Java 8 streaming API example

StreamDemo.java

package com.madadata.demo.stream;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.common.base.Charsets;
import com.google.common.base.Throwables;
import com.google.common.io.Resources;
import io.dropwizard.jackson.Jackson;
import java.io.IOException;
import java.math.BigDecimal;
import java.net.URL;
import java.util.List;
import java.util.Map;

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.partitioningBy;
import static java.util.stream.Collectors.reducing;

/**
 * Created by jiayu on 11/21/16.
 */
public class StreamDemo {

    private static final ObjectMapper objectMapper = Jackson.newObjectMapper();
    private static final List<UserOrder> userOrders;

    static {
        URL fixtureUrl = Resources.getResource("fixture/user_orders.json");
        try {
            String fixture = Resources.toString(fixtureUrl, Charsets.UTF_8);
            userOrders = objectMapper.readValue(fixture, new TypeReference<List<UserOrder>>() {
            });
        } catch (IOException e) {
            throw Throwables.propagate(e);
        }
    }

    private static Map<Boolean, Long> getGroupedByCashOnDelivery() {
        return userOrders.stream().collect(
            partitioningBy(UserOrder::isCashOnDelivery,
                mapping(UserOrder::getTotal,
                    counting())));
    }

    private static Map<String, BigDecimal> getProvincialSum() {
        return userOrders.stream().collect(
            groupingBy(UserOrder::getProvince,
                mapping(UserOrder::getTotal,
                    reducing(BigDecimal.ZERO, BigDecimal::add))));
    }

    private static Map<String, Map<String, BigDecimal>> getProvincialAndCitySum() {
        return userOrders.stream().collect(
            groupingBy(UserOrder::getProvince,
                groupingBy(UserOrder::getCity,
                    mapping(UserOrder::getTotal,
                        reducing(BigDecimal.ZERO, BigDecimal::add)))));
    }

    public static void main(String[] args) throws Exception {
        // provincial sum {四川=426.41, 河南=102.1}
        System.out.printf("provincial sum %s\n", getProvincialSum());

        // provincial and city sum {四川={德阳=31.31, 资阳=395.1}, 河南={开封=102.1}}
        System.out.printf("provincial and city sum %s\n", getProvincialAndCitySum());

        // number of orders and whether they are COD {false=1, true=2}
        System.out.printf("number of orders and whether they are COD %s\n", getGroupedByCashOnDelivery());
    }
}

user_orders.json

[
  {
    "payTime": "2016-11-10T23:11:04",
    "province": "四川",
    "city": "资阳",
    "total": 395.1,
    "isCashOnDelivery": true
  },
  {
    "payTime": "2016-11-12T04:12:44",
    "province": "四川",
    "city": "德阳",
    "total": 31.31,
    "isCashOnDelivery": false
  },
  {
    "payTime": "2016-12-20T12:04:33",
    "province": "河南",
    "city": "开封",
    "total": 102.1,
    "isCashOnDelivery": true
  }
]

UserOrder.java

package com.madadata.demo.stream;

import com.fasterxml.jackson.annotation.JsonProperty;
import java.math.BigDecimal;
import javax.validation.constraints.NotNull;
import org.hibernate.validator.constraints.NotEmpty;
import org.joda.time.LocalDateTime;

/**
 * Created by jiayu on 11/21/16.
 */
public class UserOrder {

    @JsonProperty
    @NotEmpty
    private String province;

    @JsonProperty
    @NotEmpty
    private String city;

    @JsonProperty
    @NotNull
    private BigDecimal total;

    @JsonProperty
    private boolean isCashOnDelivery;

    @JsonProperty
    @NotNull
    private LocalDateTime payTime;

    @JsonProperty
    public String getProvince() {
        return province;
    }

    @JsonProperty
    public String getCity() {
        return city;
    }

    @JsonProperty
    public BigDecimal getTotal() {
        return total;
    }

    @JsonProperty
    public boolean isCashOnDelivery() {
        return isCashOnDelivery;
    }

    public LocalDateTime getPayTime() {
        return payTime;
    }
}