Java - How to create new Entry (key, value)

Question

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?

Answer 1

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.

Answer 2

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());

Answer 3

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.

Answer 4

You could actually go with: Map.Entry<String, String> en= Maps.immutableEntry(key, value);

Answer 5

Try Maps.immutableEntry from Guava

This has the advantage of being compatible with Java 5 (unlike AbstractMap.SimpleEntry which requires Java 6.)

Answer 6

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