public class Item {

    private Long id;
    private String name;
    private Double price;

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Double getPrice() {
        return price;
    }

    public void setPrice(Double price) {
        this.price = price;
    }
}

package com.javaquery.google.firebase;

import com.google.firebase.FirebaseApp;
import com.google.firebase.FirebaseOptions;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.javaquery.bean.Item;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.concurrent.CountDownLatch;

/**
 * Example of Firebase save.
 *
 * @author javaQuery
 * @date 7th September, 2016
 * @Github: https://github.com/javaquery/Examples
 */
public class FirebaseSaveObject {

    public static void main(String[] args) {
        Item item = new Item();
        item.setId(1L);
        item.setName("MotoG");
        item.setPrice(100.12);

        // save item objec to firebase.
        new FirebaseSaveObject().save(item);
    }

    private FirebaseDatabase firebaseDatabase;

    /**
     * initialize firebase.
     */
    private void initFirebase() {
        try {
            // .setDatabaseUrl("https://fir-66f50.firebaseio.com") - Firebase project url.

            // Firebase private key(Generated while creating service account) file path.
            // .setServiceAccount(new FileInputStream(new File("filepath")))
            FirebaseOptions firebaseOptions = new FirebaseOptions.Builder()
                    .setDatabaseUrl("https://fir-66f50.firebaseio.com")
                    .setServiceAccount(new FileInputStream(new File("filepath\30f95674f4d5.json")))
                    .build();

            FirebaseApp.initializeApp(firebaseOptions);
            firebaseDatabase = FirebaseDatabase.getInstance();
        } catch (FileNotFoundException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * Save item object in Firebase.
     * @param item 
     */
    private void save(Item item) {
        if (item != null) {
            initFirebase();
            
            /* Get database root reference */
            DatabaseReference databaseReference = firebaseDatabase.getReference("/");
            
            /* Get existing child or will be created new child. */
            DatabaseReference childReference = databaseReference.child("item");

            /**
             * The Firebase Java client uses daemon threads, meaning it will not prevent a process from exiting.
             * So we'll wait(countDownLatch.await()) until firebase saves record. Then decrement `countDownLatch` value
             * using `countDownLatch.countDown()` and application will continues its execution.
             */
            CountDownLatch countDownLatch = new CountDownLatch(1);
            childReference.setValue(item, new DatabaseReference.CompletionListener() {

                @Override
                public void onComplete(DatabaseError de, DatabaseReference dr) {
                    System.out.println("Record saved!");
                    // decrement countDownLatch value and application will be continues its execution.
                    countDownLatch.countDown();
                }
            });
            try {
                //wait for firebase to saves record.
                countDownLatch.await();
            } catch (InterruptedException ex) {
                ex.printStackTrace();
            }
        }
    }
}

Record saved!