关于Java8:StreamAPI的一点记录

关于 Stream ,Functional Interface 的一点记录

stream对于集合操作的便捷度提升:

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ActiveMac {
    
    public static void main(String[] args) {
        List<Human> humans = new ArrayList<>();
        humans.add(new Human("Daniel", 23, "Google"));
        humans.add(new Human("Max", 33, "Microsoft"));
        humans.add(new Human("Jenny", 18, "Google"));
        humans.add(new Human("Alex", 28, "Facebook"));
        humans.add(new Human("Charles", 34, "Twitter"));
        humans.add(new Human("Roy", 31, "Microsoft"));
        
        // 对集合内符合条件的计数
        long nums = humans.stream().filter(human -> (human.getAge() > 20)).count();
        System.out.println("nums:" + nums);
        
        // 对集合内符合条件的筛选输出
        List<Human> nameContE = humans.stream().filter(human -> human.getName().contains("e")).collect(Collectors.toList());
        System.out.println("nameContainsE:" + nameContE.toString());
        
        // 对集合内元素中元素进行操作
        List<Integer> doubleAge = humans.stream().map(human -> human.getAge() * 2).collect(Collectors.toList());
        System.out.println("doubleAge:" + doubleAge.toString());
        
        // 对集合内元素分组
        Map<String, List<Human>> group = humans.stream().collect(Collectors.groupingBy(Human::getCompany));
        System.out.println(group.toString());
    }
}

class Human {
    private String name;
    private Integer age;
    private String company;
    
    public Human(String name, Integer age, String company) {
        super();
        this.name = name;
        this.age = age;
        this.company = company;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    public Integer getAge() {
        return age;
    }
    
    public void setAge(Integer age) {
        this.age = age;
    }
    
    public String getCompany() {
        return company;
    }
    
    public void setCompany(String company) {
        this.company = company;
    }
    
    @Override
    public String toString() {
        return name + "-" + age + "-" + company;
    }
}

新旧方法的对比:

        // 1.对集合内符合条件的计数
        long nums = humans.stream().filter(human -> (human.getAge() > 20)).count();
        System.out.println(nums);
        
        // 2.对集合内符合条件的筛选输出
        List<Human> nameContE = humans.stream().filter(human -> human.getName().contains("e")).collect(Collectors.toList());
        System.out.println(nameContE.toString());
        
        // 3.对集合内元素中元素进行操作
        List<Integer> doubleAge = humans.stream().map(human -> human.getAge() * 2).collect(Collectors.toList());
        System.out.println(doubleAge.toString());
        
        // 4.对集合内元素分组
        Map<String, List<Human>> group = humans.stream().collect(Collectors.groupingBy(Human::getCompany));
        System.out.println(group.toString());
        
        // 1
        int num = 0;
        for (Human h : humans) {
            if (h.getAge() > 20) {
                num++;
            }
        }
        System.out.println(num);
        
        
        
        
        
        //旧方法-循环
        // 2
        List<Human> eResult = new ArrayList<>();
        for (Human h : humans) {
            if (h.getName().contains("e")) {
                eResult.add(h);
            }
        }
        System.out.println(eResult.toString());
        
        // 3
        List<Integer> dounleA = new ArrayList<>();
        for (Human h : humans) {
            Integer newAge = h.getAge() * 2;
            dounleA.add(newAge);
        }
        System.out.println(dounleA.toString());
        
        // 4
        Map<String, List<Human>> maps = new HashMap<>();
        for (Human h : humans) {
            List<Human> hs = new ArrayList<>();
            String key = h.getCompany();
            if (maps.containsKey(key)) {
                hs = maps.get(key);
            }
            hs.add(h);
            maps.put(key, hs);
        }
        System.out.println(maps.toString());
        
    }

输出结果方面并不存在差异

新方法-stream
5
[Daniel-23-Google, Jenny-18-Google, Alex-28-Facebook, Charles-34-Twitter]
[46, 66, 36, 56, 68, 62]
{Google=[Daniel-23-Google, Jenny-18-Google], Twitter=[Charles-34-Twitter], Microsoft=[Max-33-Microsoft, Roy-31-Microsoft], Facebook=[Alex-28-Facebook]}


旧方法-循环
5
[Daniel-23-Google, Jenny-18-Google, Alex-28-Facebook, Charles-34-Twitter]
[46, 66, 36, 56, 68, 62]
{Google=[Daniel-23-Google, Jenny-18-Google], Twitter=[Charles-34-Twitter], Microsoft=[Max-33-Microsoft, Roy-31-Microsoft], Facebook=[Alex-28-Facebook]}

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