Java版LINQ工具包devtools-beans-streaming使用介绍

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devtools-beans-streaming包是devtools系列的工具包之一，它是一个对对象列表（或称为结果集）进行查询或聚合等操作的解决方案, 提供了类似于C#中的LINQ功能

Maven：

		<dependency><groupId>com.github.paganini2008</groupId><artifactId>devtools-beans-streaming</artifactId><version>2.0.2</version></dependency>

Jdk版本：

jdk 1.8+

API使用说明：

devtools-beans-streaming操作的对象通常是一个List

举例说明：

比如有这样一个对象：

@Getter
@Setter
@ToString
public class Product {private int id;private String name;private String location;private Date created;private Date expired;private Float price;private BigInteger sales;private boolean export;private Long number;private BigDecimal freight;private Style style;private Salesman salesman;public static enum Style {HARD, SOFT;}@Getter@Setter@ToStringpublic static class Salesman {private String name;private String password;public Salesman(String name, String password) {this.name = name;this.password = password;}}}

然后定义一个List, 生成一批对象，随机赋值
比如：

    // 定义一个Listprivate static final List<Product> products = new ArrayList<Product>(); static {String[] users = new String[] { "Petter", "Jack", "Tony" };String[] locations = new String[] { "London", "NewYork", "Tokyo", "HongKong", "Paris" };for (int i = 0; i < 10000; i++) {Product product = new Product();product.setCreated(DateUtils.valueOf(2020, RandomUtils.randomInt(1, 12), RandomUtils.randomInt(1, 28)));product.setExpired(DateUtils.addDays(product.getCreated(), 90));product.setExport(RandomUtils.randomBoolean());product.setFreight(BigDecimal.valueOf(RandomUtils.randomFloat(10, 100)).setScale(2, RoundingMode.HALF_UP));product.setId(i + 1);product.setLocation(RandomUtils.randomChoice(locations));product.setName("Product-" + product.getId());product.setNumber(RandomUtils.randomLong(1, 1000));product.setPrice(RandomUtils.randomFloat(10, 1000, 2));product.setSales(BigInteger.valueOf(RandomUtils.randomLong(10000, 100000)));product.setStyle(Style.values()[RandomUtils.randomInt(0, 2)]);product.setSalesman(new Product.Salesman(RandomUtils.randomChoice(users), "123456"));products.add(product);}}

示例1

		Predicate<Product> predicate = Restrictions.eq("location", "London");Selector.from(products).filter(predicate).list().forEach(product -> {System.out.println(product);});
// Equivalent to: select * from Product where location='London'

示例2

		Predicate<Product> predicate = Restrictions.lte("created", new Date());predicate = predicate.and(Restrictions.eq("salesman.name", "Petter"));Selector.from(products).filter(predicate).list().forEach(product -> {System.out.println(product);});
// select * from Product where created<= now() and salesman.name='Petter'

示例3

        Selector.from(products).groupBy("location", String.class).setTransformer(new View<Product>() {protected void setAttributes(Tuple tuple, Group<Product> group) {tuple.set("maxPrice", group.max("price", Float.class));tuple.set("minPrice", group.min("price", Float.class));tuple.set("avgFreight", group.avg("freight"));tuple.set("sumSales", group.sum("sales"));}}).list().forEach(tuple -> {System.out.println(tuple);});
// Equivalent to: select location,max(price) as maxPrice, min(price) as minPrice,avg(freight) as avgFreight,sum(sales) as sumSales from Product group by location

示例4

		Selector.from(products).groupBy("location", String.class).groupBy("style", Product.Style.class).having(group -> {return group.avg("freight").compareTo(BigDecimal.valueOf(55)) > 0;}).setTransformer(new View<Product>() {protected void setAttributes(Tuple tuple, Group<Product> group) {tuple.set("maxPrice", group.max("price", Float.class));tuple.set("minPrice", group.min("price", Float.class));tuple.set("avgFreight", group.avg("freight"));tuple.set("sumSales", group.sum("sales"));}}).list().forEach(tuple -> {System.out.println(tuple);});
// Equivalent to: select location,style,max(price) as maxPrice, min(price) as minPrice,avg(freight) as avgFreight,sum(sales) as sumSales from Product group by location,style having avg(freight) > 55

示例5:

		Sorter<Product> sorter = Orders.descending("price", BigDecimal.class);Selector.from(products).orderBy(sorter).list(100).forEach(product -> {System.out.println("Name: " + product.getName() + ", Price: " + product.getPrice() + ", Freight: " + product.getFreight());});
// Equivalent to: select name,price from Product order by price desc limit 100