My own MercadoPago API implementation for Java, because the official one was using very old dependencies.

SunnyMercadoPago.java

package com.mrpowergamerbr.sparklysunnyfunny.utils;

import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.time.ZonedDateTime;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import org.apache.commons.lang3.StringUtils;

import com.google.gson.Gson;

/**
 * SunnyMercadoPago - Minha própria versão do SDK do MercadoPago, drag and drop.
 * 
 * Prefira usar o Getters em vez de usar as variáveis públicas. Eu só deixo elas públicas por
 * conveniência mesmo
 * 
 * @author MrPowerGamerBR (Leonardo)
 */
public class SunnyMercadoPago {
    public static final Gson gson = new Gson();
    public String ACCESS_TOKEN;
    public String client_id;
    public String client_token;
    public static final String END_POINT = "https://api.mercadopago.com";
    // Argentina: MLA; Brasil: MLB; Mexico: MLM; Venezuela: MLV; Colombia: MCO
    public static final String ARGENTINA = "MLA";
    public static final String BRASIL = "MLB";
    public static final String MEXICO = "MLM";
    public static final String VENEZUELA = "MLV";
    public static final String COLOMBIA = "MCO";

    public SunnyMercadoPago(String client_id, String client_token) {
        this.client_id = client_id;
        this.client_token = client_token;
    }

    public static void main(String[] args) {
        SunnyMercadoPago mp = new SunnyMercadoPago(MercadoTokens.client_id, MercadoTokens.client_token);
    }    

    public String getAccessToken() {
        String response = HttpRequest.post(END_POINT + "/oauth/token")
                .part("grant_type", "client_credentials")
                .part("client_id", this.client_id)
                .part("client_secret", this.client_token)
                .body();

        AccessTokenResponse webResponse = gson.fromJson(response, AccessTokenResponse.class);

        this.ACCESS_TOKEN = webResponse.getAccessToken();

        return ACCESS_TOKEN;
    }

    public PaymentResponse createPayment(String title, int qty, String currencyId, double price) {   

        PaymentItem payment = new PaymentItem(title, qty, currencyId, price);
        PaymentRequest paymentReq = new PaymentRequest();
        paymentReq.items.add(payment);

        String response = HttpRequest.post(END_POINT + "/checkout/preferences?access_token=" + getAccessToken())
                .acceptJson()
                .contentType("application/json")
                .send(gson.toJson(paymentReq))
                .body();

        PaymentResponse webResponse = gson.fromJson(response, PaymentResponse.class);
        return webResponse;
    }

    public PaymentSearchResponse searchPayments() {
        return searchPayments(new HashMap<String, Object>());
    }

    public PaymentSearchResponse searchPayments(String country) {
        return searchPayments(new HashMap<String, Object>(), country);
    }

    public PaymentSearchResponse searchPayments(Map<String, Object> filters, String country) {
        return searchPayments(filters, country, 0, 0);
    }

    public PaymentSearchResponse searchPayments(Map<String, Object> filters) {
        return searchPayments(filters, null);
    }

    public PaymentSearchResponse searchPayments(Map<String, Object> filters, String country, long offset, long limit) {
        if (country != null) {
            filters.put("site_id", country); // Argentina: MLA; Brasil: MLB; Mexico: MLM; Venezuela: MLV; Colombia: MCO
        }

        filters.put("offset", offset);
        filters.put("limit", limit);

        String response = HttpRequest.get(END_POINT + "/collections/search?" + buildQuery(filters) + "&access_token=" + getAccessToken())
                .acceptJson()
                .contentType("application/json")
                .body();

        return gson.fromJson(response, PaymentSearchResponse.class);
    }

    /**
     * O MercadoPago só permite procurar até 1000 pagamentos por vez, esse método automatiza o processo e procura todos os pagamentos
     */
    public ArrayList<PaymentSearchResponse> searchAllPayments() {
        return searchAllPayments(new HashMap<String, Object>(), null);
    }

    /**
     * O MercadoPago só permite procurar até 1000 pagamentos por vez, esse método automatiza o processo e procura todos os pagamentos
     */
    public ArrayList<PaymentSearchResponse> searchAllPayments(Map<String, Object> filters) {
        return searchAllPayments(filters, null);
    }

