iterator in a 2Dimensional Map

iterator in a 2Dimensional Map - java

I would like to know how can I iterate in a 2Dimensional HashMap? I am creating an Object TrueStringMap2D that does the following: It will be a map 2D, i mean 2 keys and one value.
But the iterator implemented here is not functional.. i didnt know how to redefine the Iterator method in TrueStringMap2D :S (if possible should be remove in the iterator() functional)
Anyone can help?
Thankyou very much!!

I'll reinterpret the question into something similar that I enjoy answering, and then hopefully the answer to that question is useful to you.
Here's the question I'll answer:
How do I write an iterator that iterates over all values in a Map<String, Map<String, String>>?
This is how I would solve it:
class TwoDimIterator implements Iterator<String> {
Iterator<Map<String, String>> outerIter;
Iterator<String> innerIter = Collections.<String>emptyList().iterator();
public TwoDimIterator(Map<String, Map<String, String>> twoDimMap) {
outerIter = twoDimMap.values().iterator();
advanceInner();
}
#Override
public boolean hasNext() {
return innerIter.hasNext();
}
#Override
public String next() {
String toReturn = innerIter.next();
advanceInner();
return toReturn;
}
private void advanceInner() {
while (!innerIter.hasNext()) {
if (!outerIter.hasNext()) {
innerIter = Collections.<String>emptyList().iterator();
return;
}
innerIter = outerIter.next().values().iterator();
}
}
#Override
public void remove() {
throw new UnsupportedOperationException();
}
}
Test code:
class Test {
public static void main(String[] args) {
// Create a map
Map<String, Map<String, String>> twoDimMap =
new HashMap<String, Map<String, String>>();
// Fill it
Map<String, String> innerA = new HashMap<String, String>();
innerA.put("1", "A1");
innerA.put("2", "A2");
Map<String, String> innerB = new HashMap<String, String>();
innerB.put("1", "B1");
innerB.put("2", "B2");
twoDimMap.put("A", innerA);
twoDimMap.put("B", innerB);
// Create an iterator for the values:
Iterator<String> twoDimIter = new TwoDimIterator(twoDimMap);
while (twoDimIter.hasNext())
System.out.println(twoDimIter.next());
}
}
Output:
A2
A1
B2
B1

Related

How to assert Map contains Map with entry

I have a unit test that needs to check for a nested map value. I can get my assertion to work by pulling out the entry and matching the underlying Map, but I was looking for a clear way to show what the assertion is doing. Here is a very simplified test:
import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.hasEntry;
import java.util.HashMap;
import java.util.Map;
import org.junit.Test;
public class MapContainsMapTest {
#Test
public void testMapHasMap() {
Map<String, Object> outerMap = new HashMap<String, Object>();
Map<String, Object> nestedMap = new HashMap<String, Object>();
nestedMap.put("foo", "bar");
outerMap.put("nested", nestedMap);
// works but murky
assertThat((Map<String, Object>) outerMap.get("nested"), hasEntry("foo", "bar"));
// fails but clear
assertThat(outerMap, hasEntry("nested", hasEntry("foo", "bar")));
}
}
It seems the problem is the outer map is being compared using hasEntry(K key, V value) while what I want to use is hasEntry(Matcher<? super K> keyMatcher, Matcher<? super V> valueMatcher). I am not sure how to coerce the assertion to use the second form.
Thanks in advance.

If you only want to put Map<String, Object> as values in your outerMap adjust the declaration accordingly. Then you can do
#Test
public void testMapHasMap() {
Map<String, Map<String, Object>> outerMap = new HashMap<>();
Map<String, Object> nestedMap = new HashMap<String, Object>();
nestedMap.put("foo", "bar");
outerMap.put("nested", nestedMap);
Object value = "bar";
assertThat(outerMap, hasEntry(equalTo("nested"), hasEntry("foo", value)));
}
Object value = "bar"; is necessary for compile reasons. Alternatively you could use
assertThat(outerMap,
hasEntry(equalTo("nested"), Matchers.<String, Object> hasEntry("foo", "bar")));

