Java ArrayList Class

In this tutorial, we will learn about the Java ArrayList class. We will learn about different ArrayList operations and methods with the help of examples.

The ArrayList class is an implementation of the List interface that allows us to create resizable-arrays.

ArrayList类是List接口的实现，允许我们创建可调整大小的数组

Java Array Vs ArrayList

In Java, we need to declare the size of an array before we can use it. Once the size of an array is declared, it's hard to change it.

在Java中，我们需要先声明数组的大小，然后才能使用它。一旦声明了数组的大小，就很难更改它。

To handle this issue, we can use the ArrayList class. The ArrayList class present in the java.util package allows us to create resizable arrays.

要解决此问题，我们可以使用ArrayList类。java.util包中存在的ArrayList类允许我们创建可调整大小的数组。

Unlike arrays, array lists (objects of the ArrayList class) can automatically adjust its capacity when we add or remove elements from it. Hence, array lists are also known as dynamic arrays.

与数组不同，当我们向数组列表添加或删除元素时，数组列表（ArrayList类的对象）可以自动调整容量。因此，数组列表也称为动态数组。

Creating an ArrayList

Here is how we can create array lists in Java:

ArrayList<Type> arrayList= new ArrayList<>();

Here, Type indicates the type of an array list. For example,

// create Integer type arraylist
ArrayList<Integer> arrayList = new ArrayList<>();

// create String type arraylist
ArrayList<String> arrayList = new ArrayList<>();

In the above program, we have used Integer and String. Here, Integer is the corresponding wrapper class of the int type.

在这里，Integer是int类型的相应包装器类

A wrapper class is a class that wraps a primitive data type. For example, the Integer class wraps the int type, the Float class wraps the float type, etc.

包装器类是包装原始数据类型的类。例如，Integer类包装int类型，Float类包装float类型。

Note: We can not create array lists of primitive data types like int, float, char, etc. Instead, we have to use their corresponding wrapper class.

我们不能创建原始数据类型（如int, float, char等）的数组列表。相反，我们必须使用它们对应的包装器类。

In the case of strings, String is a class and doesn't have a wrapper class. Hence, we have used String as it is.

对于字符串，String是一个类，没有包装类。因此，按原样使用String

---

We can also create array lists using the List interface. It's because the ArrayList class implements the List interface.

还可以使用List接口创建数组列表。这是因为ArrayList类实现了List接口

List<String> list = new ArrayList<>();

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Methods of ArrayList

ArrayList provides various methods that allow us to perform array list operations.

ArrayList提供了各种方法，使我们能够执行数组列表操作

Add Elements to an ArrayList

1. Using the add() method

To add a single element to the array list, we use the add() method. For example,

要将单个元素添加到数组列表中，使用add()方法

package com.programiz.arraylist;

import java.util.ArrayList;

public class AddElements {
    
    public static void main(String[] args){
        
        ArrayList<String> animals = new ArrayList<>();

        // Add elements
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("ArrayList: " + animals);
    }
}

Output

ArrayList: [Dog, Cat, Horse]

2. Using index number

We can also add elements to an array list using indexes. For example,

还可使用索引将元素添加到数组列表中

package com.programiz.arraylist;

import java.util.ArrayList;

public class IndexNumber {
    
    public static void main(String[] args){
        
        ArrayList<String> animals = new ArrayList<>();

        // Add elements
        animals.add(0,"Dog");
        animals.add(1,"Cat");
        animals.add(2,"Horse");
        System.out.println("ArrayList: " + animals);
    }
}

Output

ArrayList: [Dog, Cat, Horse]

3. Add elements of an array list to another array list

将数组列表的元素添加到另一个数组列表

To add all the elements of an array list to a new array list, we use the addAll() method. For example,

要将数组列表的所有元素添加到新数组列表中，使用addAll()方法

package com.programiz.arraylist;

import java.util.ArrayList;

puublic class AddAll {
    
    public static void main(String[] args){
        
        ArrayList<String> mammals = new ArrayList<>();
        mammals.add("Dog");
        mammals.add("Cat");
        mammals.add("Horse");
        System.out.println("Mammals: " + mammals);

        ArrayList<String> animals = new ArrayList<>();
        animals.add("Crocodile");

        // Add all elements of mammals in animals
        animals.addAll(mammals);
        System.out.println("Animals: " + animals);
    }
}

Output

Mammals: [Dog, Cat, Horse]
Animals: [Crocodile, Dog, Cat, Horse]

Initialize an ArrayList Using asList()

Unlike arrays, we cannot initialize array lists directly. However, we can use the asList() method of the Arrays class to achieve the same effect.

