Java associative-array

Question

How can I create and fetch associative arrays in Java like I can in PHP?

For example:

$arr[0]['name'] = 'demo';
$arr[0]['fname'] = 'fdemo';
$arr[1]['name'] = 'test';
$arr[1]['fname'] = 'fname';

Answer 1

Java doesn't support associative arrays, however this could easily be achieved using a Map. E.g.,

Map<String, String> map = new HashMap<String, String>();
map.put("name", "demo");
map.put("fname", "fdemo");
// etc

map.get("name"); // returns "demo"

Even more accurate to your example (since you can replace String with any object that meet your needs) would be to declare:

List<Map<String, String>> data = new ArrayList<>();
data.add(0, map);
data.get(0).get("name"); 

See the official documentation for more information

Answer 2

Java doesn't have associative arrays like PHP does.

There are various solutions for what you are doing, such as using a Map, but it depends on how you want to look up the information. You can easily write a class that holds all your information and store instances of them in an ArrayList.

public class Foo{
    public String name, fname;

    public Foo(String name, String fname){
        this.name = name;
        this.fname = fname;
    }
}

And then...

List<Foo> foos = new ArrayList<Foo>();
foos.add(new Foo("demo","fdemo"));
foos.add(new Foo("test","fname"));

So you can access them like...

foos.get(0).name;
=> "demo"

Answer 3

You can accomplish this via Maps. Something like

Map<String, String>[] arr = new HashMap<String, String>[2]();
arr[0].put("name", "demo");

But as you start using Java I am sure you will find that if you create a class/model that represents your data will be your best options. I would do

class Person{
String name;
String fname;
}
List<Person> people = new ArrayList<Person>();
Person p = new Person();
p.name = "demo";
p.fname = "fdemo";
people.add(p);

Answer 4

Look at the Map interface, and at the concrete class HashMap.

To create a Map:

Map<String, String> assoc = new HashMap<String, String>();

To add a key-value pair:

assoc.put("name", "demo");

To retrieve the value associated with a key:

assoc.get("name")

And sure, you may create an array of Maps, as it seems to be what you want:

Map<String, String>[] assoc = ...

Answer 5

There is no such thing as associative array in Java. Its closest relative is a Map, which is strongly typed, however has less elegant syntax/API.

This is the closest you can get based on your example:

Map<Integer, Map<String, String>> arr = 
    org.apache.commons.collections.map.LazyMap.decorate(
         new HashMap(), new InstantiateFactory(HashMap.class));

//$arr[0]['name'] = 'demo';
arr.get(0).put("name", "demo");

System.out.println(arr.get(0).get("name"));
System.out.println(arr.get(1).get("name"));    //yields null

Answer 6

Well i also was in search of Associative array and found the List of maps as the best solution.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class testHashes {

public static void main(String args[]){
    Map<String,String> myMap1 = new HashMap<String, String>();

    List<Map<String , String>> myMap  = new ArrayList<Map<String,String>>();

    myMap1.put("URL", "Val0");
    myMap1.put("CRC", "Vla1");
    myMap1.put("SIZE", "Vla2");
    myMap1.put("PROGRESS", "Vla2");

    myMap.add(0,myMap1);
    myMap.add(1,myMap1);

    for (Map<String, String> map : myMap) {
        System.out.println(map.get("URL"));
    }

    //System.out.println(myMap);

}


}

Answer 7

Java equivalent of Perl's hash

HashMap<Integer, HashMap<String, String>> hash;

Answer 8

Java doesn't have associative arrays, the closest thing you can get is the Map interface

Here's a sample from that page.

import java.util.*;

public class Freq {
    public static void main(String[] args) {
        Map<String, Integer> m = new HashMap<String, Integer>();

        // Initialize frequency table from command line
        for (String a : args) {
            Integer freq = m.get(a);
            m.put(a, (freq == null) ? 1 : freq + 1);
        }

        System.out.println(m.size() + " distinct words:");
        System.out.println(m);
    }
}

If run with:

java Freq if it is to be it is up to me to delegate

You'll get:

8 distinct words:
{to=3, delegate=1, be=1, it=2, up=1, if=1, me=1, is=2}

Answer 9

Use ArrayList < Map < String, String > >

Here a code sample :

ArrayList<Map<String, String>> products = new ArrayList<Map<String, String>>();
while (iterator.hasNext()) {
         Map<String, String> product = new HashMap<String, String>();
         Element currentProduct = iterator.next();
         product.put("id",currentProduct.get("id"));
         product.put("name" , currentProduct.get("name") );
         products.add(product );
}
System.out.println("products : " + products);

Output :

products : [{id=0001, name=prod1}, {id=0002, name=prod2}]

Answer 10

Associative arrays in Java like in PHP :

SlotMap hmap = new SlotHashMap();
String key = "k01";
String value = "123456";
// Add key value
hmap.put( key, value );

// check if key exists key value
if ( hmap.containsKey(key)) {
    //.....        
}

// loop over hmap
Set mapkeys =  hmap.keySet();
for ( Iterator iterator = mapkeys.iterator(); iterator.hasNext();) {
  String key = (String) iterator.next();
  String value = hmap.get(key);
}

More info, see Class SoftHashMap : https://shiro.apache.org/static/1.2.2/apidocs/org/apache/shiro/util/SoftHashMap.html

Answer 11

Object[][] data = {
{"mykey1", "myval1"},
{"mykey2", "myval2"},
{new Date(), new Integer(1)},
};

Yes, this require iteration for searchting value by key, but if you need all of them, this will be the best choice.

Answer 12

In JDK 1.5 (http://tinyurl.com/3m2lxju) there is even a note: "NOTE: This class is obsolete. New implementations should implement the Map interface, rather than extending this class." Regards, N.

Answer 13

Thinking more about it, I would like to throw out tuples as a more general-purpose way of dealing with this problem. While tuples are not native to Java, I use Javatuples to provide me the same functionality which would exist in other languages. An example of how to deal with the question asked is

Map<Pair<Integer, String>, String> arr = new HashMap<Pair<Integer, String>, String>();
Pair p1 = new Pair(0, "name");
arr.put(p1, "demo");

I like this approach because it can be extended to triples and other higher ordered groupings with api provided classes and methods.

Answer 14

Actually Java does support associative arrays they are called dictionaries!

Answer 15

Regarding the PHP comment 'No, PHP wouldn't like it'. Actually, PHP would keep on chugging unless you set some very restrictive (for PHP) exception/error levels, (and maybe not even then).

What WILL happen by default is that an access to a non existing variable/out of bounds array element 'unsets' your value that you're assigning to. NO, that is NOT null. PHP has a Perl/C lineage, from what I understand. So there are: unset and non existing variables, values which ARE set but are NULL, Boolean False values, then everything else that standard langauges have. You have to test for those separately, OR choose the RIGHT evaluation built in function/syntax.