If You declare outerMap as Map<String, Map<String, Object>> you don't need the ugly cast. Like this:
public class MapContainsMapTest {
#Test
public void testMapHasMap() {
Map<String, Map<String, Object>> outerMap = new HashMap<>();
Map<String, Object> nestedMap = new HashMap<>();
nestedMap.put("foo", "bar");
outerMap.put("nested", nestedMap);
assertThat(outerMap.get("nested"), hasEntry("foo", "bar"));
}
}

I would probably extend a new Matcher for that, something like that (beware, NPEs lurking):
class SubMapMatcher extends BaseMatcher<Map<?,?>> {
private Object key;
private Object subMapKey;
private Object subMapValue;
public SubMapMatcher(Object key, Object subMapKey, Object subMapValue) {
super();
this.key = key;
this.subMapKey = subMapKey;
this.subMapValue = subMapValue;
}
#Override
public boolean matches(Object item) {
Map<?,?> map = (Map<?,?>)item;
if (!map.containsKey(key)) {
return false;
}
Object o = map.get(key);
if (!(o instanceof Map<?,?>)) {
return false;
}
Map<?,?> subMap = (Map<?,?>)o;
return subMap.containsKey(subMapKey) && subMap.get(subMapKey).equals(subMapValue);
}
#Override
public void describeTo(Description description) {
description.appendText(String.format("contains %s -> %s : %s", key, subMapKey, subMapValue));
}
public static SubMapMatcher containsSubMapWithKeyValue(String key, String subMapKey, String subMapValue) {
return new SubMapMatcher(key, subMapKey, subMapValue);
}
}

Try like this :
assertThat(nestedMap).contains(Map.entry("foo", "bar"));
assertThat(outerMap).contains(Map.entry("nested", nestedMap));

How to make class executable in forEach loop?

If I have a class
public class Op {
public Map<String, String> ops;
}
How can I make it executable in forEach loop?
for (String key : op) {
System.out.println(op.ops.get(key))
}
UPD
Here is my solution
Op op = new Op(new HashMap<String, String>() {{
put("a", "1");
put("b", "2");
put("c", "3");
}});
for (String key : op) System.out.println(op.map.get(key));
class Op implements Iterable<String> {
Map<String, String> map;
public Op(Map<String, String> map) {
this.map = map;
}
public Iterator<String> iterator() {
return map.keySet().iterator();
}
}
But I'm not sure about verbosity. Is it too verbose? Maybe there is a much concise way to implement it?

You are interested in looping over a SetEntry list.
for (Map.Entry<String, String > entry : ops.entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
// ...
}
Propable duplicate of: Iterate through a HashMap

In this exact case, use #entrySet, #keySet or #values as specified in the API docs.
In a general case, an object is iterable using the enhanced for loop, if it is implementing the interface Iterable. The most of the standard java collections are implementing this interface.

public class Op implements Iterable<Map.Entry<String,String>> {
public Map<String, String> ops;
#Override
public Iterator<Map.Entry<String,String>> iterator() {
return ops.entrySet().iterator();
}
}
Example:
for (Map.Entry<String, String> n : testOp){
System.out.println("Key: " + n.getKey() + " Value: " + n.getValue());
}

Sorting a list of maps

I have a list of maps
List<Map<String, Object>> people = new ArrayList<Map<String,Object>>();
that gets populated like so
map.put("firstName",John);
map.put("lastName",Smith);
map.put("type","1"); //type is either 1 or a 0
people.add(map);
and what I want to do with this list after it gets populated is so have all the people as type 0 at the top of the list and all with type 1 at the bottom.
I know I need to use a Comparator but I have never used one before so I dont know how to use one or how it works.
Could someone help me out

Like this
Collections.sort( people, new Comparator<Map<String, Object>>() {
#Override
public int compare( Map<String, Object> o1, Map<String, Object> o2 ) {
return (Integer.parseInt((String)o1.get( "type" ))) -
(Integer.parseInt((String)o2.get( "type" )));
}
} );
However, there are many ways to make this better. If you cannot use a Person object to represent the map as suggested by #Pshemo, then at least, use a reasonable data type for your type attribute. The best would be an enum:
public enum PersonType {
TYPE_1, TYPE_2
}
Then the comparisons are much cleaner and faster and much more readable.

