Java get List of maps of maps from config.yml

Question

I'm trying to create a plugin where I'm storing some Minecraft items' data along with some properties.

This is my YAML file's content:

rates:
- 391:
    mul: 10000
    store: 5000
- 392:
    mul: 9000
    store: 5000

So it's basically a list of maps of maps(I think so at least). This is my JAVA code where I'm trying to access the key 'mul' of '391':

List<Map<?,?>> rates;
rates= getConfig().getMapList("rates");
for(Map<?,?> mp : rates){
    Map <?,?> test = (Map<?,?>) mp.get("" + item);
    player.sendMessage(test.toString());// HERE I get null pointer exception, and the following lines if this line wasn't there in the first place
    player.sendMessage("Mul is: " + test.get("mul"));
    player.sendMessage("Store is: " + test.get("store"));
}

As per suggested answer, here is my test code, where I still get NullPointerException:

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.util.Map;

import net.sourceforge.yamlbeans.YamlException;
import net.sourceforge.yamlbeans.YamlReader;

public class Test {

public static void main(String[] args) throws FileNotFoundException, YamlException{
    YamlReader reader = new YamlReader(new FileReader("config.yml"));
    Map map = (Map) reader.read();
    Map itemMap = (Map) map.get("391");
    System.out.println(itemMap.get("mul"));//This is where I get the exception now
    System.out.println(itemMap.get("store"));
}
}

Answer 1

Parsing yaml by hand can be tedious and error prone. It might be easier to use a library like yamlbeans.

http://yamlbeans.sourceforge.net/

package com.jbirdvegas.q41267676;

import com.esotericsoftware.yamlbeans.YamlReader;

import java.io.StringReader;
import java.util.List;
import java.util.Map;

public class YamlExample {
    public static void main(String[] args) throws Exception {

        String yamlInput =
                "rates:\n" +
                        "- 391:\n" +
                        "    mul: 10000\n" +
                        "    store: 5000\n" +
                        "- 392:\n" +
                        "    mul: 9000\n" +
                        "    store: 5000";

        YamlReader reader = new YamlReader(new StringReader(yamlInput));
        Map map = (Map) reader.read();
        // rates is a list
        List<Map> rates = (List<Map>) map.get("rates");
        // each item in the list is a map
        for (Map itemMap : rates) {
            // each map contains a single map the key is [ 391|392 ]
            itemMap.forEach((key, value) -> {
                System.out.println("Key: " + key);
                // the value in in this map is itself a map
                Map embededMap = (Map) value;
                // this map contains the values you want
                System.out.println(embededMap.get("mul"));
                System.out.println(embededMap.get("store"));
            });
        }
    }
}

This prints:

Key: 391
10000
5000
Key: 392
9000
5000

This is a simple usecase but yamlbeans also provides GSON like reflective class model population if that would be better suited for your needs.

Answer 2

Your assumptions that that YAML is a list of maps is wrong. At the top level is is a map with a single key-value pair, the key of which is rates and the value is a sequence with two elements.

Each of those elements is mapping with a single key value pair, the key of which is a number (391, 392) and the value of which is a mapping with two key-value pairs each.

That there is a dash (-) at the left hand doesn't imply the that at the top level there is a sequence (there are no lists in a YAML file, that is construct in your programming language). If a sequence is a value for a specific key, those sequence elements can be at the same indentation level as the key, as in your YAML file.