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📜  Java Comparator接口

📅  最后修改于: 2020-10-13 00:42:16             🧑  作者: Mango

Java比较器界面

Java Comparator接口用于对用户定义的类的对象进行排序。

该接口在java.util包中找到，包含2个方法compare(Object obj1，Object obj2)和equals(Object element)。

它提供了多种排序顺序，即，您可以根据任何数据成员(例如，rollno，名称，年龄或其他任何内容)对元素进行排序。

Java比较器接口的方法

Method Description
public int compare(Object obj1, Object obj2) It compares the first object with the second object.
public boolean equals(Object obj) It is used to compare the current object with the specified object.
public boolean equals(Object obj) It is used to compare the current object with the specified object.

收藏类

Collections类提供用于对集合的元素进行排序的静态方法。如果集合元素是Set或Map，则可以使用TreeSet或TreeMap。但是，我们无法对List的元素进行排序。 Collections类还提供用于对List类型元素的元素进行排序的方法。

用于对List元素进行排序的Collections类的方法

public void sort(List list，Comparator c)：用于按给定的Comparator对List的元素进行排序。

Java比较器示例(非通用旧样式)

让我们看一下根据年龄和姓名对List的元素进行排序的示例。在此示例中，我们创建了4个Java类：

  • 学生.java
  • AgeComparator.java
  • NameComparator.java
  • 简单的.java

此类包含三个字段rollno，name和age和一个参数化的构造函数。


class Student{
int rollno;
String name;
int age;
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
}

此类根据年龄定义比较逻辑。如果第一个对象的年龄大于第二个对象的年龄，则我们将返回一个正值。可以是1、2、10之类的任何值。如果第一个对象的年龄小于第二个对象，我们将返回一个负值，它可以是任何负值，并且如果两个对象的年龄相等，我们返回0。

import java.util.*;
class AgeComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1=(Student)o1;
Student s2=(Student)o2;

if(s1.age==s2.age)
return 0;
else if(s1.age>s2.age)
return 1;
else
return -1;
}
}

此类提供基于名称的比较逻辑。在这种情况下，我们使用String类的compareTo()方法，该方法在内部提供比较逻辑。

import java.util.*;
class NameComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1=(Student)o1;
Student s2=(Student)o2;

return s1.name.compareTo(s2.name);
}
}

在此类中，我们将根据名称和年龄进行排序，以打印对象的值。


import java.util.*;
import java.io.*;

class Simple{
public static void main(String args[]){

ArrayList al=new ArrayList();
al.add(new Student(101,"Vijay",23));
al.add(new Student(106,"Ajay",27));
al.add(new Student(105,"Jai",21));

System.out.println("Sorting by Name");

Collections.sort(al,new NameComparator());
Iterator itr=al.iterator();
while(itr.hasNext()){
Student st=(Student)itr.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}

System.out.println("Sorting by age");

Collections.sort(al,new AgeComparator());
Iterator itr2=al.iterator();
while(itr2.hasNext()){
Student st=(Student)itr2.next();
System.out.println(st.rollno+" "+st.name+" "+st.age);
}


}
}

       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23
       
       Sorting by age       
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java比较器示例(通用)

class Student{
int rollno;
String name;
int age;
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
}

import java.util.*;
class AgeComparator implements Comparator{
public int compare(Student s1,Student s2){
if(s1.age==s2.age)
return 0;
else if(s1.age>s2.age)
return 1;
else
return -1;
}
}

此类提供基于名称的比较逻辑。在这种情况下，我们使用String类的compareTo()方法，该方法在内部提供比较逻辑。

import java.util.*;
class NameComparator implements Comparator{
public int compare(Student s1,Student s2){
return s1.name.compareTo(s2.name);
}
}

在此类中，我们将根据名称和年龄进行排序，以打印对象的值。

import java.util.*;
import java.io.*;
class Simple{
public static void main(String args[]){

ArrayList al=new ArrayList();
al.add(new Student(101,"Vijay",23));
al.add(new Student(106,"Ajay",27));
al.add(new Student(105,"Jai",21));

System.out.println("Sorting by Name");

Collections.sort(al,new NameComparator());
for(Student st: al){
System.out.println(st.rollno+" "+st.name+" "+st.age);
}

System.out.println("Sorting by age");

Collections.sort(al,new AgeComparator());
for(Student st: al){
System.out.println(st.rollno+" "+st.name+" "+st.age);
}
}
}

