Java收集器

收集器是扩展Object类的最终类。它提供归约操作, 例如将元素累积到集合中, 根据各种标准对元素进行汇总等。

Java Collectors类提供了各种处理元素的方法

Java收集器示例：以列表形式获取数据

import java.util.stream.Collectors;
import java.util.List;
import java.util.ArrayList;
class Product{
	int id;
	String name;
	float price;
	
	public Product(int id, String name, float price) {
		this.id = id;
		this.name = name;
		this.price = price;
	}
}
public class CollectorsExample {
	public static void main(String[] args) {
		List<Product> productsList = new ArrayList<Product>();
		//Adding Products
		productsList.add(new Product(1, "HP Laptop", 25000f));
		productsList.add(new Product(2, "Dell Laptop", 30000f));
		productsList.add(new Product(3, "Lenevo Laptop", 28000f));
		productsList.add(new Product(4, "Sony Laptop", 28000f));
		productsList.add(new Product(5, "Apple Laptop", 90000f));
		List<Float> productPriceList = 
				productsList.stream()
							.map(x->x.price)			// fetching price
							.collect(Collectors.toList());	// collecting as list
		System.out.println(productPriceList);
	}
}

输出：

[25000.0, 30000.0, 28000.0, 28000.0, 90000.0]

Java收集器示例：将数据转换为一组

import java.util.stream.Collectors;
import java.util.Set;
import java.util.List;
import java.util.ArrayList;
classProduct{
	intid;
	String name;
	floatprice;
	
	public Product(intid, String name, floatprice) {
		this.id = id;
		this.name = name;
		this.price = price;
	}
}
publicclass CollectorsExample {
	publicstaticvoid main(String[] args) {
		List<Product>productsList = new ArrayList<Product>();
		//Adding Products
		productsList.add(newProduct(1, "HP Laptop", 25000f));
		productsList.add(newProduct(2, "Dell Laptop", 30000f));
		productsList.add(newProduct(3, "Lenevo Laptop", 28000f));
		productsList.add(newProduct(4, "Sony Laptop", 28000f));
		productsList.add(newProduct(5, "Apple Laptop", 90000f));
		Set<Float>productPriceList = 
				productsList.stream()
							.map(x->x.price)			// fetching price
							.collect(Collectors.toSet());	// collecting as list
		System.out.println(productPriceList);
	}
}

输出：

[25000.0, 30000.0, 28000.0, 90000.0]

Java收集器示例：使用sum方法

import java.util.stream.Collectors;
import java.util.List;
import java.util.ArrayList;
class Product{
	int id;
	String name;
	float price;
	
	public Product(int id, String name, float price) {
		this.id = id;
		this.name = name;
		this.price = price;
	}
}
public class CollectorsExample {
	public static void main(String[] args) {
		List<Product> productsList = new ArrayList<Product>();
		//Adding Products
		productsList.add(new Product(1, "HP Laptop", 25000f));
		productsList.add(new Product(2, "Dell Laptop", 30000f));
		productsList.add(new Product(3, "Lenevo Laptop", 28000f));
		productsList.add(new Product(4, "Sony Laptop", 28000f));
		productsList.add(new Product(5, "Apple Laptop", 90000f));
		Double sumPrices = 
				productsList.stream()
							.collect(Collectors.summingDouble(x->x.price));	// collecting as list
		System.out.println("Sum of prices: "+sumPrices);
		Integer sumId = 
				productsList.stream().collect(Collectors.summingInt(x->x.id));
		System.out.println("Sum of id's: "+sumId);
	}
}

输出：

Sum of prices: 201000.0
Sum of id's: 15

Java收集器示例：获取产品平均价格

import java.util.stream.Collectors;
import java.util.List;
import java.util.ArrayList;
class Product{
	int id;
	String name;
	float price;
	
	public Product(int id, String name, float price) {
		this.id = id;
		this.name = name;
		this.price = price;
	}
}
public class CollectorsExample {
	public static void main(String[] args) {
		List<Product> productsList = new ArrayList<Product>();
		//Adding Products
		productsList.add(new Product(1, "HP Laptop", 25000f));
		productsList.add(new Product(2, "Dell Laptop", 30000f));
		productsList.add(new Product(3, "Lenevo Laptop", 28000f));
		productsList.add(new Product(4, "Sony Laptop", 28000f));
		productsList.add(new Product(5, "Apple Laptop", 90000f));
		Double average = productsList.stream()
						 .collect(Collectors.averagingDouble(p->p.price));
		System.out.println("Average price is: "+average);
	}
}

输出：

Average price is: 40200.0

Java收集器示例：计数元素

import java.util.stream.Collectors;
import java.util.List;
import java.util.ArrayList;
class Product{
	intid;
	String name;
	floatprice;
	
	public Product(intid, String name, floatprice) {
		this.id = id;
		this.name = name;
		this.price = price;
	}
	publicint getId() {
		returnid;
	}
	public String getName() {
		returnname;
	}
	publicfloat getPrice() {
		returnprice;
	}
}
publicclass CollectorsExample {
	publicstaticvoid main(String[] args) {
		List<Product>productsList = new ArrayList<Product>();
		//Adding Products
		productsList.add(new Product(1, "HP Laptop", 25000f));
		productsList.add(new Product(2, "Dell Laptop", 30000f));
		productsList.add(new Product(3, "Lenevo Laptop", 28000f));
		productsList.add(new Product(4, "Sony Laptop", 28000f));
		productsList.add(new Product(5, "Apple Laptop", 90000f));
		Long noOfElements = productsList.stream()
							   .collect(Collectors.counting());
		System.out.println("Total elements : "+noOfElements);
	}
}

输出：

Total elements : 5