ArrayList
- Add element
- access element
- Modify elements
- Delete element
- Calculate size
- Iterate over a list of arrays
- Other reference types
- ArrayList sorting
- Java ArrayList method
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introduction
The ArrayList class is an array that can be dynamically modified. The difference from an ordinary array is that it has no fixed size limit and we can add or delete elements. ArrayList inherits AbstractList and implements the List interface.
The ArrayList class is located in the java.util package and needs to be introduced before use. The syntax format is as follows:
import java.util.ArrayList; //Introduce the ArrayList class ArrayList<E> objectName =new ArrayList<>(); // Initialization
- E: Generic data type, used to set the data type of objectName, which can only be reference data types.
- objectName: object name.
- ArrayList is an array queue that provides related functions such as adding, deleting, modifying, and traversing.
Add element
The ArrayList class provides many useful methods. To add elements to an ArrayList, you can use the add() method:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); System.out.println(sites); } }
For the above example, the execution output is:
[Google, Runoob, Taobao, Weibo]
Access elements
To access elements in an ArrayList, use the get() method:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); System.out.println(sites.get(1)); // Access the second element } }
Note: Array index values start from 0.
For the above example, the execution output is:
Runoob
Modify elements
If you want to modify the elements in the ArrayList, you can use the set() method:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); sites.set(2, "Wiki"); // The first parameter is the index position, the second is the value to be modified System.out.println(sites); } }
For the above example, the execution output is:
[Google, Runoob, Wiki, Weibo]
Delete element
If you want to delete elements in ArrayList, you can use the remove() method:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); sites.remove(3); // Remove the fourth element System.out.println(sites); } }
For the above example, the execution output is:
[Google, Runoob, Taobao]
Calculate size
If you want to count the number of elements in an ArrayList, you can use the size() method:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); System.out.println(sites.size()); } }
For the above example, the execution output is:
4
Iterate over array list
We can use for to iterate over the elements in an array list:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); for (int i = 0; i < sites.size(); i + + ) {<!-- --> System.out.println(sites.get(i)); } } }
For the above example, the execution output is:
Runoob
Taobao
You can also use for-each to iterate over elements:
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Google"); sites.add("Runoob"); sites.add("Taobao"); sites.add("Weibo"); for (String i : sites) {<!-- --> System.out.println(i); } } }
For the above example, the execution output is:
Runoob
Taobao
Other reference types
The elements in ArrayList are actually objects. In the above example, the array list elements are all of type String.
If we want to store other types, but only reference data types, then we need to use a wrapper class of the basic type.
The packaging class table corresponding to the basic types is as follows:
In addition, BigInteger and BigDecimal are used for high-precision operations. BigInteger supports arbitrary-precision integers and is also a reference type, but they do not have corresponding basic types.
ArrayList<Integer> li=new ArrayList<>(); // Store integer elements ArrayList<Character> li=new ArrayList<>(); // Store character elements
The following example uses an ArrayList to store numbers (using the Integer type):
Example
import java.util.ArrayList; public class RunoobTest {<!-- --> 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); } } }
For the above example, the execution output is:
10
15
20
25
ArrayList sorting
The Collections class is also a very useful class located in the java.util package. It provides the sort() method to sort a list of characters or numbers.
The following example sorts letters:
Example
import java.util.ArrayList; import java.util.Collections; //Introduce Collections class public class RunoobTest {<!-- --> public static void main(String[] args) {<!-- --> ArrayList<String> sites = new ArrayList<String>(); sites.add("Taobao"); sites.add("Wiki"); sites.add("Runoob"); sites.add("Weibo"); sites.add("Google"); Collections.sort(sites); // Alphabetical sorting for (String i : sites) {<!-- --> System.out.println(i); } } }
For the above example, the execution output is:
Runoob
Taobao
Wiki
The following example sorts numbers:
Example
import java.util.ArrayList; import java.util.Collections; //Introduce Collections class public class RunoobTest {<!-- --> 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); //Number sorting for (int i : myNumbers) {<!-- --> System.out.println(i); } } }
For the above example, the execution output is:
8
12
15
20
33
34
Java ArrayList method
The list of common Java ArrayList methods is as follows:
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Version record
- 2023-10-18 First Edition