]
[Key: 12 , Value: George]
[Key: 77 , Value: Pearl]
You can see the difference in order of keys. Sorted order of keys is fine but that of keys of copied map is again in the same order of the earlier map. I dont know if this is valid to say, but for two hashmap with same keys, order of keys is same. This implies to the statement that order of keys is not guaranteed but can be same for two maps with same keys because of inherent nature of key insertion algorithm if HashMap implementation of this JVM version.
Now when I use LinkedHashMap to copy sorted Entries to HashMap, I get desired result (which was natural, but that is not the point. Point is regarding order of keys of HashMap)
HashMap<Integer, String> sortedMap = new LinkedHashMap<>();
for (Map.Entry<Integer, String> entry : entries) {
System.out.println("Putting key:"+entry.getKey());
sortedMap.put(entry.getKey(), entry.getValue());
}
System.out.println("Map after sorting:");
showMap(sortedMap);
Output:
Sorting by keys :
Putting key:6
Putting key:12
Putting key:22
Putting key:55
Putting key:66
Putting key:77
Map after sorting:
[Key: 6 , Value: Rocky]
[Key: 12 , Value: George]
[Key: 22 , Value: Apple]
[Key: 55 , Value: John]
[Key: 66 , Value: Earl]
[Key: 77 , Value: Pearl]
Hashmap does not guarantee that the order of the map will remain constant over time.
It doesn't have to be. A map's keySet function returns a Set and the set's iterator method says this in its documentation:
"Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee)."
So, unless you are using one of those classes with a guarantee, there is none.
Map is an interface and it does not define in the documentation that order should be the same. That means that you can't rely on the order. But if you control Map implementation returned by the getMap(), then you can use LinkedHashMap or TreeMap and get the same order of keys/values all the time you iterate through them.
tl;dr Yes.
I believe the iteration order for .keySet() and .values() is consistent (Java
8).
Proof 1: We load a HashMap with random keys and random values. We iterate on this HashMap using .keySet() and load the keys and it's corresponding values to a LinkedHashMap (it will preserve the order of the keys and values inserted). Then we compare the .keySet() of both the Maps and .values() of both the Maps. It always comes out to be the same, never fails.
public class Sample3 {
static final String AB = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
static SecureRandom rnd = new SecureRandom();
// from here: https://stackoverflow.com/a/157202/8430155
static String randomString(int len){
StringBuilder sb = new StringBuilder(len);
for (int i = 0; i < len; i++) {
sb.append(AB.charAt(rnd.nextInt(AB.length())));
}
return sb.toString();
}
public static void main(String[] args) throws Exception {
for (int j = 0; j < 10; j++) {
Map<String, String> map = new HashMap<>();
Map<String, String> linkedMap = new LinkedHashMap<>();
for (int i = 0; i < 1000; i++) {
String key = randomString(8);
String value = randomString(8);
map.put(key, value);
}
for (String k : map.keySet()) {
linkedMap.put(k, map.get(k));
}
if (!(map.keySet().toString().equals(linkedMap.keySet().toString()) &&
map.values().toString().equals(linkedMap.values().toString()))) {
// never fails
System.out.println("Failed");
break;
}
}
}
}
Proof 2: From here, the table is an array of Node<K,V> class. We know that iterating an array will give the same result every time.
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
*/
transient Node<K,V>[] table;
The class responsible for .values():
final class Values extends AbstractCollection<V> {
// more code here
public final void forEach(Consumer<? super V> action) {
Node<K,V>[] tab;
if (action == null)
throw new NullPointerException();
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e.value);
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
}
The class responsible for .keySet():
final class KeySet extends AbstractSet<K> {
// more code here
public final void forEach(Consumer<? super K> action) {
Node<K,V>[] tab;
if (action == null)
throw new NullPointerException();
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e.key);
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
}
Carefully look at both the inner classes. They are pretty much the same except:
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e.key); <- from KeySet class
// action.accept(e.value); <- the only change from Values class
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
They iterate on the same array table to support .keySet() in KeySet class and .values() in Values class.
Proof 3: this answer also explicitly states - So, yes, keySet(), values(), and entrySet() return values in the order the internal linked list uses.
Therefore, the .keySet() and .values() are consistent.
Logically, if the contract says "no particular order is guaranteed", and since "the order it came out one time" is a particular order, then the answer is no, you can't depend on it coming out the same way twice.
You also can store the Set instance returned by the keySet() method and can use this instance whenever you need the same order.
I would like to know: how can I iterate through a 2D map? I have a centralMap:
private final Map<String, Map<String, String>> centralMap =
new HashMap<String, Map<String, String>>();
which contains another map:
Map<String,String> nestedMap = new HashMap<String, String>();
as Value, and the second one is created in the "put" method, so that in the constructor, i have just the centralMap. Now i want to redefine this method and to get complete entry of the map (the 2 keys and the value for each element)
public Iterator<Entry> iterator()
How should I proceed? If possible, how can I remove an element through the iterator without having a problem?