Comparator is just an interface that needs to be implemented, it contains only one method that needs to be overriden.
For example:
List<Map<String, Object>> people = new ArrayList<Map<String,Object>>();
Map<String, Object> map = new HashMap<String, Object>();
map .put("firstName","John");
map.put("lastName","Smith");
map.put("type","1"); //type is either 1 or a 0
people.add(map);
Collections.sort(people, new Comparator<Map<String, Object>>() {
#Override
public int compare(Map<String, Object> o1, Map<String, Object> o2) {
// you may compare your map here
return 0;
}
});

Try this
Collections.sort(people, new Comparator<Map<String, String>>() {
#Override
public int compare(Map<String, String> m1, Map<String, String> m2) {
return m1.get("type").compareTo(m2.get("type"));
}
});

You can try like this :
class ListByType
{
private static class MyComparator implements Comparator<HashMap<String,String>>
{
#Override
public int compare(HashMap mp1 , HashMap mp2)
{
return ((String)(mp1.get("type")).compareTo((String)mp2.get("type"));
}
}
public static void main(String[] args)
{
List<Map<String, String>> people = new ArrayList<Map<String,String>>();
HashMap<String,String> map = new HashMap<String,String>();
map.put("firstName","John");
map.put("lastName","Smith");
map.put("type","1"); //type is either 1 or a 0
people.add(map);
/*...
..
...
Add more maps here..
*/
//Sort the list
Collections.sort(people,new MyComparator());
}
}

Looking for a "chained map" implementation in Java

I need a mapping from a list of keys to a value. I know I could write my own code like this:
Map<Person, Map<Daytime, Map<Food, Integer>>> eaten = ...;
Now I want to have some get and put methods like these:
Integer numberOfEggsIAteInTheMorning = eaten.get(me, morning, scrambledEggs);
eaten.put(me, evening, scrambledEggs, 1);
Do you know of an existing class that has this kind of API? I'm too lazy of writing it myself. ;)

If you look for a more generic approach, and you might have more than 2 or 3 'chain steps', I would suggest in applying some different structural approach, rather than sticking to using only basic collection classes. I have feeling that Composite Pattern could be the right choice if it's correctly applied.
EDIT: due to example requested
The full example would be somewhat time consuming, so let me just explain my idea with dirty Java/pseudocode mix (I'm not even sure if I've missed something!!!). Let's consider we have class BaseMap:
abstract class BaseMap {
public abstract Object getValue(Object.. keys);
public abstract void putValue(Object value, Object.. keys);
}
Then we could have ObjectMap that would be the 'leaf' of our composite structure:
class ObjectsMap extends BaseMap {
private Map<Object, Object> map = new [...]
public Object getValue(Object.. keys) {
// assert that keys.length == 1
return map.get(keys[0]);
}
public void putValue(Object value, Object.. keys) {
// assert that keys.length = 1
map.put(keys[0], value);
}
}
And the actual composite would be as such:
class CompositeMap extends BaseMap {
private Map<Object, BaseMap> compositeMaps = new [...]
public Object getValue(Object.. keys) {
// assert that keys.length > 1
return compositeMap.get(keys[0]).getValue(/* System.arrayCopy => subset of elements {keys_1, .. ,keys_max} */);
}
public void putValue(Object value, Object.. keys) {
// assert keys.length > 1
BaseMap newMap = null;
if (keys.length = 2) -> newMap = new ObjectsMap()
else newMap = new CompositeMap();
newMap.putValue(value, /*subset of keys {keys_1, .. , keys_max}*/);
}
}

You can use org.apache.commons.collections.keyvalue.MultiKey for that: Map<Multikey, Object>

