How to Sort a List of ArrayList in Java

To sort a list of ArrayList in Java, you can use the Collections.sort() method or the List.sort() method. Both methods allow you to sort the elements of a list in ascending order, based on their natural order or using a custom comparator. Here are the steps to sort a list in Java:

Using Collections.sort()

1. Import the necessary classes:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

2. Create an ArrayList and add elements to it:

List list = new ArrayList();
list.add("Apple");
list.add("Orange");
list.add("Banana");

3. Sort the list using the Collections.sort() method:

Collections.sort(list);

4. The list is now sorted in ascending order.

Using List.sort()

1. Import the necessary classes:

import java.util.ArrayList;
import java.util.List;

2. Create an ArrayList and add elements to it:

List list = new ArrayList();
list.add("Apple");
list.add("Orange");
list.add("Banana");

3. Sort the list using the List.sort() method and a lambda expression:

list.sort((a, b) -> a.compareTo(b));

4. The list is now sorted in ascending order.

Sorting in Descending Order

To sort the list in descending order, you can use the Collections.reverseOrder() method as a comparator. Here’s how:

1. Import the necessary classes:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

2. Create an ArrayList and add elements to it:

List list = new ArrayList();
list.add("Apple");
list.add("Orange");
list.add("Banana");

3. Sort the list in descending order using the Collections.sort() method and Collections.reverseOrder() as a comparator:

Collections.sort(list, Collections.reverseOrder());

4. The list is now sorted in descending order.

Sorting Custom Objects

If you have a list of custom objects, you can implement the Comparable interface in your object class and override the compareTo() method to specify how the objects should be sorted. Here’s an example:

public class Person implements Comparable {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Getters and setters

    @Override
    public int compareTo(Person other) {
        return this.age - other.age;
    }
}

Now, you can sort a list of Person objects based on their age:

List list = new ArrayList();
list.add(new Person("John", 25));
list.add(new Person("Alice", 30));
list.add(new Person("Bob", 20));

Collections.sort(list);

// The list is now sorted based on age in ascending order

Note: If you want to sort the list in descending order, you can modify the compareTo() method to return other.age - this.age.