与数组不同，我们不能直接初始化数组列表。但是，我们可以使用Arrays类的asList()方法来达到相同的效果。

In order to use the asList() method, we must import the java.util.Arrays package first.

为了使用asList()方法，我们必须首先导入java.util.Arrays包。

For example,

package com.programiz.arraylist;

import java.util.ArrayList;
import java.util.Arrays;

public class InitializeArrayList {
    
    public static void main(String[] args) {
        // Creating an array list
        ArrayList<String> animals = new ArrayList<>(Arrays.asList("Cat", "Cow", "Dog"));
        System.out.println("ArrayList: " + animals);

        // Access elements of the array list
        String element = animals.get(1);
        System.out.println("Accessed Element: " + element);
    }
}

Output

ArrayList: [Cat, Cow, Dog]
Accessed Elemenet: Cow

In the above example, notice the expression,

new ArrayList<>(Arrays.asList(("Cat", "Cow", "Dog"));

Here, we have first created an array of 3 elements: "Cat", "Cow", and "Dog". Then, the asList() method is used to convert the array into an array list.

在这里，我们首先创建了一个由3个元素组成的数组：“猫”，“牛”和“狗”。然后，使用asList()方法将数组转换为数组列表。

To learn more about different ways of initializing an ArrayList, visit: Initialization of an ArrayList in one Line

---

Access ArrayList Elements

1. Using get() Method

To randomly access elements of an array list, we use the get() method. For example,

要随机访问数组列表的元素，我们使用get()方法

package com.programiz.arraylist;

import java.util.ArrayList;

public class GetMethod {
    
    public static void main(String[] args) {
        ArrayList<String> animals= new ArrayList<>();

        // Add elements in the array list
        animals.add("Dog");
        animals.add("Horse");
        animals.add("Cat");
        System.out.println("ArrayList: " + animals);

        // Get the element from the array list
        String str = animals.get(0);
        System.out.print("Element at index 0: " + str);
    }
}

Output

ArrayList: [Dog, Horse, Cat]
Element at index 0: Dog

2. Using iterator() Method

To sequentially access elements of an array list, we use the iterator() method. We must import java.util.Iterator package to use this method. For example,

要顺序访问数组列表的元素，我们使用iterator()方法。我们必须导入java.util.Iterator包才能使用此方法。

package com.programiz.arraylist;

import java.util.ArrayList;
import java.util.Iterator;

public class IteratorMethod {
    
    public static void main(String[] args){
        ArrayList<String> animals = new ArrayList<>();

        // Add elements in the array list
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        animals.add("Zebra");

        // Create an object of Iterator
        Iterator<String> iterate = animals.iterator();
        System.out.print("ArrayList: ");

        // Use methods of Iterator to access elements
        while (iterate.hasNext()) {
            
            System.out.print(iterate.next());
            System.out.print(", ");
        }
    }
}

Output

ArrayList: Dog, Cat, Horse, Zebra,

Note:

• hasNext() returns true if there is a next element in the array list.

  如果数组列表中存在下一个元素，则hasNext()返回true。

• next() returns the next element in the array list

  返回数组列表中的下一个元素

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Change ArrayList Elements

To change elements of an array list, we can use the set() method. For example,

要更改数组列表的元素，可以使用set()方法。

import java.util.ArrayList;

class Main {
    public static void main(String[] args) {
        ArrayList<String> animals= new ArrayList<>();
        // Add elements in the array list
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("ArrayList: " + animals);

        // Change the element of the array list
        animals.set(2, "Zebra");
        System.out.println("Modified ArrayList: " + animals);
    }
}

Output

ArrayList: [Dog, Cat, Horse]
Modified ArrayList: [Dog, Cat, Zebra]

