package fi.tekislauta;

import static spark.Spark.*;
import fi.tekislauta.db.*;
import com.google.gson.*;
import fi.tekislauta.models.Board;
import fi.tekislauta.models.Post;

public class Main {
    public static void main(String[] args) {
        port(1234);

        Gson gson = new Gson();

        Database db = new Database();
        get("/api", (req, res) -> {
            return db.executeQuery("SELECT * FROM Board");
        });

        get("/api/boards/:id", (req,res) -> {
            return db.executeQuery("SELECT * FROM Board WHERE abbreviation='" + req.params("id") + "'");
        });

        get("/api/posts/:id", (req,res) -> {
            return "Hello world!";
        });

        get("/api/test", (req, res) -> {
            res.header("Content-Type","application/json");
            Board b = new Board(0, "Koulu", "k", "uliopisto");
            b.addThread(new Post(0, null, 12345, 123, "Joonan kissa", "Joonan kissat ovat ruotsalaisia"));


            b.addThread(new Post(1, null, 12345, 123, "Joonan kissa 2", "Joonan kissat ovat eestiläisiä"));

            return gson.toJson(b);

        });
    }
}

package fi.tekislauta;

import static spark.Spark.*;
import fi.tekislauta.db.*;
import com.google.gson.*;
import fi.tekislauta.models.Board;
import fi.tekislauta.models.Post;

public class Main {
    public static void main(String[] args) {
        port(1234);

        Gson gson = new Gson();

        Database db = new Database();
        get("/api", (req, res) -> {
            return db.executeQuery("SELECT * FROM Board");
        });

        get("/api/boards/:id", (req,res) -> {
            return db.executeQuery("SELECT * FROM Board WHERE abbreviation='" + req.params("id") + "'");
        });

        get("/api/posts/:id", (req,res) -> {
            return "Hello world!";
        });

        get("/api/test", (req, res) -> {
            res.header("Content-Type","application/json; charset=utf-8");
            Board b = new Board(0, "Koulu", "k", "uliopisto");
            b.addThread(new Post(0, null, 12345, 123, "Joonan kissa", "Joonan kissat ovat ruotsalaisia"));


            b.addThread(new Post(1, null, 12345, 123, "Joonan kissa 2", "Joonan kissat ovat eestiläisiä"));

            return gson.toJson(b);

        });
    }
}

Add charset to test response