问题
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?
回答1:
You can just implement the Map.Entry<K, V> interface yourself:
import java.util.Map;
final class MyEntry<K, V> implements Map.Entry<K, V> {
private final K key;
private V value;
public MyEntry(K key, V value) {
this.key = key;
this.value = value;
}
@Override
public K getKey() {
return key;
}
@Override
public V getValue() {
return value;
}
@Override
public V setValue(V value) {
V old = this.value;
this.value = value;
return old;
}
}
And then use it:
Map.Entry<String, Object> entry = new MyEntry<String, Object>("Hello", 123);
System.out.println(entry.getKey());
System.out.println(entry.getValue());
回答2:
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.Entryitself 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.
Caveat and tip
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:
- AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry
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.
回答3:
Try Maps.immutableEntry from Guava
This has the advantage of being compatible with Java 5 (unlike AbstractMap.SimpleEntry which requires Java 6.)
回答4:
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.
回答5:
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, ...).
回答6:
You could actually go with:
Map.Entry<String, String> en= Maps.immutableEntry(key, value);
回答7:
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
回答8:
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.
回答9:
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);
}
}
回答10:
If you are using Clojure, you have another option:
(defn map-entry
[k v]
(clojure.lang.MapEntry/create k v))
来源：https://stackoverflow.com/questions/3110547/java-how-to-create-new-entry-key-value