It would be hard to implement a general chained map.
How would the declaration of the class look like? (You can't have a variable number of type parameters.
class ChainedMap<K1..., V>
Another option would be to have a ChainedMapUtil class that performs put / get recursively.
Here is an example of a recursive get. (Quite ugly solution though I must say.)
import java.util.*;
public class Test {
public static Object chainedGet(Map<?, ?> map, Object... keys) {
Object k = keys[0];
if (!map.containsKey(k)) return null;
if (keys.length == 1) return map.get(k);
Object[] tailKeys = Arrays.copyOfRange(keys, 1, keys.length);
return chainedGet((Map<?,?>) map.get(k), tailKeys);
}
public static void main(String[] arg) {
Map<String, String> m1 = new HashMap<String, String>();
m1.put("ipsum", "dolor");
Map<Integer, Map<String, String>> m2 =
new HashMap<Integer, Map<String, String>>();
m2.put(17, m1);
Map<String, Map<Integer, Map<String, String>>> chained =
new HashMap<String, Map<Integer, Map<String, String>>>();
chained.put("lorem", m2);
System.out.println(chainedGet(chained, "lorem", 17, "ipsum")); // dolor
System.out.println(chainedGet(chained, "lorem", 19, "ipsum")); // null
}
}

If you are going to write your own, I would suggest
eaten.increment(me, evening, scrambledEggs);
You could use a composite key
eaten.increment(Key.of(me, evening, scrambledEggs));
(TObjectIntHashMap supports increment and adjust)
You may not even need a custom key.
eaten.increment(me + "," + evening + "," + scrambledEggs);
It is fairly easy to decompose the key with split()

I once made a map using 3 keys just for fun.May be you can use it instead of using chained maps:
public class ThreeKeyMap<K1,K2,K3,V>{
class wrap{
K1 k1;
K2 k2;
K3 k3;
public wrap(K1 k1,K2 k2,K3 k3) {
this.k1=k1;this.k2=k2;this.k3=k3;
}
#Override
public boolean equals(Object arg0) {
// TODO Auto-generated method stub
wrap o=(wrap)arg0;
if(!this.k1.equals(o.k1))
return false;
if(!this.k2.equals(o.k2))
return false;
if(!this.k2.equals(o.k2))
return false;
return true;
}
#Override
public int hashCode() {
int result=17;
result=37*result+k1.hashCode();
result=37*result+k2.hashCode();
result=37*result+k3.hashCode();
return result;
}
}
HashMap<wrap,V> map=new HashMap<wrap, V>();
public V put(K1 k1,K2 k2,K3 k3,V arg1) {
return map.put(new wrap(k1,k2,k3), arg1);
}
public V get(Object k1,Object k2,Object k3) {
return map.get(new wrap((K1)k1,(K2)k2,(K3)k3));
}
public static void main(String[] args) {
ThreeKeyMap<Integer,Integer,Integer,String> birthDay=new ThreeKeyMap<Integer, Integer, Integer, String>();
birthDay.put(1, 1,1986,"Emil");
birthDay.put(2,4,2009, "Ansih");
birthDay.put(1, 1,1986,"Praveen");
System.out.println(birthDay.get(1,1,1986));
}
}
UPDATE:
As #Arturs Licis suggested.I looked up in net for composite pattern and I wrote a sample using it.I guess this is composite..Please comment if it is not so.
Person class:
public class Person {
private final String name;
private Map<Time, Food> map = new HashMap<Time, Food>();
public Person(String name) {
this.name = name;
}
void addTimeFood(Time time, Food food) {
map.put(time, food);
}
public String getName() {
return name;
}
Food getFood(Time time) {
Food tmp = null;
return (tmp = map.get(time)) == null ? Food.NoFood : tmp;
}
// main to test the person class
public static void main(String[] args) {
Person p1 = new Person("Jack");
p1.addTimeFood(Time.morning, Food.Bread);
p1.addTimeFood(Time.evening, Food.Chicken);
Person p2 = new Person("Jill");
p2.addTimeFood(Time.morning, Food.Egg);
p2.addTimeFood(Time.evening, Food.Rice);
Map<String, Person> map = new HashMap<String, Person>();
map.put(p1.getName(), p1);
map.put(p2.getName(), p2);
System.out.println(map.get("Jack").getFood(Time.evening));
}
#Override
public String toString() {
StringBuilder b = new StringBuilder();
b.append(name).append("\n");
b.append(map);
return b.toString();
}
}
Food class:
public enum Food {
Rice,
Egg,
Chicken,
Bread,
NoFood;
}
Time class:
public enum Time {
morning,
evening,
night
}

How do I access nested HashMaps in Java?

