Java List Sorting: Methods and Examples

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問題:

Introduce Java List Sorting and give examples

答案:

In Java, the List interface is part of the Java Collections Framework and represents an ordered collection (also known as a sequence). The List interface provides various methods to manipulate the elements in the list, including sorting. Sorting a list can be done using the Collections.sort() method or the List.sort() method, both of which allow you to sort the elements in natural order or using a custom comparator.

Sorting a List in Java

  1. Using Collections.sort(): The Collections.sort() method sorts the specified list into ascending order, according to the natural ordering of its elements.

    Example:

    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.List;
    
    public class ListSortingExample {
        public static void main(String[] args) {
            List<String> fruits = new ArrayList<>();
            fruits.add("Banana");
            fruits.add("Apple");
            fruits.add("Orange");
            fruits.add("Mango");
    
            // Sorting the list in natural order
            Collections.sort(fruits);
    
            System.out.println("Sorted fruits: " + fruits);
        }
    }
    

    Output:

    Sorted fruits: [Apple, Banana, Mango, Orange]
    
  2. Using List.sort(): The List.sort() method is a default method in the List interface that sorts the list using a specified comparator.

    Example:

    import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.List;
    
    public class ListSortingWithComparator {
        public static void main(String[] args) {
            List<String> fruits = new ArrayList<>();
            fruits.add("Banana");
            fruits.add("Apple");
            fruits.add("Orange");
            fruits.add("Mango");
    
            // Sorting the list in reverse order using a comparator
            fruits.sort(Comparator.reverseOrder());
    
            System.out.println("Sorted fruits in reverse order: " + fruits);
        }
    }
    

    Output:

    Sorted fruits in reverse order: [Orange, Mango, Banana, Apple]
    
  3. Sorting a List of Custom Objects: You can also sort a list of custom objects by implementing the Comparable interface or by using a Comparator.

    Example:

    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Comparator;
    import java.util.List;
    
    class Person {
        String name;
        int age;
    
        Person(String name, int age) {
            this.name = name;
            this.age = age;
        }
    
        @Override
        public String toString() {
            return name + " (" + age + ")";
        }
    }
    
    public class CustomObjectSorting {
        public static void main(String[] args) {
            List<Person> people = new ArrayList<>();
            people.add(new Person("Alice", 30));
            people.add(new Person("Bob", 25));
            people.add(new Person("Charlie", 35));
    
            // Sorting by age using a comparator
            Collections.sort(people, Comparator.comparingInt(person -> person.age));
    
            System.out.println("Sorted people by age: " + people);
        }
    }
    

    Output:

    Sorted people by age: [Bob (25), Alice (30), Charlie (35)]
    

Summary

  • You can sort lists in Java using Collections.sort() or List.sort().
  • Sorting can be done in natural order or using custom comparators.
  • You can sort lists of custom objects by implementing the Comparable interface or by providing a Comparator.

These methods provide a flexible way to sort data in Java applications.