Java ArrayList Tutorial with Examples

Posted 2020-12-13 PrimerPlus

ArrayList in Java is used to store dynamically sized collection of elements. Contrary to Arrays that are fixed in size, an ArrayList grows its size automatically when new elements are added to it.

Java中的ArrayList用于存储动态调整大小的元素集合。与固定大小的数组相反，当向其添加新元素时，ArrayList会自动增加其大小

ArrayList is part of Java’s collection framework and implements Java’s List interface.

ArrayList是Java集合框架的一部分，并实现Java的List接口

Following are few key points to note about ArrayList in Java -

以下是有关Java中ArrayList的几点注意事项

• An ArrayList is a re-sizable array, also called a dynamic array. It grows its size to accommodate new elements and shrinks the size when the elements are removed.

  ArrayList是可调整大小的数组，也称为动态数组。它会增加其大小以容纳新元素，并在删除元素时缩小其大小

• ArrayList internally uses an array to store the elements. Just like arrays, It allows you to retrieve the elements by their index.

  ArrayList在内部使用数组存储元素，就像数组一样，它允许您按元素索引检索元素

• Java ArrayList allows duplicate and null values.

  Java ArrayList 允许重复和空值

• Java ArrayList is an ordered collection. It maintains the insertion order of the elements.

  Java ArrayList是一个有序集合。它保持元素的插入顺序

• You cannot create an ArrayList of primitive types like int, char etc. You need to use boxed types like Integer, Character, Boolean etc.

  您不能创建基本类型（如int, char等）的ArrayList 您需要装箱的类型（如Integer, Character, Boolean等）

• Java ArrayList is not synchronized. If multiple threads try to modify an ArrayList at the same time, then the final outcome will be non-deterministic. You must explicitly synchronize access to an ArrayList if multiple threads are gonna modify it.、

  Java ArrayList不同步。如果多个线程尝试同时修改ArrayList，则最终结果将是不确定的。如果要修改多个线程，则必须显式同步对ArrayList的访问

Creating an ArrayList and adding new elements to it

创建一个ArrayList并向其中添加新元素

This example shows:

• How to create an ArrayList using the ArrayList() constructor.

  如何使用ArrayList()构造函数创建ArrayList

• Add new elements to an ArrayList using the add() method.

  使用add()方法将新元素添加到ArrayList

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;

public class CreateArrayListExample {

    public static void main(String[] args) {

        // Creating an ArrayList of String
        // 创建字符串的ArrayList
        List<String> animals = new ArrayList<>();

        // Adding new elements to the ArrayList
        // 向ArrayList中添加新元素
        animals.add("Lion");
        animals.add("Tiger");
        animals.add("Cat");
        animals.add("Dog");

        System.out.println(animals);

        // Adding an element at a particular index in an ArrayList
        // 在ArrayList中的特定索引处添加元素
        animals.add(2, "Elephant");

        System.out.println(animals);
    }
}

Output

[Lion, Tiger, Cat, Dog]
[Lion, Tiger, Elephant, Cat, Dog]

Creating an ArrayList from another collection

从另一个集合创建ArrayList

This example shows:

• How to create an ArrayList from another collection using the ArrayList(Collection c) constructor.

  如何使用ArrayList(Collection c)构造函数从另一个集合创建ArrayList

• How to add all the elements from an existing collection to the new ArrayList using the addAll() method.

