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Thủ Thuật Hướng dẫn ArrayList to List Android 2022
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Java ArrayList
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Java ArrayList
The ArrayList class is a resizable array, which can be found in the java.util package.
Nội dung chính
- Java ArrayListJava ArrayListAccess an ItemChange an ItemRemove an ItemArrayList SizeLoop Through an ArrayListOther TypesSort an ArrayListVideo liên quan
The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified (if you want to
add or remove elements to/from an array, you have to create a new one). While elements can be added and removed from an ArrayList whenever you want. The syntax is also slightly different:
Example
Create an ArrayList object called cars that will store strings:
import java.util.ArrayList; // import the ArrayList class
ArrayList cars = new ArrayList(); // Create an ArrayList object
If you don’t know what a package is, read our Java Packages Tutorial.
Add Items
The ArrayList class has many useful methods. For example, to add elements to the ArrayList, use the add() method:
Example
import java.util.ArrayList;
public class Main
public static void main(String[] args)
ArrayList cars = new ArrayList();
cars.add(“Volvo”);
cars.add(“BMW”);
cars.add(“Ford”);
cars.add(“Mazda”);
System.out.println(cars);
Try it Yourself »
Access an Item
To access an element in the ArrayList, use the get() method and refer to the index number:
Example
cars.get(0);
Try it Yourself »
Remember: Array indexes start with 0: [0] is the first element. [1] is the second
element, etc.
Change an Item
To modify an element, use the set() method
and refer to the index number:
Example
cars.set(0, “Opel”);
Try it Yourself »
Remove an Item
To remove an element, use the remove() method
and refer to the index number:
Example
cars.remove(0);
Try it Yourself »
To remove all the elements in the ArrayList, use the clear() method:
Example
cars.clear();
Try it Yourself »
ArrayList Size
To find out how many elements an ArrayList have, use the size method:
Example
cars.size();
Try it Yourself »
Loop Through an ArrayList
Loop through the elements of an ArrayList with a for loop, and use the
size() method to specify how many times the loop should run:
Example
public class Main
public static void main(String[] args)
ArrayList cars = new ArrayList();
cars.add(“Volvo”);
cars.add(“BMW”);
cars.add(“Ford”);
cars.add(“Mazda”);
for (int i = 0; i < cars.size(); i++)
System.out.println(cars.get(i));
Try it Yourself »
You can also loop through an ArrayList with the for-each loop:
Example
public class Main
public static void main(String[] args)
ArrayList cars = new ArrayList();
cars.add(“Volvo”);
cars.add(“BMW”);
cars.add(“Ford”);
cars.add(“Mazda”);
for (String i : cars)
System.out.println(i);
Try it Yourself »
Other Types
Elements in an ArrayList are actually objects. In the examples above, we created elements
(objects) of type “String”. Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types,
use: Boolean for boolean, Character for char, Double for double,
etc:
Example
Create an ArrayList to store numbers (add elements of type Integer):
import java.util.ArrayList;
public class Main
public static void main(String[] args)
ArrayList myNumbers = new ArrayList();
myNumbers.add(10);
myNumbers.add(15);
myNumbers.add(20);
myNumbers.add(25);
for (int i : myNumbers)
System.out.println(i);
Try it Yourself »
Sort an ArrayList
Another useful class in the java.util package is the Collections class, which include the sort() method for sorting lists
alphabetically or numerically:
Example
Sort an ArrayList of Strings:
import java.util.ArrayList;
import java.util.Collections; // Import the Collections class
public class Main
public static void main(String[] args)
ArrayList cars = new ArrayList();
cars.add(“Volvo”);
cars.add(“BMW”);
cars.add(“Ford”);
cars.add(“Mazda”);
Collections.sort(cars); // Sort cars
for (String i : cars)
System.out.println(i);
Try it Yourself »
Example
Sort an ArrayList of Integers:
import java.util.ArrayList;
import java.util.Collections; // Import the Collections class
public class Main
public static void main(String[] args)
ArrayList myNumbers = new ArrayList();
myNumbers.add(33);
myNumbers.add(15);
myNumbers.add(20);
myNumbers.add(34);
myNumbers.add(8);
myNumbers.add(12);
Collections.sort(myNumbers); // Sort myNumbers
for (int i : myNumbers)
System.out.println(i);
Try it Yourself »
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