Iterators are meant to operate on a collection, such as the keyset of your first map, or the values (which are a collection of maps) of your nested one. You cannot expect the iterator renove method to understand your complex structure.
I would suggest that you build your own class for this, with your own convenience methods that do what you described.
Also, going on a limb here: make sure you didn't just want to have a multimap. If so, have a look, for example, at guava's HashMultimap
You apply the same procedure as if you were iterating over a single map, you just do it twice:
public void printNestedMap(Map<String, Map<String, String>> map)
Iterator it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pairs = (Map.Entry)it.next(); // <- pairs.getValue() is a map
System.out.println("Key1: " + pairs.getKey());
//print the inner Map
printMap((Map<String, String>)pairs.getValue());
it.remove(); // avoids a ConcurrentModificationException
}
}
EDIT
It would actually be better to move the iteration over a single map to a different method to be called in this scenario.
public void printMap(Map<String, String>> map)
{
Iterator it = map.entrySet().iterator();
while(it.hasNext())
{
Map.Entry pairs = (Map.Entry)it.next(); // <- pairs.getValue() is a String
System.out.println("Key2: " + pairs.getKey() + " Value2: " + pairs.getValue());
it.remove();
}
}
EDIT 2: Test Program
import java.util.*;
public class TestMap
{
public static void main(String[] args)
{
Map<String, String> innerMap = new HashMap<>();
Map<String, Map<String, String>> outerMap = new HashMap<>();
innerMap.put("Key1", "Val1");
innerMap.put("Key2", "Val2");
innerMap.put("Key3", "Val3");
innerMap.put("Key4", "Val4");
outerMap.put("OuterKey1", innerMap);
printNestedMap(outerMap);
}
public static void printNestedMap(Map<String, Map<String, String>> map)
{
Iterator it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pairs = (Map.Entry)it.next(); // <- pairs.getValue() is a map
System.out.println("Key1: " + pairs.getKey());
//print the inner Map
printMap((Map<String, String>)pairs.getValue());
it.remove(); // avoids a ConcurrentModificationException
}
}
public static void printMap(Map<String, String> map)
{
Iterator it = map.entrySet().iterator();
while(it.hasNext())
{
Map.Entry pairs = (Map.Entry)it.next(); // <- pairs.getValue() is a String
System.out.println("Key2: " + pairs.getKey() + " Value2: " + pairs.getValue());
it.remove();
}
}
}
Output:
Key1: OuterKey1
Key2: Key2 Value2: Val2
Key2: Key1 Value2: Val1
Key2: Key4 Value2: Val4
Key2: Key3 Value2: Val3
If you want to get Map.Entry elements containing the two keys and the value, it will really be much more natural to create a class Pair<String, String> that combines the two keys in a single element and use that as the key in a single map rather than nesting maps.
If you do this, your main structure will be a Map<Pair<String, String>, String> and using the Map.entrySet() method will give you a Set<Map.Entry<String, String>, String> from which you can get an iterator that gives approximately what you're after.
If you need to have a Map<String, Map<String, String>> for other reasons, it is also possible to convert this into the structure described above by reasonably simple code, and this might be the most sensible way of getting the information out of it.
Edit Note:
The Pair class described above is essentially the same as Map.Entry, so you could avoid creating a new class for the key by building a Map<Map.Entry<String, String>, String>. I think it makes the code a bit less clear, but it can certainly be made functionally equivalent.
Sample Code
In the code below, I have defined the Pair class as an inner static (for real use, you might want to extract as an independent class), and written a conversion that takes a nested map as you describe, converts it to the form I've suggested, and uses an iterator on the entries of the converted map to print the values.
The iterator could of course be used for other things, and the convert method and Pair class are generic.
import java.util.*;
public class TestMap
{
public static void main(String[] args)
{
Map<String, String> innerMap1 = new HashMap<String, String>();
Map<String, String> innerMap2 = new HashMap<String, String>();
Map<String, Map<String, String>> outerMap = new HashMap<String, Map<String, String>>();
innerMap1.put("InnerKey1", "Val1");
innerMap1.put("InnerKey2", "Val2");
innerMap1.put("InnerKey3", "Val3");
innerMap1.put("InnerKey4", "Val4");
innerMap2.put("InnerKey5", "Val5");
innerMap2.put("InnerKey6", "Val6");
innerMap2.put("InnerKey7", "Val7");
innerMap2.put("InnerKey8", "Val8");
outerMap.put("OuterKey1", innerMap1);
outerMap.put("OuterKey2", innerMap2);
Map<Pair<String, String>, String> convertedMap = convert(outerMap);
for (Map.Entry<Pair<String, String>, String> entry: convertedMap.entrySet()) {
System.out.println(String.format("OuterKey: %s, InnerKey: %s, Value: %s",
entry.getKey().getFirst(),
entry.getKey().getSecond(),
entry.getValue()
));
}
}
private static <K1,K2,V> Map<Pair<K1, K2>,V> convert(Map<K1, Map<K2,V>> nestedMap) {
Map<Pair<K1, K2>, V> result = new HashMap<Pair<K1, K2>, V>();
for (Map.Entry<K1, Map<K2, V>> outerEntry: nestedMap.entrySet()) {
final K1 outerKey = outerEntry.getKey();
for (Map.Entry<K2, V> innerEntry: outerEntry.getValue().entrySet()) {
final K2 innerKey = innerEntry.getKey();
final V value = innerEntry.getValue();
result.put(new Pair<K1, K2>(outerKey, innerKey), value);
}
}
return result;
}
public static class Pair<T1, T2> {
private T1 first;
private T2 second;
public Pair(T1 first, T2 second) {
this.first = first;
this.second = second;
}
public T1 getFirst() {
return first;
}
public T2 getSecond() {
return second;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Pair pair = (Pair) o;
if (first != null ? !first.equals(pair.first) : pair.first != null) return false;
if (second != null ? !second.equals(pair.second) : pair.second != null) return false;
return true;
}
#Override
public int hashCode() {
int result = first != null ? first.hashCode() : 0;
result = 31 * result + (second != null ? second.hashCode() : 0);
return result;
}
}
}
Note on Usage in context:
In your current code, you have a class with a field centralMap which is the map in your old nested form, and an integer counter for the size of the map.