I have a HashMap in Java, the contents of which (as you all probably know) can be accessed by
HashMap.get("keyname");
If a have a HashMap inside another HashMap i.e. a nested HashMap, how would i access the contents? Can i do this like this, inline:
HashMap.get("keyname").get("nestedkeyname");
Thank you.

You can do it like you assumed. But your HashMap has to be templated:
Map<String, Map<String, String>> map =
new HashMap<String, Map<String, String>>();
Otherwise you have to do a cast to Map after you retrieve the second map from the first.
Map map = new HashMap();
((Map)map.get( "keyname" )).get( "nestedkeyname" );

You can get the nested value by repeating .get(), but with deeply nested maps you have to do a lot of casting into Map. An easier way is to use a generic method for getting a nested value.
Implementation
public static <T> T getNestedValue(Map map, String... keys) {
Object value = map;
for (String key : keys) {
value = ((Map) value).get(key);
}
return (T) value;
}
Usage
// Map contents with string and even a list:
{
"data": {
"vehicles": {
"list": [
{
"registration": {
"owner": {
"id": "3643619"
}
}
}
]
}
}
}
List<Map> list = getNestedValue(mapContents, "data", "vehicles", "list");
Map first = list.get(0);
String id = getNestedValue(first, "registration", "owner", "id");

Yes.
See:
public static void main(String args[]) {
HashMap<String, HashMap<String, Object>> map = new HashMap<String, HashMap<String,Object>>();
map.put("key", new HashMap<String, Object>());
map.get("key").put("key2", "val2");
System.out.println(map.get("key").get("key2"));
}

If you plan on constructing HashMaps with variable depth, use a recursive data structure.
Below is an implementation providing a sample interface:
class NestedMap<K, V> {
private final HashMap<K, NestedMap> child;
private V value;
public NestedMap() {
child = new HashMap<>();
value = null;
}
public boolean hasChild(K k) {
return this.child.containsKey(k);
}
public NestedMap<K, V> getChild(K k) {
return this.child.get(k);
}
public void makeChild(K k) {
this.child.put(k, new NestedMap());
}
public V getValue() {
return value;
}
public void setValue(V v) {
value = v;
}
}
and example usage:
class NestedMapIllustration {
public static void main(String[] args) {
NestedMap<Character, String> m = new NestedMap<>();
m.makeChild('f');
m.getChild('f').makeChild('o');
m.getChild('f').getChild('o').makeChild('o');
m.getChild('f').getChild('o').getChild('o').setValue("bar");
System.out.println(
"nested element at 'f' -> 'o' -> 'o' is " +
m.getChild('f').getChild('o').getChild('o').getValue());
}
}

As others have said you can do this but you should define the map with generics like so:
Map<String, Map<String, String>> map = new HashMap<String, Map<String,String>>();
However, if you just blindly run the following:
map.get("keyname").get("nestedkeyname");
you will get a null pointer exception whenever keyname is not in the map and your program will crash. You really should add the following check:
String valueFromMap = null;
if(map.containsKey("keyname")){
valueFromMap = map.get("keyname").get("nestedkeyname");
}

Yes, if you use the proper generic type signature for the outer hashmap.
HashMap<String, HashMap<String, Foo>> hm = new HashMap<String, HashMap<String, Foobar>>();
// populate the map
hm.get("keyname").get("nestedkeyname");
If you're not using generics, you'd have to do a cast to convert the object retrieved from the outer hash map to a HashMap (or at least a Map) before you could call its get() method. But you should be using generics ;-)

I prefer creating a custom map that extends HashMap. Then just override get() to add extra logic so that if the map doesnt contain your key. It will a create a new instance of the nested map, add it, then return it.
public class KMap<K, V> extends HashMap<K, V> {
public KMap() {
super();
}
#Override
public V get(Object key) {
if (this.containsKey(key)) {
return super.get(key);
} else {
Map<K, V> value = new KMap<K, V>();
super.put((K)key, (V)value);
return (V)value;
}
}
}
Now you can use it like so:
Map<Integer, Map<Integer, Map<String, Object>>> nestedMap = new KMap<Integer, Map<Integer, Map<String, Object>>>();
Map<String, Object> map = (Map<String, Object>) nestedMap.get(1).get(2);
Object obj= new Object();
map.put(someKey, obj);

