Java Comparator thenComparingDouble()方法及示例

Java中比较器接口的 thenComparingDouble( java.util.function.ToDoubleFunction ) 方法返回一个具有提取double排序键的函数的词表顺序比较器。如果你想对那些在比较方法中相等的值再进行一次比较，那么这个方法将在比较方法之后应用。

语法

default Comparator <T> thenComparingDouble(
    ToDoubleFunction <T> keyExtractor)

参数： 该方法接受 keyExtractor ，它是用来提取Double sort key的函数。

返回值： 该方法返回一个由this和Double sort key组成的lexicographic-order比较器。

异常： 如果参数为空，该方法会抛出NullPointerException。

下面的程序说明了thenComparingDouble(java.util.function.ToDoubleFunction)方法：

程序 1 :

// Java program to demonstrate Comparator
// thenComparingDouble(ToDoubleFunction) method
  
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
  
public class GFG {
  
    public static void main(String[] args)
    {
        List<Student> list = getStudentList();
  
        Comparator<Student> comparator
            = Comparator
                  .comparing(Student::getSchool)
                  .thenComparingDouble(Student::getpercentageMarks);
  
        Collections.sort(list, comparator);
        System.out.println("After sort");
        list.forEach(s -> System.out.println(s));
    }
  
    public static List<Student> getStudentList()
    {
  
        Student s1 = new Student("Ram", 85.5, "SJV");
        Student s2 = new Student("Shyam", 83.25, "MSH");
        Student s3 = new Student("Mohan", 86.55, "SJV");
        Student s4 = new Student("Sohan", 81.00, "MSH");
        Student s5 = new Student("Rabi", 55.6, "SJV");
        List<Student> list = Arrays.asList(s1, s2, s3, s4, s5);
  
        return list;
    }
}
  
class Student {
  
    private String name;
    private double percentageMarks;
    private String school;
  
    public Student(String name,
                   double percentageMarks,
                   String school)
    {
        this.name = name;
        this.percentageMarks = percentageMarks;
        this.school = school;
    }
  
    public String getName()
    {
        return name;
    }
  
    public void setName(String name)
    {
        this.name = name;
    }
  
    public double getpercentageMarks()
    {
        return percentageMarks;
    }
  
    public void setpercentageMarks(int percentageMarks)
    {
        this.percentageMarks = percentageMarks;
    }
  
    public String getSchool()
    {
        return school;
    }
  
    public void setSchool(String school)
    {
        this.school = school;
    }
  
    @Override
    public String toString()
    {
        return "Student [name=" + name + ", 
            percentageMarks
            = " + percentageMarks 
              + ", school=" + school + "]";
    }
}

输出：

Java中的比较器thenComparingDouble方法及示例
你可以看到，在这个例子中，首先对学校进行了分类，如果学校相同，则对百分数进行分类。

程序2 :

// Java program to demonstrate Comparator
// thenComparingDouble(ToDoubleFunction)  method
  
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
  
public class GFG {
  
    public static void main(String... args)
    {
  
        List<Integer> list
            = Arrays.asList(1, 2, 3, 4, 5, 6);
  
        try {
  
            // apply thenComparingDouble
            Comparator.comparing(list::get)
                .thenComparingDouble(null);
        }
        catch (Exception e) {
  
            System.out.printf("Exception:" + e);
        }
    }
}

打印在控制台的输出如下。

输出：

Java中的比较器thenComparingDouble方法及示例