I hope you were joking about writing your own parser. :-)

For such a simple mapping, most tools from http://json.org (section java) would work. For one of them (Jackson https://github.com/FasterXML/jackson-databind/#5-minute-tutorial-streaming-parser-generator), you'd do:

Map<String,Object> result =
        new ObjectMapper().readValue(JSON_SOURCE, HashMap.class);

(where JSON_SOURCE is a File, input stream, reader, or json content String)

In recursive way:

public static Map<String, Object> jsonToMap(JSONObject json) throws JSONException {
    Map<String, Object> retMap = new HashMap<String, Object>();
    
    if(json != JSONObject.NULL) {
        retMap = toMap(json);
    }
    return retMap;
}

public static Map<String, Object> toMap(JSONObject object) throws JSONException {
    Map<String, Object> map = new HashMap<String, Object>();

    Iterator<String> keysItr = object.keys();
    while(keysItr.hasNext()) {
        String key = keysItr.next();
        Object value = object.get(key);
        
        if(value instanceof JSONArray) {
            value = toList((JSONArray) value);
        }
        
        else if(value instanceof JSONObject) {
            value = toMap((JSONObject) value);
        }
        map.put(key, value);
    }
    return map;
}

public static List<Object> toList(JSONArray array) throws JSONException {
    List<Object> list = new ArrayList<Object>();
    for(int i = 0; i < array.length(); i++) {
        Object value = array.get(i);
        if(value instanceof JSONArray) {
            value = toList((JSONArray) value);
        }

        else if(value instanceof JSONObject) {
            value = toMap((JSONObject) value);
        }
        list.add(value);
    }
    return list;
}

Using Jackson library:

import com.fasterxml.jackson.databind.ObjectMapper;

Map<String, Object> mapping = new ObjectMapper().readValue(jsonStr, HashMap.class);

Parse the JSONObject and create HashMap

public static void jsonToMap(String t) throws JSONException {

        HashMap<String, String> map = new HashMap<String, String>();
        JSONObject jObject = new JSONObject(t);
        Iterator<?> keys = jObject.keys();

        while( keys.hasNext() ){
            String key = (String)keys.next();
            String value = jObject.getString(key); 
            map.put(key, value);

        }

        System.out.println("json : "+jObject);
        System.out.println("map : "+map);
    }

Tested output:

json : {"phonetype":"N95","cat":"WP"}
map : {cat=WP, phonetype=N95}

[Update Sept 2020] Although my original answer here, from many years ago, seems to be helpful and is still getting upvotes, I now use the GSON library from Google, which I find to be more intuitive.

I've got the following code:

public void testJackson() throws IOException {  
    ObjectMapper mapper = new ObjectMapper(); 
    File from = new File("albumnList.txt"); 
    TypeReference<HashMap<String,Object>> typeRef 
            = new TypeReference<HashMap<String,Object>>() {};

    HashMap<String,Object> o = mapper.readValue(from, typeRef); 
    System.out.println("Got " + o); 
}   

It's reading from a file, but mapper.readValue() will also accept an InputStream and you can obtain an InputStream from a string by using the following:

new ByteArrayInputStream(astring.getBytes("UTF-8")); 

There's a bit more explanation about the mapper on my blog.

This is method to read Map from json string using Jackson:

public final class JsonUtils {
    public static <T> Map<String, T> readMap(String json) throws Exception {
        if (json == null)
            return null;

        ObjectReader reader = new ObjectMapper().readerFor(Map.class);
        MappingIterator<Map<String, T>> it = reader.readValues(json);

        if (it.hasNextValue()) {
            Map<String, T> res = it.next();
            return res.isEmpty() ? Collections.emptyMap() : res;
        }

        return Collections.emptyMap();
    }
}

This is how to read Map from json using given utilit method:

Map<String, String> map = flatMap(new LinkedHashMap<>(), "", JsonUtils.readMap(json));

And finally, this is how to transfrom Map into required Map (probably this could be done within Jackson engine, with provided custom deserializers or so, but I do not know exactly how, and thats why it is easier to me to implement it manually):

public static Map<String, String> flatMap(Map<String, String> res, String prefix, Map<String, Object> map) {
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        String key = prefix + entry.getKey();
        Object value = entry.getValue();

        if (value instanceof Map)
            flatMap(res, key + '.', (Map<String, Object>)value);
        else
            res.put(key, String.valueOf(value));
    }

    return res;
}