Java中List对象集合按照对象某一属性字段排序

ゝ一纸荒年。 2022-09-11 09:21 361阅读 0赞

Java中，List集合对象，默认有一个排序方法sort(Comparator<? super E> c)，如果传递null，那是对简单类型的排序，如果是对象类型，并且是需要按照对象类型的某一个属性字段排序，就需要我们传入排序的规则。

show me the code:

package com.xxx.huali.hualitest.sort;
    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.List;
    public class ListSort {
    	public static void main(String[] args) {
    		List<Person> list = new ArrayList<Person>();
    		list.add(new Person(1,"jack",18));
    		list.add(new Person(2,"paul",28));
    		list.add(new Person(3,"zion",20));
    		list.add(new Person(4,"jone",30));
    		list.forEach(System.out::println);
    		//利用Collections.sort方法按照年龄排序，默认升序
    		Collections.sort(list, (a,b)->{
    			return a.getAge() - b.getAge();
    		});
    		System.out.println("================================");
    		list.forEach(System.out::println);
    		//直接使用List集合sort方法按照姓名排序，默认字母升序
    		list.sort((a,b)->a.getName().compareTo(b.getName()));
    		System.out.println("================================");
    		list.forEach(System.out::println);
    	}
    }
    
    class Person{
    	private int id ;
    	private String name ;
    	private int age;
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	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 "Person [id=" + id + ", name=" + name + ", age=" + age + "]";
    	}
    	
    	public Person() {
    		
    	}
    	
    	public Person(int id,String name,int age) {
    		this.id = id;
    		this.name = name;
    		this.age = age;
    	}
    }

示例中，定义了Person对象，分别按照年龄和姓名排序，运行程序，打印结果如下：

Person [id=1, name=jack, age=18]
    Person [id=2, name=paul, age=28]
    Person [id=3, name=zion, age=20]
    Person [id=4, name=jone, age=30]
    ================================
    Person [id=1, name=jack, age=18]
    Person [id=3, name=zion, age=20]
    Person [id=2, name=paul, age=28]
    Person [id=4, name=jone, age=30]
    ================================
    Person [id=1, name=jack, age=18]
    Person [id=4, name=jone, age=30]
    Person [id=2, name=paul, age=28]
    Person [id=3, name=zion, age=20]

排序思路很简单，就是利用List集合自带的sort函数，传入相应的比较函数。

有意思的是，Set集合，是一个不重复的集合，但是它就没有sort排序方法。

上面的排序算法，我们传入了排序函数，是使用的lambda表达式的形式，这个是在jdk8以后的版本中才能使用，传统的做法这里其实是需要传入一个匿名内部类Comparator。

list.sort(new Comparator<Person>() {
    
    	@Override
    	public int compare(Person o1, Person o2) {
    		return o1.getId() - o2.getId();
    	}
    			
    });

另外，数组对象排序，可以使用Arrays.sort()方法，这个方法的对象不能是集合，只能是数组。

package com.xxx.huali.hualitest.sort;
    import java.util.Arrays;
    public class ArraySort {
    	public static void main(String[] args) {
    		Employee[] emps = new Employee[] {new Employee(1, "jack", 5000),
    				new Employee(2,"ivan",2888),
    				new Employee(4, "lisa", 3000),
    				new Employee(3, "lucy", 9999)};
    		for (int i = 0; i < emps.length; i++) {
    			System.out.println(emps[i]);
    		}
    		Arrays.sort(emps);
    	}
    }
    
    class Employee{
    	private int id;
    	private String name;
    	private double salary;
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	public double getSalary() {
    		return salary;
    	}
    	public void setSalary(double salary) {
    		this.salary = salary;
    	}
    	@Override
    	public String toString() {
    		return "Employee [id=" + id + ", name=" + name + ", salary=" + salary + "]";
    	}
    	
    	public Employee() {
    		// TODO Auto-generated constructor stub
    	}
    	
    	public Employee(int id,String name,double salary) {
    		this.id = id;
    		this.name = name;
    		this.salary = salary;
    	}
    }

运行代码，报错：

Exception in thread "main" java.lang.ClassCastException: com.xxx.huali.hualitest.sort.Employee cannot be cast to java.lang.Comparable  
    at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)  
    at java.util.ComparableTimSort.sort(ComparableTimSort.java:188)  
    at java.util.Arrays.sort(Arrays.java:1246)  
    at com.xxx.huali.hualitest.sort.ArraySort.main(ArraySort.java:12)

