I'd like to create new item that similarly to Util.Map.Entry
that will contain the structure key
, value
.
The problem is that I can't instantiate a Map.Entry
because it's an interface.
Does anyone know how to create a new generic key/value object for Map.Entry?
This question is related to
java
dictionary
collections
key-value
org.apache.commons.lang3.tuple.Pair
implements java.util.Map.Entry
and can also be used standalone.
Also as others mentioned Guava's com.google.common.collect.Maps.immutableEntry(K, V)
does the trick.
I prefer Pair
for its fluent Pair.of(L, R)
syntax.
If you look at the documentation of Map.Entry you will find that it is a static interface (an interface which is defined inside the Map interface an can be accessed through Map.Entry) and it has two implementations
All Known Implementing Classes:
AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry
The class AbstractMap.SimpleEntry provides 2 constructors:
Constructors and Description
AbstractMap.SimpleEntry(K key, V value)
Creates an entry representing a mapping from the specified key to the
specified value.
AbstractMap.SimpleEntry(Map.Entry<? extends K,? extends V> entry)
Creates an entry representing the same mapping as the specified entry.
An example use case:
import java.util.Map;
import java.util.AbstractMap.SimpleEntry;
public class MyClass {
public static void main(String args[]) {
Map.Entry e = new SimpleEntry<String, String>("Hello","World");
System.out.println(e.getKey()+" "+e.getValue());
}
}
If you are using Clojure, you have another option:
(defn map-entry
[k v]
(clojure.lang.MapEntry/create k v))
Example of AbstractMap.SimpleEntry:
import java.util.Map;
import java.util.AbstractMap;
import java.util.AbstractMap.SimpleEntry;
Instantiate:
ArrayList<Map.Entry<Integer, Integer>> arr =
new ArrayList<Map.Entry<Integer, Integer>>();
Add rows:
arr.add(new AbstractMap.SimpleEntry(2, 3));
arr.add(new AbstractMap.SimpleEntry(20, 30));
arr.add(new AbstractMap.SimpleEntry(2, 4));
Fetch rows:
System.out.println(arr.get(0).getKey());
System.out.println(arr.get(0).getValue());
System.out.println(arr.get(1).getKey());
System.out.println(arr.get(1).getValue());
System.out.println(arr.get(2).getKey());
System.out.println(arr.get(2).getValue());
Should print:
2
3
20
30
2
4
It's good for defining edges of graph structures. Like the ones between neurons in your head.
Starting from Java 9, there is a new utility method allowing to create an immutable entry which is Map#entry(Object, Object)
.
Here is a simple example:
Entry<String, String> entry = Map.entry("foo", "bar");
As it is immutable, calling setValue
will throw an UnsupportedOperationException
. The other limitations are the fact that it is not serializable and null
as key or value is forbidden, if it is not acceptable for you, you will need to use AbstractMap.SimpleImmutableEntry
or AbstractMap.SimpleEntry
instead.
NB: If your need is to create directly a Map
with 0 to up to 10 (key, value) pairs, you can instead use the methods of type Map.of(K key1, V value1, ...)
.
Why Map.Entry
? I guess something like a key-value pair is fit for the case.
Use java.util.AbstractMap.SimpleImmutableEntry
or java.util.AbstractMap.SimpleEntry
I defined a generic Pair class that I use all the time. It's great. As a bonus, by defining a static factory method (Pair.create) I only have to write the type arguments half as often.
public class Pair<A, B> {
private A component1;
private B component2;
public Pair() {
super();
}
public Pair(A component1, B component2) {
this.component1 = component1;
this.component2 = component2;
}
public A fst() {
return component1;
}
public void setComponent1(A component1) {
this.component1 = component1;
}
public B snd() {
return component2;
}
public void setComponent2(B component2) {
this.component2 = component2;
}
@Override
public String toString() {
return "<" + component1 + "," + component2 + ">";
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result
+ ((component1 == null) ? 0 : component1.hashCode());
result = prime * result
+ ((component2 == null) ? 0 : component2.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
final Pair<?, ?> other = (Pair<?, ?>) obj;
if (component1 == null) {
if (other.component1 != null)
return false;
} else if (!component1.equals(other.component1))
return false;
if (component2 == null) {
if (other.component2 != null)
return false;
} else if (!component2.equals(other.component2))
return false;
return true;
}
public static <A, B> Pair<A, B> create(A component1, B component2) {
return new Pair<A, B>(component1, component2);
}
}
You could actually go with:
Map.Entry<String, String> en= Maps.immutableEntry(key, value);
There's public static class AbstractMap.SimpleEntry<K,V>
. Don't let the Abstract
part of the name mislead you: it is in fact NOT an abstract
class (but its top-level AbstractMap
is).
The fact that it's a static
nested class means that you DON'T need an enclosing AbstractMap
instance to instantiate it, so something like this compiles fine:
Map.Entry<String,Integer> entry =
new AbstractMap.SimpleEntry<String, Integer>("exmpleString", 42);
As noted in another answer, Guava also has a convenient static
factory method Maps.immutableEntry
that you can use.
You said:
I can't use
Map.Entry
itself because apparently it's a read-only object that I can't instantiate newinstanceof
That's not entirely accurate. The reason why you can't instantiate it directly (i.e. with new
) is because it's an interface Map.Entry
.
As noted in the documentation, AbstractMap.SimpleEntry
is @since 1.6
, so if you're stuck to 5.0, then it's not available to you.
To look for another known class that implements Map.Entry
, you can in fact go directly to the javadoc. From the Java 6 version
Interface Map.Entry
All Known Implementing Classes:
Unfortunately the 1.5 version does not list any known implementing class that you can use, so you may have be stuck with implementing your own.
Try Maps.immutableEntry from Guava
This has the advantage of being compatible with Java 5 (unlike AbstractMap.SimpleEntry
which requires Java 6.)
Source: Stackoverflow.com