Comparable
接口主要用于对自定义对象的数组(或列表)进行排序。
实现Comparable
接口的对象列表(和数组)可以由Collections.sort
(和Arrays.sort
)自动排序。在我们看到如何对自定义对象的对象进行排序之前,让我们看看如何对已经实现 Comparable
的数组元素和包装器类进行排序。
示例:对数组和包装器类进行排序
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class Demo {
public static void main(String[] args) {
/*
* Integer class implements Comparable
* Interface so we can use the sort method
*/
int[] arr = {11,55,22,0,89};
Arrays.sort(arr);
System.out.print("Sorted Int Array: ");
System.out.println(Arrays.toString(arr));
/*
* String class implements Comparable
* Interface so we can use the sort method
*/
System.out.print("Sorted String Array: ");
String[] names = {"Steve", "Ajeet", "Kyle"};
Arrays.sort(names);
System.out.println(Arrays.toString(names));
/*
* String class implements Comparable
* Interface so we can use the sort method
*/
System.out.print("Sorted List: ");
List fruits = new ArrayList();
fruits.add("Orange");
fruits.add("Banana");
fruits.add("Apple");
fruits.add("Guava");
fruits.add("Grapes");
Collections.sort(fruits);
for(String s: fruits) System.out.print(s+", ");
}
}
输出:
Sorted Int Array: [0, 11, 22, 55, 89]
Sorted String Array: [Ajeet, Kyle, Steve]
Sorted List: Apple, Banana, Grapes, Guava, Orange,
在上面的示例中,您已经看到对实现Comparable
接口的数组和对象列表进行排序是多么容易,您只需要调用Collections.sort
(和Arrays.sort
)。
但是,如果要对自定义类的对象进行排序,则需要在自定义类中实现Comparable
接口。
该接口只有一个方法:
public abstract int compareTo(T obj)
由于此方法是抽象的,因此如果实现Comparable
接口,则必须在类中实现此方法。
让我们举个例子来更好地理解这个:
示例:通过实现Comparable
接口对自定义对象进行排序
正如您所看到的,我在Author
类中实现了Comparable
接口,因为我想对这个类的对象进行排序。我已经在compareTo()
方法中编写了排序逻辑,您可以根据需求编写逻辑。我想先按姓氏排序作者姓名,如果姓氏相同,则按名字排序。如果您只想按姓氏排序,那么compareTo()
方法中的第一行就足够了。
Author
类
public class Author implements Comparable<Author> {
String firstName;
String lastName;
String bookName;
Author(String first, String last, String book){
this.firstName = first;
this.lastName = last;
this.bookName = book;
}
@Override
/*
* This is where we write the logic to sort. This method sort
* automatically by the first name in case that the last name is
* the same.
*/
public int compareTo(Author au){
/*
* Sorting by last name. compareTo should return < 0 if this(keyword)
* is supposed to be less than au, > 0 if this is supposed to be
* greater than object au and 0 if they are supposed to be equal.
*/
int last = this.lastName.compareTo(au.lastName);
//Sorting by first name if last name is same d
return last == 0 ? this.firstName.compareTo(au.firstName) : last;
}
}
排序类:SortAuthByNames
import java.util.ArrayList;
import java.util.Collections;
public class SortAuthByNames{
public static void main(String args[]){
// List of objects of Author class
ArrayList<Author> al=new ArrayList<Author>();
al.add(new Author("Henry","Miller", "Tropic of Cancer"));
al.add(new Author("Nalo","Hopkinson", "Brown Girl in the Ring"));
al.add(new Author("Frank","Miller", "300"));
al.add(new Author("Deborah","Hopkinson", "Sky Boys"));
al.add(new Author("George R. R.","Martin", "Song of Ice and Fire"));
/*
* Sorting the list using Collections.sort() method, we
* can use this method because we have implemented the
* Comparable interface in our user defined class Author
*/
Collections.sort(al);
for(Author str:al){
System.out.println(str.firstName+" "+
str.lastName+" "+"Book: "+str.bookName);
}
}
}
输出:
Deborah Hopkinson Book: Sky Boys
Nalo Hopkinson Book: Brown Girl in the Ring
George R. R. Martin Book: A Song of Ice and Fire
Frank Miller Book: 300
Henry Miller Book: Tropic of Cancer
注意:我们应该以这样的方式编写
compareTo()
方法:如果这个(我在这里指的是this
关键字)小于传递的对象那么它应该返回负数,如果大于正数则为零,如果相等则返回 0。
你可能想知道为什么我没有写那个逻辑?因为名字和姓氏是字符串,所以我调用了字符串类的compareTo()
方法,它完全相同。
但是如果我们比较的东西是其他类型的东西,比如int
那么你就可以编写这样的逻辑:
假设Employee
类的对象是(empId
,empName
,empAge
),我们想通过empAge
对对象进行排序]。
public int compareTo(Employee e){
if(this.empAge==e.empAge)
return 0;
else if(this.empAge>e.empAge)
return 1;
else
return -1;
}
或
public int compareTo(Employee e){
return this.empAge > e.empAge ? 1 : this.empAge < e.empAge ? -1 : 0;
}