This containing class has a method for adding entries that looks like this:
#Override
public String put(final String row, final String column, final String value) {
/**
* Second map which is contained by centralMap, that contain Strings as Keys
* and Values.
*/
Map<String, String> nestedMap;
if (centralMap.containsKey(row))
nestedMap = centralMap.get(row);
else
nestedMap = new HashMap<String, String>();
if (!nestedMap.containsKey(column))
counter++;
centralMap.put(row, nestedMap);
return nestedMap.put(column, value);
}
If instead of using the nested map at all, you change this field to a map of the suggested form, this method would become a bit simpler:
#Override
public String put(final String row, final String column, final String value) {
Pair<String, String> key = new Pair(row, column);
if (centralMap.contains(key)
counter++;
centralMap.put(key, value);
}
And you actually wouldn't need the counter anymore, as it will always contain the same value as centralMap.size().
Update:
From edits put in yesterday but now deleted, it's now clear to me (from edit history) that you want to build a single iterator that delegates to all the iterators of the map in correct sequence, and returns a simple structure containing both keys and the value.
This is certainly possible, and if I have time later, I might add some sample code for it. As was noted in another response, the iterator.remove() method may be impossible or unnatural.
Meanwhile, your requirements (as noted a comment on the same other response) is rather similar to what is supplied by guava's Table. That's open source, and looking at it may give you ideas. You can download the source for guava here.
Specifically, in guava's StandardTable, there is an inner class CellIterator, which looks like:
private class CellIterator implements Iterator<Cell<R, C, V>> {
final Iterator<Entry<R, Map<C, V>>> rowIterator
= backingMap.entrySet().iterator();
Entry<R, Map<C, V>> rowEntry;
Iterator<Entry<C, V>> columnIterator
= Iterators.emptyModifiableIterator();
#Override public boolean hasNext() {
return rowIterator.hasNext() || columnIterator.hasNext();
}
#Override public Cell<R, C, V> next() {
if (!columnIterator.hasNext()) {
rowEntry = rowIterator.next();
columnIterator = rowEntry.getValue().entrySet().iterator();
}
Entry<C, V> columnEntry = columnIterator.next();
return Tables.immutableCell(
rowEntry.getKey(), columnEntry.getKey(), columnEntry.getValue());
}
#Override public void remove() {
columnIterator.remove();
if (rowEntry.getValue().isEmpty()) {
rowIterator.remove();
}
}
}
You can't just copy this code as it depends on other things in guava, but it shows the basic pattern of what you have to do.
I have a Hashtable in Java and want to iterate over all the values in the table and delete a particular key-value pair while iterating.
How may this be done?
You need to use an explicit java.util.Iterator to iterate over the Map's entry set rather than being able to use the enhanced For-loop syntax available in Java 6. The following example iterates over a Map of Integer, String pairs, removing any entry whose Integer key is null or equals 0.
Map<Integer, String> map = ...
Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Integer, String> entry = it.next();
// Remove entry if key is null or equals 0.
if (entry.getKey() == null || entry.getKey() == 0) {
it.remove();
}
}
You can use Enumeration:
Hashtable<Integer, String> table = ...
Enumeration<Integer> enumKey = table.keys();
while(enumKey.hasMoreElements()) {
Integer key = enumKey.nextElement();
String val = table.get(key);
if(key==0 && val.equals("0"))
table.remove(key);
}
You can use a temporary deletion list:
List<String> keyList = new ArrayList<String>;
for(Map.Entry<String,String> entry : hashTable){
if(entry.getValue().equals("delete")) // replace with your own check
keyList.add(entry.getKey());
}
for(String key : keyList){
hashTable.remove(key);
}
You can find more information about Hashtable methods in the Java API
So you know the key, value pair that you want to delete in advance? It's just much clearer to do this, then:
table.delete(key);
for (K key: table.keySet()) {
// do whatever you need to do with the rest of the keys
}