       Sorting by Name
       106 Ajay 27
       105 Jai 21
       101 Vijay 23

       Sorting by age     
       105 Jai 21
       101 Vijay 23
       106 Ajay 27

Java 8比较器界面

Java 8 Comparator接口是仅包含一个抽象方法的功能接口。现在，我们可以将Comparator接口用作lambda表达式或方法引用的分配目标。

Java 8比较器接口的方法


Method Description
int compare(T o1, T o2) It compares the first object with second object.
static >
Comparator comparing(Function keyExtractor)		 It accepts a function that extracts a Comparable sort key from a type T, and returns a Comparator that compares by that sort key.
static Comparatorcomparing(Function keyExtractor, Comparator keyComparator) It accepts a function that extracts a sort key from a type T, and returns a Comparator that compares by that sort key using the specified Comparator.
static Comparator comparingDouble(ToDoubleFunction keyExtractor) It accepts a function that extracts a double sort key from a type T, and returns a Comparator that compares by that sort key.
static Comparator comparingInt(ToIntFunction keyExtractor) It accepts a function that extracts an int sort key from a type T, and returns a Comparator that compares by that sort key.
static Comparator comparingLong(ToLongFunction keyExtractor) It accepts a function that extracts a long sort key from a type T, and returns a Comparator that compares by that sort key.
boolean equals(Object obj) It is used to compare the current object with the specified object.
static >
Comparator naturalOrder()		 It returns a comparator that compares Comparable objects in natural order.
static Comparator nullsFirst(Comparator comparator) It returns a comparator that treats null to be less than non-null elements.
static Comparator nullsLast(Comparator comparator) It returns a comparator that treats null to be greater than non-null elements.
default Comparator reversed() It returns comparator that contains reverse ordering of the provided comparator.
static > Comparator reverseOrder() It returns comparator that contains reverse of natural ordering.
default Comparator thenComparing(Comparator other) It returns a lexicographic-order comparator with another comparator.
default >
Comparator thenComparing(Function keyExtractor)		 It returns a lexicographic-order comparator with a function that extracts a Comparable sort key.
default Comparator thenComparing(Function keyExtractor, Comparator keyComparator) It returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator.
default Comparator thenComparingDouble(ToDoubleFunction keyExtractor) It returns a lexicographic-order comparator with a function that extracts a double sort key.
default Comparator thenComparingInt(ToIntFunction keyExtractor) It returns a lexicographic-order comparator with a function that extracts a int sort key.
default Comparator thenComparingLong(ToLongFunction keyExtractor) It returns a lexicographic-order comparator with a function that extracts a long sort key.

Java 8比较器示例

让我们看一下根据年龄和姓名对List的元素进行排序的示例。

档案：Student.java

class Student {  
   int rollno;  
   String name;  
  int age;  
    Student(int rollno,String name,int age){  
    this.rollno=rollno;  
    this.name=name;  
    this.age=age;  
    }

public int getRollno() {
return rollno;
}

public void setRollno(int rollno) {
this.rollno = rollno;
}

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;
}

    }

文件：TestSort1.java

    import java.util.*;  
    public class TestSort1{  
    public static void main(String args[]){  
    ArrayList al=new ArrayList();  
    al.add(new Student(101,"Vijay",23));  
    al.add(new Student(106,"Ajay",27));  
    al.add(new Student(105,"Jai",21)); 
//Sorting elements on the basis of name
    Comparator cm1=Comparator.comparing(Student::getName);
     Collections.sort(al,cm1);
     System.out.println("Sorting by Name");
     for(Student st: al){
     System.out.println(st.rollno+" "+st.name+" "+st.age);
     }
     //Sorting elements on the basis of age
      Comparator cm2=Comparator.comparing(Student::getAge);
 Collections.sort(al,cm2);
     System.out.println("Sorting by Age");
     for(Student st: al){
     System.out.println(st.rollno+" "+st.name+" "+st.age);
     }  
    }  
    }  

Sorting by Name
106 Ajay 27
105 Jai 21
101 Vijay 23
Sorting by Age
105 Jai 21
101 Vijay 23
106 Ajay 27

Java 8 Comparator示例：nullsFirst()和nullsLast()方法

在这里，我们对也包含null的元素列表进行排序。

档案：Student.java

class Student {  
   int rollno;  
   String name;  
  int age;  
    Student(int rollno,String name,int age){  
    this.rollno=rollno;  
    this.name=name;  
    this.age=age;  
    }
public int getRollno() {
return rollno;
}
public void setRollno(int rollno) {
this.rollno = rollno;
}
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;
}
    }

文件：TestSort2.java

   import java.util.*;  
    public class TestSort2{  
    public static void main(String args[]){  
    ArrayList al=new ArrayList();  
    al.add(new Student(101,"Vijay",23));  
    al.add(new Student(106,"Ajay",27));  
    al.add(new Student(105,null,21));  
    Comparator cm1=Comparator.comparing(Student::getName,Comparator.nullsFirst(String::compareTo));
     Collections.sort(al,cm1);
     System.out.println("Considers null to be less than non-null");
     for(Student st: al){
     System.out.println(st.rollno+" "+st.name+" "+st.age);
     }
     Comparator cm2=Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo));
     Collections.sort(al,cm2);
     System.out.println("Considers null to be greater than non-null");
     for(Student st: al){
     System.out.println(st.rollno+" "+st.name+" "+st.age);
     }
    }  
    }   

Considers null to be less than non-null
105 null 21
106 Ajay 27
101 Vijay 23
Considers null to be greater than non-null
106 Ajay 27
101 Vijay 23
105 null 21