I came to this StackOverflow page looking for a something ala valueForKeyPath known from objc. I also came by another post - "Key-Value Coding" for Java, but ended up writing my own.
I'm still looking for at better solution than PropertyUtils.getProperty in apache's beanutils library.
Usage
Map<String, Object> json = ...
public String getOptionalFirstName() {
return MyCode.getString(json, "contact", "firstName");
}
Implementation
public static String getString(Object object, String key0, String key1) {
if (key0 == null) {
return null;
}
if (key1 == null) {
return null;
}
if (object instanceof Map == false) {
return null;
}
#SuppressWarnings("unchecked")
Map<Object, Object> map = (Map<Object, Object>)object;
Object object1 = map.get(key0);
if (object1 instanceof Map == false) {
return null;
}
#SuppressWarnings("unchecked")
Map<Object, Object> map1 = (Map<Object, Object>)object1;
Object valueObject = map1.get(key1);
if (valueObject instanceof String == false) {
return null;
}
return (String)valueObject;
}

import java.util.*;
public class MyFirstJava {
public static void main(String[] args)
{
Animal dog = new Animal();
dog.Info("Dog","Breezi","Lab","Chicken liver");
dog.Getname();
Animal dog2= new Animal();
dog2.Info("Dog", "pumpkin", "POM", "Pedigree");
dog2.Getname();
HashMap<String, HashMap<String, Object>> dogs = new HashMap<>();
dogs.put("dog1", new HashMap<>() {{put("Name",dog.name);
put("Food",dog.food);put("Age",3);}});
dogs.put("dog2", new HashMap<>() {{put("Name",dog2.name);
put("Food",dog2.food);put("Age",6);}});
//dogs.get("dog1");
System.out.print(dogs + "\n");
System.out.print(dogs.get("dog1").get("Age"));
}
}

Example Map:
{
"data": {
"userData": {
"location": {
"city": "Banja Luka"
}
}
}
}
Implementation:
public static Object getValueFromMap(final Map<String, Object> map, final String key) {
try {
final String[] tmpKeys = key.split("\\.");
Map<String, Object> currentMap = map;
for (int i = 0; i < tmpKeys.length - 1; i++) {
currentMap = (Map<String, Object>) currentMap.get(tmpKeys[i]);
}
return currentMap.get(tmpKeys[tmpKeys.length - 1]);
} catch (Exception exception) {
return null;
}
}
Usage:
final Map<String, Object> data = new HashMap<>();
final Map<String, Object> userData = new HashMap<>();
final Map<String, Object> location = new HashMap<>();
location.put("city", "Banja Luka");
userData.put("location", location);
data.put("userData", userData);
System.out.println(getValueFromMap(data, "userData.location.city"));
Result:
Banja Luka
Process finished with exit code 0

I hit this discussion while trying to figure out how to get a value from a nested map of unknown depth and it helped me come up with the following solution to my problem. It is overkill for the original question but maybe it will be helpful to someone that finds themselves in a situation where you have less knowledge about the map being searched.
private static Object pullNestedVal(
Map<Object, Object> vmap,
Object ... keys) {
if ((keys.length == 0) || (vmap.size() == 0)) {
return null;
} else if (keys.length == 1) {
return vmap.get(keys[0]);
}
Object stageObj = vmap.get(keys[0]);
if (stageObj instanceof Map) {
Map<Object, Object> smap = (Map<Object, Object>) stageObj;
Object[] skeys = Arrays.copyOfRange(keys, 1, keys.length);
return pullNestedVal(smap, skeys);
} else {
return null;
}
}

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iterator in a 2Dimensional Map - java

Related

How to assert Map contains Map with entry

How to make class executable in forEach loop?

Sorting a list of maps

Looking for a "chained map" implementation in Java

How do I access nested HashMaps in Java?

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