    /**
     * O MercadoPago só permite procurar até 1000 pagamentos por vez, esse método automatiza o processo e procura todos os pagamentos
     */
    public ArrayList<PaymentSearchResponse> searchAllPayments(Map<String, Object> filters, String country) {
        return searchAllPayments(filters, country, 0);
    }

    private ArrayList<PaymentSearchResponse> searchAllPayments(Map<String, Object> filters, String country, int offset) {
        ArrayList<PaymentSearchResponse> paymentList = new ArrayList<PaymentSearchResponse>();
        PaymentSearchResponse psr = searchPayments(filters, country, 0, 1000);
        paymentList.add(psr);
        if (psr.paging.total >= offset) {
            offset = offset + 1000;
            psr = searchPayments(filters, country, offset, 1000);
            paymentList.add(psr);
        }
        return paymentList;
    }

    private String buildQuery(Map<String, Object> params) {
        String[] query = new String[params.size()];
        int index = 0;
        for (String key : params.keySet()) {
            String val = String.valueOf(params.get(key) != null ? params.get(key) : "");
            try {
                val = URLEncoder.encode(val, "UTF-8");
            } catch (UnsupportedEncodingException e) {
            }
            query[index++] = key+"="+val;
        }

        return StringUtils.join(query, "&");
    }

    public static class AccessTokenResponse {
        public String access_token;
        public String refresh_token;
        public boolean live_mode;
        public int user_id;
        public String token_type;
        public int expires_in;
        public String scope;

        public String getAccessToken() {
            return access_token;
        }
    }

    public static class PaymentSearchResponse {
        public PaymentPaging paging;
        public ArrayList<PaymentSearchWrapper> results;
    }

    public static class PaymentSearchWrapper {
        public GeneratedPaymentItem collection;
    }

    public static class PaymentPaging {
        public int total;
        public int limit;
        public int offset;
    }

    public static class PaymentRequest {
        public ArrayList<PaymentItem> items = new ArrayList<PaymentItem>();
    }

    public static class PaymentItem {
        public String id;
        public String title;
        public String description;
        public String category_id;
        public String picture_url;
        public String currency_id;
        public int quantity;
        public double unit_price;

        public PaymentItem(String title, int quantity, String currency_id, double unit_price) {
            this.title = title;
            this.quantity = quantity;
            this.currency_id = currency_id;
            this.unit_price = unit_price;
        }
    }

    public static class GeneratedPaymentItem {
        public String id;
        public String site_id;
        public String date_created;
        public String date_approved;
        public String last_modified;
        public String money_release_date;
        public String operation_type;
        public int collector_id;
        public String sponsor_id;
        public GeneratedPayer payer; // MercadoPago adora criar negócios diferentes
        public String external_reference;
        public int merchant_order_id;
        public String reason;
        public String currency_id;
        public double transaction_amount;
        public double total_paid_amount;
        public double shipping_cost;
        public double account_money_amount;
        public double mercadopago_fee;
        public double net_received_amount;
        public double marketplace_fee;
        public int coupon_id;
        public int coupon_amount;
        public double coupon_fee;
        public double finance_fee;
        public String status;
        public String status_detail;
        public String status_code;
        public String released;
        public String payment_type;
        public String installments;
        public double installment_amount;
        public String deferred_period;
        public Cardholder cardholder;
        public String statement_descriptor;
        public String last_four_digits;
        public String payment_method_id;
        public String marketplace;
        public ArrayList<String> tags;
        public ArrayList<Refund> refunds;
        public double amount_refunded;
        public String notification_url;

        public String getPaymentTitle() {
            return reason;
        }

        public PaymentStatus getPaymentStatus() {
            return PaymentStatus.fromString(status);
        }

        public ZonedDateTime getDateApproved() {
            if (date_approved == null) {
                return null;
            }
            return ZonedDateTime.parse(date_approved);
        }

        public ZonedDateTime getDateCreated() {
            if (date_created == null) {
                return null;
            }
            return ZonedDateTime.parse(date_created);
        }
    }

