排序集合的对象 (Sorting objects of the Collection)

• This concept is related to sorting and here we will see how to sort objects on the Collection?

  这个概念与排序有关，在这里我们将看到如何对Collection上的对象进行排序？

• In java, we have utility class Collections which provide various methods to perform various task and one of the methods of Collection class is related to sorting like sort().

  在Java中，我们有实用程序类Collections，它提供了执行各种任务的各种方法，并且Collection类的方法之一与sort()之类的排序有关。

• We can implement sorting on Collection object in two ways:

  我们可以通过两种方式对Collection对象实施排序：

  1. By using Comparable
  2. By using Comparator

• When we call Collections.sort(). It sorts an object based on natural sorting or default sorting(i.e Ascending order) that's is specified in compareTo() method.

  当我们调用Collections.sort()时 。 它根据compareTo()方法中指定的自然排序或默认排序(即升序)对对象进行排序。

• When we call Collections.sort(Comparator). It sorts an object based on customized sorting (i.e Ascending order or Descending order) that's is specified in compare() method of Comparator.

  当我们调用Collections.sort(Comparator)时 。 它根据在Comparator的compare()方法中指定的自定义排序(即升序或降序)对对象进行排序。

We will see the sorting ways one by one...

我们将一一看到排序方式...

1)通过使用比较器 (1) By using Comparator)

• If we pass the Comparator object in Collection class constructor then our compare() method will be executed.

  如果我们在Collection类构造函数中传递Comparator对象，则将执行compare()方法。

• When we want customize sorting then we should go for Comparator.

  当我们想要自定义排序时，我们应该选择比较器。

• It is possible to implement customized sorting by using Comparator interface. (Customized sorting means that according to our need whether it is ascending or descending).

  使用Comparator接口可以实现自定义排序。 (自定义排序意味着根据我们的需要是升序还是降序)。

Example:

例：

import java.util.*;
class TreeSetClass {public static void main(String[] args) {// Here we are passing Comparator object in Collection
// class constructor for custoize sorting
TreeSet ts = new TreeSet(new CustomizeSorting());
// adding elements to TreeSet
ts.add(10);
ts.add(40);
ts.add(30);
ts.add(20);
// Customized Sorted List
System.out.println("Customize sorting :" + ts);
}
}
// Here we are implementing Comparator interface
class CustomizeSorting implements Comparator {// Here we are overrding compare() method of Comparator
public int compare(Object obj1, Object obj2) {Integer i1 = (Integer) obj1;
Integer i2 = (Integer) obj2;
return -i1.compareTo(i2);
}
}

Output

输出量

E:\Programs>javac TreeSetClass.java
E:\Programs>java TreeSetClass
Customize sorting :[40, 30, 20, 10]

2)使用可比接口 (2) By using Comparable interface)

• For predefined Comparable classes default natural sorting is already available.

  对于预定义的可比较类，默认的自然排序已可用。

• For predefined Non-Comparable classes default natural sorting is not already available.

  对于预定义的“不可比较”类，默认自然排序尚不可用。

• For our customized classes to define natural sorting then we should go for Comparable.

  为了让我们的自定义类定义自然排序，我们应该选择Comparable。

• In case of default natural sorting compulsory object should be homogenous and Comparable otherwise we will get CCE (ClassCastException).

  在默认情况下，自然排序强制对象应该是同质且可比较的，否则我们将获得CCE(ClassCastException)。

Example:

例：

import java.util.*;
class TreeSetClass {public static void main(String[] args) {Student s1 = new Student(10);
Student s2 = new Student(30);
Student s3 = new Student(70);
Student s4 = new Student(20);
// Here we are not passing Comparator object in Collection
// class constructor for default sorting
TreeSet ts = new TreeSet();
// adding elements to TreeSet
ts.add(s1);
ts.add(s2);
ts.add(s3);
ts.add(s4);
// Customized Sorted List
System.out.println("Default sorting :" + ts);
}
}
// Here we are implementing Comparable interface
class Student implements Comparable {int code;
Student(int code) {this.code = code;
}
public String toString() {return " Code - " + code;
}
// Here we are overrding compare() method of Comparable interface
public int compareTo(Object obj) {int code1 = this.code;
Student intermediate = (Student) obj;
int code2 = intermediate.code;
if (code1 < code2)
return -1;
else if (code1 > code2)
return +1;
else
return 0;
}
}

Output

输出量

E:\Programs>javac TreeSetClass.java
E:\Programs>java TreeSetClass
Default sorting :[ Code - 10,  Code - 20,  Code - 30,  Code - 70]

翻译自: https://www.includehelp.com/java/how-to-sort-objects-of-the-collection-in-java.aspx