Fontes do projeto GsonExample

Cliente.java

/*
 * Copyright (C) 2017 haynes
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package gsonexample.cliente;

/**
 *
 * @author haynes
 */
public class Cliente {

    private int id;
    private String nome;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getNome() {
        return nome;
    }

    public void setNome(String nome) {
        this.nome = nome;
    }

}

dependencies.gradle

dependencies {
    ...
    
    compile 'com.google.code.gson:gson:2.8.1'
    
    ...
}

dependencies.xml

<dependency> 
    <groupId>com.google.code.gson</groupId> 
    <artifactId>gson</artifactId> 
    <version>2.8.1</version> 
</dependency>

GsonExample.java

/*
 * Copyright (C) 2017 haynes
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package gsonexample;

import gsonexample.cliente.Cliente;
import gsonexample.pedido.Pedido;
import gsonexample.pedido.itens.ItemPedido;

import com.google.gson.Gson;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.Reader;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author haynes
 */
public class GsonExample {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        /**
         * Como escrever um arquivo JSON
         */
        createJSON();
        
        System.out.println("\n\n");
        
        /**
         * Como ler arquivo json
         */
        try {
            File filePath = new File("dist/pedido.json");
            loadJSON(filePath);
        } catch (FileNotFoundException ex) {
            Logger.getLogger(GsonExample.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public static void createJSON() {
        Pedido pedido = new Pedido();

        Cliente cliente = new Cliente();
        cliente.setId(12);
        cliente.setNome("Guilherme Haynes");

        pedido.setId(1234);
        pedido.setCliente(cliente);

        for (int i = 1; i <= 3; i++) {
            ItemPedido item = new ItemPedido();
            item.setId(i);
            item.setDescricao("Item " + i);
            item.setQtd(10);
            pedido.getItens().add(item);
        }

        Gson gson = new Gson();
        String json = gson.toJson(pedido);

        System.out.println(json);
    }

    public static void loadJSON(File filePath) throws FileNotFoundException {
        Reader reader = new FileReader(filePath);

        Gson gson = new Gson();
        Pedido pedido = gson.fromJson(reader, Pedido.class);

        System.out.println("Pedido nº: " + pedido.getId());
        System.out.println("Cliente nº: " + pedido.getCliente().getId() + " | Nome: " + pedido.getCliente().getNome());
        System.out.println("Itens: ");
        for (ItemPedido item : pedido.getItens()) {
            System.out.println("----------------------------------------------------------");
            System.out.println("   Ped. nº: " + item.getId());
            System.out.println("   Item: " + item.getDescricao());
            System.out.println("   Qtd: " + item.getQtd());
        }

    }

}

ItemPedido.java

/*
 * Copyright (C) 2017 haynes
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package gsonexample.pedido.itens;

/**
 *
 * @author haynes
 */
public class ItemPedido {

    private int id;
    private String descricao;
    private int qtd;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getDescricao() {
        return descricao;
    }

    public void setDescricao(String descricao) {
        this.descricao = descricao;
    }

    public int getQtd() {
        return qtd;
    }

    public void setQtd(int qtd) {
        this.qtd = qtd;
    }

}

Pedido.java

/*
 * Copyright (C) 2017 haynes
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package gsonexample.pedido;

import gsonexample.cliente.Cliente;
import gsonexample.pedido.itens.ItemPedido;
import java.util.ArrayList;
import java.util.List;

/**
 *
 * @author haynes
 */
public class Pedido {

    private int id;
    private Cliente cliente;
    private List<ItemPedido> itens;

    public Pedido() {
        itens = new ArrayList<>();
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Cliente getCliente() {
        return cliente;
    }

    public void setCliente(Cliente cliente) {
        this.cliente = cliente;
    }

    public List<ItemPedido> getItens() {
        return itens;
    }

    public void setItens(List<ItemPedido> itens) {
        this.itens = itens;
    }

}

pedido.json

{
    "id": 1234,
    "cliente": {
        "id": 24,
        "nome": "Guilherme Haynes Howe"
    },
    "itens": [
        {
            "id": 1,
            "descricao": "Cerveja Brahma Litrão",
            "qtd": 6
        },
        {
            "id": 2,
            "descricao": "Mendorato 500g",
            "qtd": 2
        },
        {
            "id": 3,
            "descricao": "Paçoquita caixa com 24",
            "qtd": 12
        }
    ]
}