java 如何在java中对由pojo类组成的Arraylist进行排序
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How to sort Arraylist consisting of pojo class in java
提问by programminglover
I have POJO class Student like this
我有这样的 POJO 班学生
class Student
{
private int score;
private String FirstName;
//Getters and setters .................
}
I am creating ArrayList like this
我正在像这样创建 ArrayList
public static void main(String[] args)
{
List<Student> al_students= new ArrayList<Student>();
Student s1= new Student();
s1.setScore(90);
s1.setFirstName("abc");
al_students.add(s1);
Student s2= new Student();
s2.setScore(95);
s2.setFirstName("def");
al_students.add(s2);
Student s3= new Student();
s3.setScore(85);
s3.setFirstName("xyz");
al_students.add(s3);
}
Now I want to sort it based on scores in descending order i.e
output
现在我想根据降序的分数对其进行排序，即
输出
1)def 95
2)abc 90
3)xyz 85
回答by Glorfindel
Use a Comparator:
使用比较器：
Collections.sort(al_students, new Comparator<Student>() {
@Override
public int compare(Student o1, Student o2) {
return Integer.compare(o2.getScore(), o1.getScore());
}
});
Alternatively, have Student implement the Comparableinterface:
或者，让 Student 实现Comparable接口：
class Student implements Comparable<Student> {
...
@Override
public int compareTo(Student s) {
return Integer.compare(s.getScore(), getScore());
}
}
Then you can just sort without the Comparator:
然后你可以在没有比较器的情况下进行排序：
Collections.sort(al_students);
回答by Mena
You can use a custom Comparator.
您可以使用自定义Comparator.
Here's a full example (imports excluded):
这是一个完整的示例（不包括进口）：
public class Main {
// main method setting up and printing students
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>();
Student s1 = new Student();
s1.setScore(90);
s1.setFirstName("abc");
students.add(s1);
Student s2 = new Student();
s2.setScore(95);
s2.setFirstName("def");
students.add(s2);
Student s3 = new Student();
s3.setScore(85);
s3.setFirstName("xyz");
students.add(s1);
System.out.printf("Unordered: %s%n", students);
// sorting using anonymous Comparator
Collections.sort(students, new Comparator<Student>() {
public int compare(Student s1, Student s2) {
// notice the cast to (Integer) to invoke compareTo
return ((Integer)s1.getScore()).compareTo(s2.getScore());
}
});
System.out.printf("Ordered: %s%n", students);
}
// Student class
static class Student {
private int score;
private String firstName;
// boring stuff
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String name) {
this.firstName = name;
}
// for printing
@Override
public String toString() {
return String.format("Student \"%s\" with score: %d%n", firstName,
score);
}
}
}
Output
输出
Unordered: [Student "abc" with score: 90
, Student "def" with score: 95
, Student "abc" with score: 90
]
Ordered: [Student "abc" with score: 90
, Student "abc" with score: 90
, Student "def" with score: 95
]
Note
笔记
As others mention, you can also implement Comparable<Student>in your Studentclass, if the only (or default) sorting will be by score.#
正如其他人提到的，你也可以implement Comparable<Student>在你的Student班级中，如果唯一（或默认）排序是按分数。#
Second edit
第二次编辑
In order to sort in a decreasing order, you can replace the returnstatement in your Comparatorwith the following:
为了按降序排序，您可以将您的return语句替换Comparator为以下内容：
return ((Integer)s2.getScore()).compareTo(s1.getScore());
Thanks programmingloverfor spotting this / apologies for mistakenly rejecting the edit!
感谢编程爱好者发现这一点/为错误地拒绝编辑而道歉！
回答by Pshemo
If you are using Java 8 then your code can look like
如果您使用的是 Java 8，那么您的代码可能如下所示
al_students.sort(Comparator.comparingInt(Student::getScore).reversed());
回答by Sonu Gupta
You can write the custom Comparatorto solve this.
您可以编写自定义Comparator来解决此问题。
回答by Kishore Kumar Korada
See, Have this logic.
Student is a class which you've created. And If you want to allow some other class to sort your custom objects in your own requirements, you should tell it. And that's gonna happen if you implement either Comparable or Comparator interface and override those method in your custom class and in those methods you specify how you are gonna compare.
看，有这个逻辑。
Student 是您创建的类。如果您想允许其他类根据您自己的要求对自定义对象进行排序，您应该告诉它。如果您实现 Comparable 或 Comparator 接口并在您的自定义类中覆盖这些方法，并且在这些方法中您指定要比较的方式，就会发生这种情况。
回答by FatherMathew
Almost the entire question is answered by others....
几乎整个问题都由其他人回答......
Since there are 2 options, either to use Comparable interface or Comparator interface, I would like you to go to this postexplaining when to use Comparable and when to use Comparator.
由于有 2 个选项，要么使用 Comparable 接口，要么使用 Comparator 接口，我希望你去这篇文章解释何时使用 Comparable 以及何时使用 Comparator。
That will be really helpful to you.
这对你真的很有帮助。
