java8 之 stream流

package com.mozi.stream;

public class User {
	  private String name;
	  private int age;
	 
	  public User() {
	  }
	 
	  public User(String name, int age) {
	    this.name = name;
	    this.age = age;
	  }
	 
	  public int getAge() {
	    return age;
	  }
	 
	  public void setAge(int age) {
	    this.age = age;
	  }
	 
	  public String getName() {
	    return name;
	  }
	 
	  public void setName(String name) {
	    this.name = name;
	  }
	 
	  @Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		User other = (User) obj;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	@Override
	  public String toString() {
	    return "User{" +
	        "name='" + name + '\'' +
	        ", age=" + age +
	        '}';
	  }
}

           

```java
package com.mozi.stream;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.junit.Before;
import org.junit.Test;

public class StreamTest {

	private List<User> userList;

	@Before
	public void streamNew() {
		userList = new ArrayList<User>();
		User user1 = new User("小花", 13);
		User user2 = new User("大花", 16);
		User user3 = new User("小米", 23);
		User user4 = new User("大米", 24);
		User user5 = new User("張三", 18);
		User user6 = new User("李四", 15);
		User user7 = new User("王五", 22);
		User user8 = new User("李珊珊", 29);
		User user9 = new User("朝夕夕", 24);
		userList.add(user1);
		userList.add(user2);
		userList.add(user3);
		userList.add(user4);
		userList.add(user5);
		userList.add(user6);
		userList.add(user7);
		userList.add(user8);
		userList.add(user9);
	}
	
	/**
	 * 12、使用collect()做字元串joining()
	 */
	@Test
	public void streamCollect() {
	    Stream<String> stream = Stream.of("張三","李四","王五","趙六");
	 // 張三李四王五趙六
	    //String string = stream.collect(Collectors.joining());
	    //System.out.println(string);
	 // 張三-李四-王五-趙六
	    String string2 = stream.collect(Collectors.joining("-"," "," "));
	    System.out.println(string2);
	}
	
	/**
	 * 11、reduce() ：多面手
	 */
	@Test
	public void streamReduce() {
		//userList.stream().max(Comparator.comparing(keyExtractor, keyComparator))
		Integer reduce = userList.stream().map(user -> user.getAge())
		.reduce(10,(x,y) -> x+y);
		System.out.println(reduce);
	}
	
	/**
	 * findFirst() ：使用該方法擷取第一個元素
	 */
	@Test
	public void streamFindFirst() {
		Optional<User> findFirst = userList.stream().findFirst();
		User user = findFirst.get();
		System.out.println(user);
		
		//Stream<String> map = userList.stream().map(user -> user.getName());
		
	}
	
	// flatMap與map的差別在于 flatMap是将一個流中的每個值都轉成一個個流，然後再将這些流扁平化成為一個流 。
	/**
	 * flatMap()：對每個元素執行mapper指定的操作，
	 * 并用所有mapper傳回的Stream中的元素組成一個新的Stream作為最終傳回結果，
	 * 通俗易懂就是将原來的stream中的所有元素都展開組成一個新的stream
	 */
	@Test
	public void streamFlatMap() {
		List<String> list1 = Arrays.asList("aa","bb","cc");
		List<String> list2 = Arrays.asList("dd","ee");
		Stream<List<String>> stream = Stream.of(list1,list2);
		//stream.flatMap()
		
	}
	
	/**
	 * 8、map()：接收一個方法作為參數，
	 * 該函數會被應用到每個元素上，并将其映射成一個新的元素
	 */
	@Test
	public void streamMap() {
//		userList.stream().map(User::getAge)
//		.forEach(System.out::println);
		
	}
	/**
	 * max，min，sum，avg，count 求平均值
	 */
	@Test
	public void StreamArith() {
		IntSummaryStatistics num 
			= userList.stream().
			mapToInt(user -> user.getAge())
			.summaryStatistics();
	    System.out.println("總共人數：" + num.getCount());
	    System.out.println("平均年齡：" + num.getAverage());
	    System.out.println("最大年齡：" + num.getMax());
	    System.out.println("最小年齡：" + num.getMin());
	    System.out.println("年齡之和：" + num.getSum());
	}
	
	/**
	 * demo: 去重+按照年齡大于20以後從小到大+隻取前二
	 */
	@Test
	public void streamDemo() {
		
		List<User> list = userList.stream().distinct()
		.filter(user -> user.getAge() > 20)
		.sorted(Comparator.comparingInt(User::getAge))
		.limit(2L).collect(Collectors.toList());
		list.stream().forEach(System.out::println);
		
	}

	/**
	 * distinct()： 使用該方法去重，注意：必須重寫對應泛型的hashCode()和equals()方法
	 */
	@Test
	public void streamDistinct() {
		List<User> list = userList.stream().distinct().collect(Collectors.toList());
		list.stream().forEach(System.out::println);
	}

	/**
	 * skip()：與limit互斥，使用該方法跳過元素
	 */
	@Test
	public void streamSkip() {
		userList.stream().skip(2L).forEach(System.out::println);
	}

	/**
	 * limit()：使用該方法截斷(隻保留前幾個資料)
	 */
	@Test
	public void streamLimit() {
		userList.stream().limit(2).forEach(System.out::println);
	}

	@Test
	public void StreamFilter() {
		// 輸出年齡大于20的人
		List<User> list = userList.stream().filter(user -> user.getAge() > 20).collect(Collectors.toList());
		list.forEach(user -> System.out.println(user));
	}

	@Test
	public void streamForEach() {
		 userList.forEach(user -> System.out.println(user));
		 userList.stream().forEach(System.out::print);
	}

	@Test
	public void streamSorted() {
		userList.stream().sorted(Comparator.comparingInt(User::getAge)).forEach(System.out::print);
	}

}