---

Remove ArrayList Elements

1. Using remove() Method

To remove an element from an array list, we can use the remove() method. For example,

要从数组列表中删除一个元素，我们可以使用remove()方法

package com.programiz.arraylist;

import java.util.ArrayList;

class Main {
    public static void main(String[] args) {
        ArrayList<String> animals = new ArrayList<>();

        // Add elements in the array list
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("Initial ArrayList: " + animals);

        // Remove element from index 2
        String str = animals.remove(2);
        System.out.println("Final ArrayList: " + animals);
        System. out.println("Removed Element: " + str);
    }
}

Output

Initial ArrayList: [Dog, Cat, Horse]
Final ArrayList: [Dog, Cat]
Removed Element: Horse

---

2. Using removeAll() method

To remove all elements from an array list, we use the removeAll() method. For example,

要从数组列表中删除所有元素，我们使用removeAll()方法

package com.programiz.arraylist;

import java.util.ArrayList;

public class RemoveAllMethod {
    
    public static void main(String[] args) {
        
        ArrayList<String> animals = new ArrayList<>();

        // Add elements in the ArrayList
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("Initial ArrayList: " + animals);

        // Remove all the elements
        animals.removeAll(animals);
        System.out.println("Final ArrayList: " + animals);
    }
}

Output

Initial ArrayList: [Dog, Cat, Horse]
Final ArrayList: []

---

3. Using clear() Method

We can also use the clear() method to remove all elements from an array list. For example,

我们还可以使用clear()方法从数组列表中删除所有元素

import java.util.ArrayList;

public class ClearMethod {
    
    public static void main(String[] args) {
        ArrayList<String> animals= new ArrayList<>();

        // Add elements in the array list
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("Initial ArrayList: " + animals);

        // Remove all the elements
        animals.clear();
        System.out.println("Final ArrayList: " + animals);
    }
}

Output

Initial ArrayList: [Dog, Cat, Horse]
Final ArrayList: []

Note: The clear() method is more efficient than the removeAll() method.

clear()方法比removeAll()方法更有效

---

Loop Through an ArrayList

1. Using for loop

package com.programiz.arraylist;

import java.util.ArrayList;

public class ForLoop {
    
    public static void main(String[] args) {
        // Creating an array list
        ArrayList<String> animals = new ArrayList<>();
        animals.add("Cow");
        animals.add("Cat");
        animals.add("Dog");
        System.out.println("ArrayList: " + animals);

        // Using for loop
        System.out.println("Accessing individual elements: ");

        for(int i = 0; i < animals.size(); i++) {
            System.out.print(animals.get(i));
            System.out.print(", ");
        }
    }
}

Output

ArrayList: [Cow, Cat, Dog]
Accessing individual elements:
Cow, Cat, Dog,

---

2. Using the forEach loop

package com.programiz.arraylist;

import java.util.ArrayList;

public class ForEachLoop {
    
    public static void main(String[] args) {
        // Creating an array list
        ArrayList<String> animals = new ArrayList<>();
        animals.add("Cow");
        animals.add("Cat");
        animals.add("Dog");
        System.out.println("ArrayList: " + animals);

        // Using forEach loop
        System.out.println("Accessing individual elements:  ");
        for(String animal : animals) {
            System.out.print(animal);
            System.out.print(", ");
        }
    }
}

Output

ArrayList: [Cow, Cat, Dog]
Accessing individual elements:
Cow, Cat, Dog,

In both examples, we have accessed individual elements of an array list using loops.

在两个示例中，我们都使用循环访问了数组列表的各个元素。

---

Get ArrayList Length

To get the length of the array list, we use the size() method. For example,

package com.programiz.arraylist;

import java.util.ArrayList;

public class SizeMethod {
    
    public static void main(String[] args) {
        
        ArrayList<String> animals= new ArrayList<>();

        // Adding elements in the arrayList
        animals.add("Dog");
        animals.add("Horse");
        animals.add("Cat");
        System.out.println("ArrayList: " + animals);

        // getting the size of the arrayList
        System.out.println("Size: " + animals.size());
    }
}

Output

ArrayList: [Dog, Horse, Cat]
Size: 3

---

Sort Elements of an ArrayList

To sort elements of an array list, we use the sort() method of the Collections class. In order to use it, we must import the java.util.Collections package first.

