Java中的比较器接口及其示例-yiteyi-C++库

比较器接口用于对用户定义类的对象进行排序。比较器对象能够比较同一类的两个对象 . 以下功能比较obj1和obj2。

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语法：

public int compare(Object obj1, Object obj2):

假设我们有一个自己类类型的数组/数组列表，其中包含卷号、名称、地址、DOB等字段，我们需要根据卷号或名称对数组进行排序？

方法1 ：一种显而易见的方法是使用标准算法之一编写我们自己的sort（）函数。该解决方案需要根据卷号和名称等不同标准重写整个分拣代码。

方法2： 使用比较器接口——比较器接口用于对用户定义类的对象进行排序。这个接口是用java实现的。util包，包含两个方法比较（对象obj1、对象obj2）和等于（对象元素）。使用比较器，我们可以根据数据成员对元素进行排序。例如，它可能在卷号、姓名、年龄或其他任何内容上。

用于排序列表元素的Collections类的方法用于通过给定的比较器对列表元素进行排序。

public void sort(List list, ComparatorClass c)

要对给定列表进行排序，ComparatorClass必须实现一个Comparator接口。

Collections类的sort（）方法是如何工作的？

在内部，Sort方法会调用它正在排序的类的Compare方法。要比较两个元素，它会问“哪个更大？”比较方法返回值-1、0或1，判断它是否小于、等于或大于另一个。它使用这个结果来确定是否应该将它们替换为它们的排序。

实例

JAVA

                         // Java Program to Demonstrate Working of                       
                         // Comparator Interface                       
           
                         // Importing required classes                       
                         import                                     java.io.*;                       
                         import                                     java.lang.*;                       
                         import                                     java.util.*;                       
           
                         // Class 1                       
                         // A class to represent a Student                       
                         class                                     Student {                       
           
                                                 // Attributes of a student                       
                                                 int                                     rollno;                       
                                                 String name, address;                       
           
                                                 // Constructor                       
                                                 public                                     Student(                                     int                                     rollno, String name, String address)                       
                                                 {                       
           
                                                 // This keyword refers to current instance itself                       
                                                 this                                     .rollno = rollno;                       
                                                 this                                     .name = name;                       
                                                 this                                     .address = address;                       
                                                 }                       
           
                                                 // Method of Student class                       
                                                 // To print student details in main()                       
                                                 public                                     String toString()                       
                                                 {                       
           
                                                 // Returning attributes of Student                       
                                                 return                                     this                                     .rollno +                                     " "                                     +                                     this                                     .name +                                     " "                       
                                                 +                                     this                                     .address;                       
                                                 }                       
                         }                       
           
                         // Class 2                       
                         // Helper class implementing Comparator interface                       
                         class                                     Sortbyroll                                     implements                                     Comparator<Student> {                       
           
                                                 // Method                       
                                                 // Sorting in ascending order of roll number                       
                                                 public                                     int                                     compare(Student a, Student b)                       
                                                 {                       
           
                                                 return                                     a.rollno - b.rollno;                       
                                                 }                       
                         }                       
           
                         // Class 3                       
                         // Helper class implementing Comparator interface                       
                         class                                     Sortbyname                                     implements                                     Comparator<Student> {                       
           
                                                 // Method                       
                                                 // Sorting in ascending order of name                       
                                                 public                                     int                                     compare(Student a, Student b)                       
                                                 {                       
           
                                                 return                                     a.name.compareTo(b.name);                       
                                                 }                       
                         }                       
           
                         // Class 4                       
                         // Main class                       
                         class                                     GFG {                       
           
                                                 // Main driver method                       
                                                 public                                     static                                     void                                     main(String[] args)                       
                                                 {                       
           
                                                 // Creating an empty ArrayList of Student type                       
                                                 ArrayList<Student> ar =                                     new                                     ArrayList<Student>();                       
           
