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The Dropbox API for Java is a Dropbox supported client library for accessing the JSON+REST interface to Dropbox. It supports OAuth proxied authentication. It is designed to be simple and easy to use, as well as instructional in case you want to write your own. It is MIT licensed.

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package com.dropbox.client2.jsonextract;

import java.util.Iterator;
import java.util.Map;

/**
 * (Internal class for extracting JSON.)
 *
 * A JSON "object" (a mapping of string keys to arbitrary JSON values).
 */
public final class JsonMap extends JsonBase> implements Iterable> {

    public JsonMap(Map internal, String path) {
        super(internal, path);
    }

    public JsonMap(Map internal) {
        super(internal);
    }

    private static boolean isIdentLike(String s) {
        if (s.length() == 0) return false;
        if (!isEnglishLetter(s.charAt(0))) return false;
        for (int i = 1; i < s.length(); i++) {
            char c = s.charAt(i);
            if (!isEnglishLetter(c) && !isEnglishDigit(c)) return false;
        }
        return true;
    }

    private static boolean isEnglishLetter(char c) {
        return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
    }

    private static boolean isEnglishDigit(char c) {
        return c >= '0' && c <= '9';
    }

    private static String pathConcatField(String path, String fieldName) {
        String suffix = fieldName;
        if (!isIdentLike(fieldName)) {
            suffix = '"' + fieldName + '"'; // TODO: Proper JSON quoting.
        }
        return JsonThing.pathConcat(path, suffix);
    }

    public JsonThing get(String fieldName) throws JsonExtractionException {
        if (!internal.containsKey(fieldName)) {
            throw error("expecting object to have field \"" + fieldName + "\", but it does not");
        }
        return new JsonThing(internal.get(fieldName), pathConcatField(path, fieldName));
    }

    public JsonThing getOrNull(String fieldName) {
        if (!internal.containsKey(fieldName)) {
            return null;
        }
        return new JsonThing(internal.get(fieldName), pathConcatField(path, fieldName));
    }

    /**
     * A key+value iterator that automatically wraps every value in a JsonThing.
     */
    private static final class WrapperIterator implements Iterator> {
        private final String path;
        private final Iterator> internal;

        private WrapperIterator(String path, Iterator> internal) {
            this.path = path;
            this.internal = internal;
        }

        public boolean hasNext() { return internal.hasNext(); }
        public Map.Entry next() {
            return new WrappedEntry(path, internal.next());
        }
        public void remove() { throw new UnsupportedOperationException("can't remove"); }
    }

    private static final class WrappedEntry implements Map.Entry {
        private final String key;
        private final JsonThing value;

        private WrappedEntry(String path, Map.Entry original) {
            this.key = original.getKey();
            this.value = new JsonThing(original.getValue(), pathConcatField(path, key));
        }

        public String getKey() { return key; }
        public JsonThing getValue() { return value; }
        public JsonThing setValue(JsonThing jsonThing) { throw new UnsupportedOperationException(); }
    }

    public Iterator> iterator() {
        return new WrapperIterator(path, internal.entrySet().iterator());
    }
}