Java ArrayList
Java ArrayList
The ArrayList class is a resizable array, which can be found in the java.util package.
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
示例
Create an ArrayList object called cars that will store strings
import java.util.ArrayList; // import the ArrayList class
ArrayList<String> cars = new ArrayList<String>(); // 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 list, use the add() method
示例
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
System.out.println(cars);
}
}You can also add an item at a specified position by referring to the index number
示例
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add(0, "Mazda"); // Insert element at the beginning of the list (0)
System.out.println(cars);
}
}Remember: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Access an Item
To access an element in the ArrayList, use the get() method and refer to the index number
Change an Item
To modify an element, use the set() method and refer to the index number
Remove an Item
To remove an element, use the remove() method and refer to the index number
To remove all the elements in the ArrayList, use the clear() method
ArrayList Size
To find out how many elements an ArrayList have, use the size method
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
示例
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
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));
}
}
}
You can also loop through an ArrayList with the for-each loop
示例
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
for (String i : cars) {
System.out.println(i);
}
}
}
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
示例
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<Integer> myNumbers = new ArrayList<Integer>();
myNumbers.add(10);
myNumbers.add(15);
myNumbers.add(20);
myNumbers.add(25);
for (int i : myNumbers) {
System.out.println(i);
}
}
}
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
示例
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<String> cars = new ArrayList<String>();
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);
}
}
}
示例
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<Integer> myNumbers = new ArrayList<Integer>();
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);
}
}
}
Complete ArrayList Reference
For a complete reference of ArrayList methods, go to our Java ArrayList Reference.