                                                 // Adding entries in above List                       
                                                 // using add() method                       
                                                 ar.add(                                     new                                     Student(                                     111                                     ,                                     "Mayank"                                     ,                                     "london"                                     ));                       
                                                 ar.add(                                     new                                     Student(                                     131                                     ,                                     "Anshul"                                     ,                                     "nyc"                                     ));                       
                                                 ar.add(                                     new                                     Student(                                     121                                     ,                                     "Solanki"                                     ,                                     "jaipur"                                     ));                       
                                                 ar.add(                                     new                                     Student(                                     101                                     ,                                     "Aggarwal"                                     ,                                     "Hongkong"                                     ));                       
           
                                                 // Display message on console for better readability                       
                                                 System.out.println(                                     "Unsorted"                                     );                       
           
                                                 // Iterating over entries to print them                       
                                                 for                                     (                                     int                                     i =                                     0                                     ; i < ar.size(); i++)                       
                                                 System.out.println(ar.get(i));                       
           
                                                 // Sorting student entries by roll number                       
                                                 Collections.sort(ar,                                     new                                     Sortbyroll());                       
           
                                                 // Display message on console for better readability                       
                                                 System.out.println(                                     "Sorted by rollno"                                     );                       
           
                                                 // Again iterating over entries to print them                       
                                                 for                                     (                                     int                                     i =                                     0                                     ; i < ar.size(); i++)                       
                                                 System.out.println(ar.get(i));                       
           
                                                 // Sorting student entries by name                       
                                                 Collections.sort(ar,                                     new                                     Sortbyname());                       
           
                                                 // Display message on console for better readability                       
                                                 System.out.println(                                     "Sorted by name"                                     );                       
           
                                                 // // Again iterating over entries to print them                       
                                                 for                                     (                                     int                                     i =                                     0                                     ; i < ar.size(); i++)                       
                                                 System.out.println(ar.get(i));                       
                                                 }                       
                         }                       

输出

Unsorted111 Mayank london131 Anshul nyc121 Solanki jaipur101 Aggarwal HongkongSorted by rollno101 Aggarwal Hongkong111 Mayank london121 Solanki jaipur131 Anshul nycSorted by name101 Aggarwal Hongkong131 Anshul nyc111 Mayank london121 Solanki jaipur

通过更改compare方法内的返回值，您可以按照您希望的任何顺序进行排序，例如：对于降序，只需更改上述compare方法中“a”和“b”的位置。

按多个字段对集合排序

在上一个示例中，我们讨论了如何使用Comparable and Comparator接口在单个字段的基础上对对象列表进行排序，但是，如果我们需要根据多个字段对ArrayList对象进行排序，比如第一，根据学生姓名进行排序，第二，根据学生年龄进行排序，该怎么办。

实例

JAVA

                         // Java Program to Demonstrate Working of                       
                         // Comparator Interface Via More than One Field                       
           
                         // Importing required classes                       
                         import                                     java.util.ArrayList;                       
                         import                                     java.util.Collections;                       
                         import                                     java.util.Comparator;                       
                         import                                     java.util.Iterator;                       
                         import                                     java.util.List;                       
           
                         // Class 1                       
                         // Helper class representing a Student                       
                         class                                     Student {                       
           
                                                 // Attributes of student                       
                                                 String Name;                       
                                                 int                                     Age;                       
           
                                                 // Parameterized constructor                       
                                                 public                                     Student(String Name, Integer Age)                       
                                                 {                       
           
                                                 // This keyword refers to current instance itself                       
                                                 this                                     .Name = Name;                       
                                                 this                                     .Age = Age;                       
                                                 }                       
           
                                                 // Getter setter methods                       
                                                 public                                     String getName() {                                     return                                     Name; }                       
           
                                                 public                                     void                                     setName(String Name) {                                     this                                     .Name = Name; }                       
           
                                                 public                                     Integer getAge() {                                     return                                     Age; }                       
           
                                                 public                                     void                                     setAge(Integer Age) {                                     this                                     .Age = Age; }                       
           
                                                 // Method                       
                                                 // Overriding toString() method                       
                                                 @Override                                     public                                     String toString()                       
                                                 {                       
                                                 return                                     "Customer{"                       
                                                 +                                     "Name="                                     + Name +                                     ", Age="                                     + Age +                                     '}'                                     ;                       
                                                 }                       
           
