import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import com.tumblr.jumblr.JumblrClient;
import com.tumblr.jumblr.types.Blog;
import com.tumblr.jumblr.types.PhotoPost;
import com.tumblr.jumblr.types.Post;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

/**
 * Example usage of Jumblr
 * @author jc
 */
public class App {

    public static void main(String[] args) throws FileNotFoundException, IOException, InstantiationException, IllegalAccessException {

        // Read in the JSON data for the credentials
        FileReader fr = new FileReader("credentials.json");
        BufferedReader br = new BufferedReader(fr);
        String json = "";
        while (br.ready()) { json += br.readLine(); }

        // Parse the credentials
        JsonParser parser = new JsonParser();
        JsonObject obj = (JsonObject) parser.parse(json);

        // Create a client
        JumblrClient client = new JumblrClient(
            obj.getAsJsonPrimitive("consumer_key").getAsString(),
            obj.getAsJsonPrimitive("consumer_secret").getAsString()
        );

        // Give it a token
        client.setToken(
            obj.getAsJsonPrimitive("oauth_token").getAsString(),
            obj.getAsJsonPrimitive("oauth_token_secret").getAsString()
        );

        // And make some calls - 2 posts for each blog
        for (Blog blog : client.user().getBlogs()) {
            System.out.println(blog.getTitle());

            Map<String, Integer> map = new HashMap<String, Integer>();
            map.put("limit", 2);
            for (Post post : blog.posts(map)) {
                System.out.println(post.toString());
            }
        }

        // @TODO play more with usage to ensure usability

        PhotoPost post = (PhotoPost) client.newPost("apeyes.tumblr.com", PhotoPost.class);
        post.setSource("http://www.petfinder.com/wp-content/uploads/2012/11/99059361-choose-cat-litter-632x475.jpg");
        post.save();

    }

}

import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import com.tumblr.jumblr.JumblrClient;
import com.tumblr.jumblr.types.Blog;
import com.tumblr.jumblr.types.Post;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

/**
 * Example usage of Jumblr
 * @author jc
 */
public class App {

    public static void main(String[] args) throws FileNotFoundException, IOException, InstantiationException, IllegalAccessException {

        // Read in the JSON data for the credentials
        FileReader fr = new FileReader("credentials.json");
        BufferedReader br = new BufferedReader(fr);
        String json = "";
        while (br.ready()) { json += br.readLine(); }

        // Parse the credentials
        JsonParser parser = new JsonParser();
        JsonObject obj = (JsonObject) parser.parse(json);

        // Create a client
        JumblrClient client = new JumblrClient(
            obj.getAsJsonPrimitive("consumer_key").getAsString(),
            obj.getAsJsonPrimitive("consumer_secret").getAsString()
        );

        // Give it a token
        client.setToken(
            obj.getAsJsonPrimitive("oauth_token").getAsString(),
            obj.getAsJsonPrimitive("oauth_token_secret").getAsString()
        );

        // Get a blog by name
        Blog blog = client.blogInfo("seejohnrun");

        // Get the most recent post
        Post post = blog.posts().get(0);
        System.out.println(post.getId());

        // Then re-get it by id
        Post postById = blog.getPost(post.getId());
        System.out.println(postById.getId());

    }

}

Update example