根据提示，Employee对象不能转为Comparable类型。修改代码，实现Comparable接口，并覆盖方法。

package com.xxx.huali.hualitest.sort;
    import java.util.Arrays;
    public class ArraySort {
    	public static void main(String[] args) {
    		Employee[] emps = new Employee[] {new Employee(1, "jack", 5000),
    				new Employee(2,"ivan",2888),
    				new Employee(4, "lisa", 3000),
    				new Employee(3, "lucy", 9999)};
    		for (int i = 0; i < emps.length; i++) {
    			System.out.println(emps[i]);
    		}
    		Arrays.sort(emps);
    		System.out.println("=========================================");
    		for (int i = 0; i < emps.length; i++) {
    			System.out.println(emps[i]);
    		}
    	}
    }
    
    class Employee implements Comparable<Employee>{
    	private int id;
    	private String name;
    	private double salary;
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	public double getSalary() {
    		return salary;
    	}
    	public void setSalary(double salary) {
    		this.salary = salary;
    	}
    	@Override
    	public String toString() {
    		return "Employee [id=" + id + ", name=" + name + ", salary=" + salary + "]";
    	}
    	
    	public Employee() {
    		// TODO Auto-generated constructor stub
    	}
    	
    	public Employee(int id,String name,double salary) {
    		this.id = id;
    		this.name = name;
    		this.salary = salary;
    	}
    	@Override
    	public int compareTo(Employee o) {
    		return (int)(this.salary - o.getSalary());
    	}
    }

运行代码，打印结果如下：

Employee [id=1, name=jack, salary=5000.0]
    Employee [id=2, name=ivan, salary=2888.0]
    Employee [id=4, name=lisa, salary=3000.0]
    Employee [id=3, name=lucy, salary=9999.0]
    =========================================
    Employee [id=2, name=ivan, salary=2888.0]
    Employee [id=4, name=lisa, salary=3000.0]
    Employee [id=1, name=jack, salary=5000.0]
    Employee [id=3, name=lucy, salary=9999.0]

从上面的这个数组集合排序的例子中，我们可以得出一个结论，我们如果要让Arrays.sort()方法发挥作用，需要让对象实现Comparable接口，那么实现了Comparable接口的对象，是否在List.sort()方法中，可以默认排序呢，答案是肯定的。

package com.xxx.huali.hualitest.sort;
    import java.util.Arrays;
    import java.util.List;
    public class ArraySort {
    	public static void main(String[] args) {
    		Employee[] emps = new Employee[] {new Employee(1, "jack", 5000),
    				new Employee(2,"ivan",2888),
    				new Employee(4, "lisa", 3000),
    				new Employee(3, "lucy", 9999)};
    		/*
    		for (int i = 0; i < emps.length; i++) {
    			System.out.println(emps[i]);
    		}
    		Arrays.sort(emps);
    		System.out.println("=========================================");
    		for (int i = 0; i < emps.length; i++) {
    			System.out.println(emps[i]);
    		}*/
    		
    		List<Employee> list = Arrays.asList(emps);
    		list.forEach(System.out::println);
    		System.out.println("=========================================");
    		list.sort(null);
    		list.forEach(System.out::println);
    	}
    }
    
    class Employee implements Comparable<Employee>{
    	private int id;
    	private String name;
    	private double salary;
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	public double getSalary() {
    		return salary;
    	}
    	public void setSalary(double salary) {
    		this.salary = salary;
    	}
    	@Override
    	public String toString() {
    		return "Employee [id=" + id + ", name=" + name + ", salary=" + salary + "]";
    	}
    	
    	public Employee() {
    		// TODO Auto-generated constructor stub
    	}
    	
    	public Employee(int id,String name,double salary) {
    		this.id = id;
    		this.name = name;
    		this.salary = salary;
    	}
    	@Override
    	public int compareTo(Employee o) {
    		return (int)(this.salary - o.getSalary());
    	}
    }

排序结果：

结果与上面通过Arrays.sort()对数组对象排序结果是一样的。

通过前面的介绍，我们知道，对象类型排序，需要实现排序函数，也就是指定排序规则，他们都需要实现Comparable接口或者Comparator接口。

那么，Comparable vs Comparator：

1、Comprable接口一旦实现，只能提供一种排序规则，而Comparator可以以匿名内部类的形式提供多种排序方法。

2、使用Comparable接口，需要实现它，而使用Comparator接口，我们无需对原始类作改动，在类中添加额外代码。

3、Comparable接口是在java.lang包中，而Comparator接口在java.util包中。

4、 Comparable接口已经侵入类中，所以类默认有了排序规则，所以我们在使用排序算法的时候，不需要传入排序函数，自动使用默认排序compareTo()函数，而使用Comparator接口，我们需要在排序的地方提供compare()函数。

本文编写过程中参照了一些其他资料：

[Comparable and Comparator in Java Example - JournalDev][]

[Comparable and Comparator in Java Example - JournalDev]: https://www.journaldev.com/780/comparable-and-comparator-in-java-example