                                                 // Class 2                       
                                                 // Helper class implementing Comparator interface                       
                                                 static                                     class                                     CustomerSortingComparator                       
                                                 implements                                     Comparator<Student> {                       
           
                                                 // Method 1                       
                                                 // To compare customers                       
                                                 @Override                       
                                                 public                                     int                                     compare(Student customer1,                       
                                                 Student customer2)                       
                                                 {                       
           
                                                 // Comparing customers                       
                                                 int                                     NameCompare = customer1.getName().compareTo(                       
                                                 customer2.getName());                       
           
                                                 int                                     AgeCompare = customer1.getAge().compareTo(                       
                                                 customer2.getAge());                       
           
                                                 // 2nd level comparison                       
                                                 return                                     (NameCompare ==                                     0                                     ) ? AgeCompare                       
                                                 : NameCompare;                       
                                                 }                       
                                                 }                       
           
                                                 // Method 2                       
                                                 // Main driver method                       
                                                 public                                     static                                     void                                     main(String[] args)                       
                                                 {                       
           
                                                 // Create an empty ArrayList                       
                                                 // to store Student                       
                                                 List<Student> al =                                     new                                     ArrayList<>();                       
           
                                                 // Create customer objects                       
                                                 // using constructor initialization                       
                                                 Student obj1 =                                     new                                     Student(                                     "Ajay"                                     ,                                     27                                     );                       
                                                 Student obj2 =                                     new                                     Student(                                     "Sneha"                                     ,                                     23                                     );                       
                                                 Student obj3 =                                     new                                     Student(                                     "Simran"                                     ,                                     37                                     );                       
                                                 Student obj4 =                                     new                                     Student(                                     "Ajay"                                     ,                                     22                                     );                       
                                                 Student obj5 =                                     new                                     Student(                                     "Ajay"                                     ,                                     29                                     );                       
                                                 Student obj6 =                                     new                                     Student(                                     "Sneha"                                     ,                                     22                                     );                       
           
                                                 // Adding customer objects to ArrayList                       
                                                 // using add() method                       
                                                 al.add(obj1);                       
                                                 al.add(obj2);                       
                                                 al.add(obj3);                       
                                                 al.add(obj4);                       
                                                 al.add(obj5);                       
                                                 al.add(obj6);                       
           
                                                 // Iterating using Iterator                       
                                                 // before Sorting ArrayList                       
                                                 Iterator<Student> custIterator = al.iterator();                       
           
                                                 // Display message                       
                                                 System.out.println(                                     "Before Sorting:"                                     );                       
           
                                                 // Holds true till there is single element                       
                                                 // remaining in List                       
                                                 while                                     (custIterator.hasNext()) {                       
           
                                                 // Iterating using next() method                       
                                                 System.out.println(custIterator.next());                       
                                                 }                       
           
                                                 // Sorting using sort method of Collections class                       
                                                 Collections.sort(al,                       
                                                 new                                     CustomerSortingComparator());                       
           
                                                 // Display message only                       
                                                 System.out.println(                                     "After Sorting:"                                     );                       
           
                                                 // Iterating using enhanced for-loop                       
                                                 // after Sorting ArrayList                       
                                                 for                                     (Student customer : al) {                       
                                                 System.out.println(customer);                       
                                                 }                       
                                                 }                       
                         }                       

输出

Before Sorting:Customer{Name=Ajay, Age=27}Customer{Name=Sneha, Age=23}Customer{Name=Simran, Age=37}Customer{Name=Ajay, Age=22}Customer{Name=Ajay, Age=29}Customer{Name=Sneha, Age=22}After Sorting:Customer{Name=Ajay, Age=22}Customer{Name=Ajay, Age=27}Customer{Name=Ajay, Age=29}Customer{Name=Simran, Age=37}Customer{Name=Sneha, Age=22}Customer{Name=Sneha, Age=23}

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