【Java 8 新特性】Java 8 Stream使用sorted()方法自定义排序的示例

2023-03-09 18:47:27

文章目录


在本页中，我们将提供 java 8 Stream sorted()排序的示例。

我们可以按照自然顺序和比较器提供的顺序对流进行排序。

在Java8中，可以使用lambda表达式实例化比较器(Comparator)。

我们还可以颠倒自然顺序和比较器(Comparator)提供的顺序。

自然排序使用Comparable提供的排序，该排序必须由实例为流元素的类实现。

在本页中，我们将使用java 8 Stream sorted()方法对列表List, Map和Set进行排序。

1.使用Stream sorted()完成自然排序、比较器和反向排序

下面是sorted()方法的语法

  1. sorted()：它使用自然顺序对流中的元素进行排序。元素类必须实现Comparable接口。
  2. sorted(Comparator<? super T> comparator)：这里我们使用lambda表达式创建一个Comparator实例。我们可以按升序和降序对流元素进行排序。

下面的代码行将按自然顺序对列表进行排序。

list.stream().sorted()

要反转自然顺序，Comparator提供reverseOrder()方法。

list.stream().sorted(Comparator.reverseOrder())

下面的代码行使用Comparator对列表进行排序。

list.stream().sorted(Comparator.comparing(Student::getAge))

为了颠倒顺序，Comparator提供reversed()方法。

list.stream().sorted(Comparator.comparing(Student::getAge).reversed())

2.在List中使用Stream sorted()方法

下面我们对Student列表进行排序操作。首先，我们将按自然顺序排序，然后使用比较器(Comparator)。

下面是颠倒自然排序和比较器提供的排序的例子。

SortList.java

package com.concretepage;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
public class SortList {
	public static void main(String[] args) {
		List<Student> list = new ArrayList<Student>();
		list.add(new Student(1, "Mahesh", 12));
		list.add(new Student(2, "Suresh", 15));
		list.add(new Student(3, "Nilesh", 10));
		
		System.out.println("---Natural Sorting by Name---");
		List<Student> slist = list.stream().sorted().collect(Collectors.toList());
		slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
		
		System.out.println("---Natural Sorting by Name in reverse order---");
		slist = list.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
		slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));		
		
		System.out.println("---Sorting using Comparator by Age---");
		slist = list.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
		slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
		
		System.out.println("---Sorting using Comparator by Age with reverse order---");
		slist = list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());
		slist.forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
	}
}

Student.java

package com.concretepage;
public class Student implements Comparable<Student> {
	private int id;
	private String name;
	private int age;
	public Student(int id, String name, int age) {
		this.id = id;
		this.name = name;
		this.age = age;
	}
	public int getId() {
		return id;
	}
	public String getName() {
		return name;
	}
	public int getAge() {
		return age;
	}
	@Override
	public int compareTo(Student ob) {
		return name.compareTo(ob.getName());
	}
        @Override
        public boolean equals(final Object obj) {
          if (obj == null) {
             return false;
          }
          final Student std = (Student) obj;
          if (this == std) {
             return true;
          } else {
             return (this.name.equals(std.name) && (this.age == std.age));
          }
        }
        @Override
        public int hashCode() {
          int hashno = 7;
          hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
          return hashno;
        }	
}

输出

---Natural Sorting by Name---
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10
Id:2, Name: Suresh, Age:15
---Natural Sorting by Name in reverse order---
Id:2, Name: Suresh, Age:15
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
---Sorting using Comparator by Age---
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
Id:2, Name: Suresh, Age:15
---Sorting using Comparator by Age with reverse order---
Id:2, Name: Suresh, Age:15
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10

3.在Set中使用Stream sorted()方法

下面我们对Student类的集合(Set)进行排序操作，此类必须重写equals()和hashCode()方法来标识唯一的元素。

对于自然排序，学生类需要实现Comparable接口。

在下面的例子中，我们将使用自然排序和比较器提供的排序对集合进行排序。

SortSet.java

package com.concretepage;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Set;
public class SortSet {
	public static void main(String[] args) {
		Set<Student> set = new HashSet<Student>();
		set.add(new Student(1, "Mahesh", 12));
		set.add(new Student(2, "Suresh", 15));
		set.add(new Student(3, "Nilesh", 10));
		
		System.out.println("---Natural Sorting by Name---");
		set.stream().sorted().forEach(e -> System.out.println("Id:"
		          + e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
		
		System.out.println("---Natural Sorting by Name in reverse order---");
		set.stream().sorted(Comparator.reverseOrder()).forEach(e -> System.out.println("Id:"
		          + e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));		
		
		System.out.println("---Sorting using Comparator by Age---");
		set.stream().sorted(Comparator.comparing(Student::getAge))
		  .forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
		
		System.out.println("---Sorting using Comparator by Age in reverse order---");
		set.stream().sorted(Comparator.comparing(Student::getAge).reversed())
		  .forEach(e -> System.out.println("Id:"+ e.getId()+", Name: "+e.getName()+", Age:"+e.getAge()));
	}
}

输出

---Natural Sorting by Name---
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10
Id:2, Name: Suresh, Age:15
---Natural Sorting by Name in reverse order---
Id:2, Name: Suresh, Age:15
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
---Sorting using Comparator by Age---
Id:3, Name: Nilesh, Age:10
Id:1, Name: Mahesh, Age:12
Id:2, Name: Suresh, Age:15
---Sorting using Comparator by Age in reverse order---
Id:2, Name: Suresh, Age:15
Id:1, Name: Mahesh, Age:12
Id:3, Name: Nilesh, Age:10

4.在Map中使用Stream sorted()方法

这里我们将按键和值对Map进行排序。

SortMap.java

package com.concretepage;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
public class SortMap {
	public static void main(String[] args) {
		Map<Integer, String> map = new HashMap<>();
		map.put(15, "Mahesh");
		map.put(10, "Suresh");
		map.put(30, "Nilesh");
		
		System.out.println("---Sort by Map Value---");
	        map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
	          .forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));

		System.out.println("---Sort by Map Key---");
	        map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getKey))
	          .forEach(e -> System.out.println("Key: "+ e.getKey() +", Value: "+ e.getValue()));
	}
}

输出

---Sort by Map Value---
Key: 15, Value: Mahesh
Key: 30, Value: Nilesh
Key: 10, Value: Suresh
---Sort by Map Key---
Key: 10, Value: Suresh
Key: 15, Value: Mahesh
Key: 30, Value: Nilesh

下面我们要对值为自定义对象的Map进行排序。

SortMapOfCustomObject.java

package com.concretepage;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
public class SortMapOfCustomObject {
	public static void main(String[] args) {
		Map<Integer, Student> map = new HashMap<>();
		map.put(1, new Student(1, "Mahesh", 12));
		map.put(2, new Student(2, "Suresh", 15));
		map.put(3, new Student(3, "Nilesh", 10));
                //Map Sorting by Value i.e student's natural ordering i.e by name
	        map.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue))
	    	.forEach(e -> {
				Integer key = (Integer)e.getKey();
				Student std = (Student)e.getValue();
	    		System.out.println("Key: " + key +", value: ("+ std.getId() +", "+ std.getName()+", "+ std.getAge()+")"); 
	    	});
	}
}

输出

Key: 1, value: (1, Mahesh, 12)
Key: 3, value: (3, Nilesh, 10)
Key: 2, value: (2, Suresh, 15)

参考文献

【1】Java 8 Stream sorted() Example

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