为了对数组列表的元素进行排序，我们使用Collections类的sort()方法。为了使用它，我们必须首先导入java.util.Collections包。

By default, the sorting occurs either alphabetically or numerically in ascending order. For example,

默认情况下，排序以字母或数字升序进行。例如，

package com.programiz.arraylist;

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

public class SortMethod {
    
    public static void main(String[] args){
        
        ArrayList<String> animals= new ArrayList<>();

        // Add elements in the array list
        animals.add("Horse");
        animals.add("Zebra");
        animals.add("Dog");
        animals.add("Cat");

        System.out.println("Unsorted ArrayList: " + animals);

        // Sort the array list
        Collections.sort(animals);
        System.out.println("Sorted ArrayList: " + animals);
    }
}

Output

Unsorted ArrayList: [Horse, Zebra, Dog, Cat]
Sorted ArrayList: [Cat, Dog, Horse, Zebra]

To learn more about sorting array list, visit Java ArrayList sort.

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Java ArrayList To Array

In Java, we can convert array lists into arrays using the toArray() method. For example,

在Java中，我们可以使用toArray()方法将数组列表转换为数组。例如，

package com.programiz.arraylist;

import java.util.ArrayList;

public class ToArrayMethod {
    
    public static void main(String[] args) {
        
        ArrayList<String> animals= new ArrayList<>();

        // Add elements in the array list
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("ArrayList: " + animals);

        // Create a new array of String type
        String[] arr = new String[animals.size()];

        // Convert ArrayList into an array
        animals.toArray(arr);
        System.out.print("Array: ");
        
        for(String item : arr) {
            System.out.print(item + ", ");
        }
    }
}

Output

ArrayList: [Dog, Cat, Horse]
Array: Dog, Cat, Horse,

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Java Array to ArrayList

We can also convert arrays into array lists. For that, we can use the asList() method of the Arrays class.

我们还可以将数组转换为数组列表。为此，我们可以使用Arrays类的asList()方法。

To use asList(), we must import the java.util.Arrays package first. For example,

package com.programiz.arraylist;

import java.util.ArrayList;
import java.util.Arrays;

public class AsListMethod {
    
    public static void main(String[] args) {
        
        // Create an array of String type
        String[] arr = {"Dog", "Cat", "Horse"};
        System.out.print("Array: ");

        // Print array
        for(String str : arr) {
            
            System.out.print(str);
            System.out.print(" ");
        }

        // Create an ArrayList from an array
        ArrayList<String> animals = new ArrayList<>(Arrays.asList(arr));
        System.out.println("
ArrayList: " + animals);
    }
}

Output

Array: Dog, Cat, Horse
ArrayList: [Dog, Cat, Horse]

In the above program, we first created an array arr of the String type.

在上面的程序中，我们首先创建了String类型的数组arr

We then converted the array into an array list using the asList() method.

然后，使用asList()方法将数组转换为数组列表

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Java ArrayList to String

To convert an array list into a String, we can use the toString() method. For example,

package com.programiz.arraylist;

import java.util.ArrayList;

public class ToStringMethod {
    
    public static void main(String[] args) {
        
        ArrayList<String> animals = new ArrayList<>();

        // Add elements in the ArrayList
        animals.add("Dog");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("ArrayList: " + animals);

        // Convert ArrayList into an String
        String str = animals.toString();
        System.out.println("String: " + str);
    }
}

Output

ArrayList: [Dog, Cat, Horse]
String: [Dog, Cat, Horse]

Note: toString() converts the whole array list into a single String.

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Other ArrayList Methods

Methods	Descriptions
clone()	Creates a new array list with the same element, size, and capacity.
contains()	Searches the array list for the specified element and returns a boolean result.
ensureCapacity()	Specifies the total element the array list can contain.
isEmpty()	Checks if the array list is empty.
indexOf()	Searches a specified element in an array list and returns the index of the element.
trimToSize()	Reduces the capacity of an array list to its current size.