java8新特性Stream之筛选与切片

在文章开始之前，我们创建一个Student类，方便下面操作使用：

public class Student {	
	
	 private String name;
	 private int age;
	 
	public Student(){}
	
	public Student(String name, int age)
	{
		super();
		this.name = name;
		this.age = age;
	}
	
	public String getName()
	{
		return name;
	}

	public void setName(String name)
	{
		this.name = name;
	}
	public int getAge()
	{
		return age;
	}

	public void setAge(int age)
	{
		this.age = age;
	}
	
	@Override
	public String toString()
	{
		return "Student [name=" + name + ", age=" + age + "]";
	}
}

filter

先看一下 Stream< T > filter(Predicate predicate);的源码：

/**
 * Returns a stream consisting of the elements of this stream that match
 * the given predicate.
 *
 * 
This is an intermediate
 * operation.
 *
 * @param predicate a non-interfering,
 *                  stateless
 *                  predicate to apply to each element to determine if it
 *                  should be included
 * @return the new stream
 */
Stream filter(Predicate predicate);

其实就是接受lambda，从流中排除某些元素。

比如筛选出年龄大于20岁的学生：

import java.util.Arrays;
import java.util.List;
import java.util.stream.Stream;

class Test{
	public static void main(String []args) {
		 List<Student> stuList = Arrays.asList(new Student("xiaoming",18),
				 							   new Student("xiaoli",22),
				 							   new Student("xiaozhang",19),
				 							   new Student("Tom",26),
				 							   new Student("Kevin",20),
				 							   new Student("Lucy",26)
				 							   );
		 
		 Stream<Student> stream1 = stuList.stream().filter((stu) -> stu.getAge()>20);
		 stream1.forEach(System.out::println);
	}
}

Output：

Student [name=xiaoli, age=22]
Student [name=Tom, age=26]
Student [name=Lucy, age=26]

limit

截断流，使元素不超过limit中传入的数

比如筛选出年龄大于20岁，但不超过2个人数

Stream<Student> stream1 = stuList.stream().filter((stu) -> stu.getAge()>20).limit(2);
stream1.forEach(System.out::println);

output:

Student [name=xiaoli, age=22]
Student [name=Tom, age=26]

找到两个符合条件的就不继续找，有利于效率提高。

skip

跳过元素，返回一个抛弃掉了前n个元素的流。若流中元素不足n个，则返回一个空流

Stream stream1 = stuList.stream().filter((stu) -> stu.getAge()>20).skip(1);
stream1.forEach(System.out::println);

distinct

去除重复元素，前提是重写hashcode和equals方法

我们新加入了三个重复的元素new Student("Lucy",26)，并且在Student类重写了hashcode和equals方法。

class Test{
	public static void main(String []args) {
		 List<Student> stuList = Arrays.asList(new Student("xiaoming",18),
				 							   new Student("xiaoli",22),
				 							   new Student("xiaozhang",19),
				 							   new Student("Tom",26),
				 							   new Student("Kevin",20),
				 							   new Student("Lucy",26),
				 							   new Student("Lucy",26),
				 							   new Student("Lucy",26)
				 							   );
		 
		 Stream<Student> stream1 = stuList.stream().filter((stu) -> stu.getAge()>20).skip(1).distinct();
		 stream1.forEach(System.out::println);
	}
}

Output：

Student [name=Tom, age=26]
Student [name=Lucy, age=26]