Java8 Collectors类

1 Java8 Collectors类的介绍

Collectors类是扩展Object类的final类。它提供归总操作，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。

2 Java8 Collectors类的方法

3 Java8 Collectors类案例：列表形式获取数据

/**
 * 小牛知识库网： https://www.xnip.cn
 */
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);  
    }  
}

输出结果为：

[25000.0, 30000.0, 28000.0, 28000.0, 90000.0]

4 Java8 Collectors类案例：将数据转换为一组

/**
 * 小牛知识库网： https://www.xnip.cn
 */
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);  
    }  
}  

输出结果为：

[25000.0, 30000.0, 28000.0, 90000.0]

5 Java8 Collectors类案例：使用sum方法

/**
 * 小牛知识库网： https://www.xnip.cn
 */
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);  
    }  
}  

输出结果为：

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

6 Java8 Collectors类案例：求产品平均价格

/**
 * 小牛知识库网： https://www.xnip.cn
 */
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);  
    }  
}  

输出结果为：

Average price is: 40200.0

7 Java8 Collectors类案例：计数元素

/**
 * 小牛知识库网： https://www.xnip.cn
 */
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);  
    }  
}  

输出结果为：

Total elements : 5