Java ArrayList

Java 集合框架Java Collections Framework

The ArrayList class is an array that can be dynamically modified. The difference from ordinary arrays is that it has no fixed size limit, and we can add or delete elements.

ArrayList inherits from AbstractList and implements the List interface.

The ArrayList class is located in the java.util package. You need to import it before use. The syntax format is as follows:

import java.util.ArrayList; // 引入 ArrayList 类

ArrayList<E> objectName =new ArrayList<>();　 // 初始化
  • E: generic data type, used to set the data type of objectName,can only be reference data types。
  • objectName: object name.

ArrayList is an array queue that provides related functions such as add, delete, modify, and traverse.

Add Element

The ArrayList class provides many useful methods. You can use the add() method to add elements to an ArrayList:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites);
    }
}

The above example executes and the output result is:

[Google, Example, Taobao, Weibo]

Access Element

To access an element in an ArrayList, you can use the get() method:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites.get(1));  // Access the second element
    }
}

Note: The index value of an array starts from 0.

The above example executes and the output result is:

Example

Modify Element

To modify an element in an ArrayList, you can use the set() method. The first parameter of the set(int index, E element) method is the index, indicating the position of the element to be replaced; the second parameter is the new element, indicating the new value to be set:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        sites.set(2, "Wiki"); // The first parameter is the index position, the second is the value to modify
        System.out.println(sites);
    }
}

The above example executes and the output result is:

[Google, Example, Wiki, Weibo]

Delete Element

To delete an element in an ArrayList, you can use the remove() method:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        sites.remove(3); // Delete the fourth element
        System.out.println(sites);
    }
}

The above example executes and the output result is:

[Google, Example, Taobao]

Calculate Size

To calculate the number of elements in an ArrayList, you can use the size() method:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        System.out.println(sites.size());
    }
}

The above example executes and the output result is:

4

Iterate ArrayList

We can use for to iterate over the elements in the array list:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        for (int i = 0; i < sites.size(); i++) {
            System.out.println(sites.get(i));
        }
    }
}

The above example executes and the output result is:

Google
Example
Taobao
Weibo

You can also use for-each to iterate over elements:

Example

import java.util.ArrayList;

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Example");
        sites.add("Taobao");
        sites.add("Weibo");
        for (String i : sites) {
            System.out.println(i);
        }
    }
}

The above example executes and the output result is:

Google
Example
Taobao
Weibo

Other Reference Types

The elements in an ArrayList are actually objects. In the above examples, the array list elements are all String types.

If we want to store other types, and <E> can only be reference data types, then we need to use the wrapper classes of basic types.

The wrapper class table corresponding to basic types is as follows:

Basic TypeReference Type
booleanBoolean
byteByte
shortShort
intInteger
longLong
floatFloat
doubleDouble
charCharacter

In addition, BigInteger and BigDecimal are used for high-precision operations. BigInteger supports arbitrary precision integers. They are also reference types, but they do not have corresponding basic types.

ArrayList<Integer> li=new ArrayList<>();     // 存放整数元素
ArrayList<Character> li=new ArrayList<>();   // 存放字符元素

The following example uses ArrayList to store numbers (using Integer type):

Example

import java.util.ArrayList;

public class ExampleTest {
    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);
        }
    }
}

The above example executes and the output result is:

10
15
20
25

ArrayList Sorting

The Collections class is also a very useful class, located in the java.util package. Its sort() method can sort character or number lists.

The following example sorts letters:

Example

import java.util.ArrayList;
import java.util.Collections;  // Import the Collections class

public class ExampleTest {
    public static void main(String[] args) {
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Taobao");
        sites.add("Wiki");
        sites.add("Example");
        sites.add("Weibo");
        sites.add("Google");
        Collections.sort(sites);  // Sort letters
        for (String i : sites) {
            System.out.println(i);
        }
    }
}

The above example executes and the output result is:

Google
Example
Taobao
Weibo
Wiki

The following example sorts numbers:

Example

import java.util.ArrayList;
import java.util.Collections;  // Import the Collections class

public class ExampleTest {
    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 numbers

        for (int i : myNumbers) {
            System.out.println(i);
        }
    }
}

The above example executes and the output result is:

8
12
15
20
33
34

Java ArrayList Methods

The list of commonly used Java ArrayList methods is as follows:

Method Description
add() Insert an element into the arraylist at the specified position
addAll() Add all elements in a collection to the arraylist
clear() Remove all elements from the arraylist
clone() Copy a arraylist
contains() Determine whether an element is in the arraylist
get() Get the element in the arraylist by index value
indexOf() Return the index value of an element in the arraylist
removeAll() Remove all elements in the arraylist that exist in the specified collection
remove() Remove a single element from the arraylist
size() Return the number of elements in the arraylist
isEmpty() Determine whether the arraylist is empty
subList() Extract a portion of the arraylist elements
set() Replace the element at the specified index in the arraylist
sort() Sort the arraylist elements
toArray() Convert the arraylist to an array
toString() Convert the arraylist to a string
ensureCapacity() Set the arraylist with the specified capacity
lastIndexOf() Return the last occurrence position of the specified element in the arraylist
retainAll() Retain those elements in the arraylist that also exist in the specified collection
containsAll() Check whether the arraylist contains all elements in the specified collection
trimToSize() Adjust the capacity of the arraylist to the number of elements in the array
removeRange() Delete elements between the specified indexes in the arraylist
replaceAll() Replace each element in the array with the given operation content
removeIf() Remove all arraylist elements that satisfy specific conditions
forEach() Traverse each element in the arraylist and perform a specific operation

For more API methods, you can check:https://www.example.com/manual/jdk11api/java.base/java/util/ArrayList.html

Java 集合框架Java Collections Framework

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