    public static class Refund {
        public double amount;
        public int id;
        public Source source;
        public String date_created;
        public long collection_id;
        public long movement_id;
    }

    public static class Source {
        public String name;
        public String type;
        public int id;
    }

    public static class Cardholder {
        public String name;
        public Identification identification;
    }

    public static class GeneratedPayer {
        public String nickname;
        public String first_name;
        public String last_name;
        public Phone phone;
        public String email;
        public int id;
        public Identification identification;

        public String getEmail() {
            return email;
        }
    }

    public static class PaymentResponseRoot {
        public int status;
        public PaymentResponse response;
    }

    public static class PaymentResponse {
        public int collector_id;
        public String operation_type;
        public ArrayList<PaymentItem> paymentItems;
        public BackUrls back_urls;
        public String auto_return;
        public PaymentMethods payment_methods;
        public int client_id;
        public String marketplace;
        public String marketplace_fee;
        public Shipments shipments;
        public String notification_url;
        public String external_reference;
        public String additional_info;
        public boolean expires;
        public String expiration_date_from;
        public String expiration_date_to;
        public String date_created;
        public String id;
        public String init_point;
        public String sandbox_init_point;

        public String getPaymentLink(boolean isSandbox) {
            return (isSandbox ? sandbox_init_point : init_point);
        }
    }

    public static class Payer {
        public String name;
        public String surname;
        public String email;
        public String date_created;
        public Phone phone;
        public Identification identification;
        public Address address;
    }

    public static class Phone {
        public String area_code;
        public String number;
        public String extension;
    }

    public static class Identification {
        public String type;
        public String number;
    }

    public static class Address {
        public  String street_name;
        public String street_number;
        public  String zip_code;
    }

    public static class BackUrls {
        public String success;
        public String pending;
        public  String failure;
    }

    public static class PaymentMethods {
        public ArrayList<PaymentMethod> excluded_payment_methods;
        public ArrayList<PaymentType> excluded_payment_types;
        public String installments;
        public String default_payment_method_id;
        public String default_installments;
    }

    public static class Shipments {
        public ShipmentAddress receiver_address;
    }

    public static class ShipmentAddress {
        public String zip_code;
        public String street_number;
        public String street_name;
        public String floor;
        public String apartment;
    }

    public static class PaymentMethod {
        String id;
    }

    public static class PaymentType {
        String id;
    }

    public static enum PaymentStatus {
        /**
         * O usuário ainda não completou o processo de pagamento.
         */
        PENDING,
        /**
         * O pagamento foi aprovado e acreditado.
         */
        APPROVED,
        /**
         * O pagamento foi cancelado por uma das parte ou porque o tempo expirou.
         */
        CANCELLED,
        /**
         * Foi feito um chargeback no cartão do comprador.
         */
        CHARGED_BACK,
        /**
         * O pagamento foi rejeitado. O usuário pode tentar novamente.
         */
        REJECTED,
        /**
         * O pagamento foi autorizado, mas ainda não capturado.
         */
        AUTHORIZED,
        /**
         * O pagamento está em revisão.
         */
        IN_PROCESS,
        /**
         * O usuário iniciou uma disputa.
         */
        IN_MEDIATION,
        UNKNOWN;

        public static PaymentStatus fromString(String status) {
            if (status.equalsIgnoreCase("approved")) {
                return PaymentStatus.APPROVED;
            }
            if (status.equalsIgnoreCase("cancelled")) {
                return PaymentStatus.CANCELLED;
            }
            if (status.equalsIgnoreCase("pending")) {
                return PaymentStatus.PENDING;
            }
            if (status.equalsIgnoreCase("charged_back")) {
                return PaymentStatus.CHARGED_BACK;
            }
            if (status.equalsIgnoreCase("rejected")) {
                return PaymentStatus.REJECTED;
            }
            if (status.equalsIgnoreCase("authorized")) {
                return PaymentStatus.AUTHORIZED;
            }
            if (status.equalsIgnoreCase("in_process")) {
                return PaymentStatus.IN_PROCESS;
            }
            if (status.equalsIgnoreCase("in_mediation")) {
                return PaymentStatus.IN_MEDIATION;
            }
            return PaymentStatus.UNKNOWN;
        }
    }
}