  如何使用addAll()方法将现有集合中的所有元素添加到新的ArrayList中

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;

public class CreateArrayListFromCollectionExample {

    public static void main(String[] args) {

        List<Integer> firstFivePrimeNumbers = new ArrayList<>();
        firstFivePrimeNumbers.add(2);
        firstFivePrimeNumbers.add(3);
        firstFivePrimeNumbers.add(5);
        firstFivePrimeNumbers.add(7);
        firstFivePrimeNumbers.add(11);

        // Create an ArrayList from another collection
        // 从另一个集合创建一个ArrayList
        List<Integer> firstTenPrimeNumbers = new ArrayList<>(firstFivePrimeNumbers);

        List<Integer> nextFivePrimeNumbers = new ArrayList<>();
        nextFivePrimeNumbers.add(13);
        nextFivePrimeNumbers.add(17);
        nextFivePrimeNumbers.add(19);
        nextFivePrimeNumbers.add(23);
        nextFivePrimeNumbers.add(29);

        // Adding an entries collection to an ArrayList
        // 将整个集合添加到ArrayList
        firstTenPrimeNumbers.addAll(nextFivePrimeNumbers);

        System.out.println(firstTenPrimeNumbers);
    }
}

Output

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Accessing elements from an ArrayList

从ArrayList访问元素

This example shows:

• How to check if an ArrayList is empty using the isEmpty() method.

  如何使用isEmpty()方法检查ArrayList是否为空

• How to find the size of an ArrayList using the size() method.

  如何使用size()方法查找ArrayList的大小

• How to access the element at a particular index in an ArrayList using the get() method.

  如何使用get()方法访问ArrayList中特定索引处的元素

• How to modify the element at a particular index in an ArrayList using the set() method.

  如何使用set()方法修改ArrayList中特定索引处的元素

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;

public class AccessElementsFromArrayListExample {

    public static void main(String[] args) {
        List<String> topCompanies = new ArrayList<>();

        // Check if an ArrayList is empty
        // 检查ArrayList是否为空
        // topCompanies列表是否为空
        System.out.println("Is the topCompanies list empty? : " + topCompanies.isEmpty());

        topCompanies.add("Google");
        topCompanies.add("Apple");
        topCompanies.add("Microsoft");
        topCompanies.add("Amazon");
        topCompanies.add("Facebook");

        // Find the size of ArrayList
        // 查找ArrayList的大小
        // 这是世界排名前五的公司
        System.out.println("Here are the top " + topCompanies.size() + " companies in the world");
        System.out.println(topCompanies);

        // Retrieve the element at a given index
        // 给定索引检查元素
        String bestCompany = topCompanies.get(0);
        String secondBestCompany = topCompanies.get(1);
        String lastCompany = topCompanies.get(topCompanies.size() - 1);

        System.out.println("Best Company: " + bestCompany);
        System.out.println("Second Best Company: " + secondBestCompany);
        System.out.println("Last Company in the list: " + lastCompany);

        // Modify the element at a given index
        // 修改给定索引处的元素
        topCompanies.set(4, "Walmart");
        System.out.println("Modified top companies list: " + topCompanies);
    }
}

Output

Is the topCompanies list empty? : true
Here are the top 5 companies in the world
[Google, Apple, Microsoft, Amazon, Facebook]
Best Company: Google
Second Best Company: Apple
Last Company in the list: Facebook
Modified top companies list: [Google, Apple, Microsoft, Amazon, Walmart]

Removing elements from an ArrayList

从ArrayList中删除元素

This example shows:

1. How to remove the element at a given index in an ArrayList | remove(int index)

   如何删除ArrayList中给定索引处的元素

2. How to remove an element from an ArrayList | remove(Object o)

   如何从ArrayList中删除元素

3. How to remove all the elements from an ArrayList that exist in a given collection | removeAll()

   如何从ArrayList中删除给定集合中存在的所有元素

4. How to remove all the elements matching a given predicate | removeIf()

   如何删除与给定谓词匹配的所有元素

5. How to clear an ArrayList | clear()

   如何清除ArrayList

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;

public class RemoveElementsFromArrayListExample {

    public static void main(String[] args) {

        List<String> programmingLanguages = new ArrayList<>();
        programmingLanguages.add("C");
        programmingLanguages.add("C++");
        programmingLanguages.add("Java");
        programmingLanguages.add("Kotlin");
        programmingLanguages.add("Python");
        programmingLanguages.add("Perl");
        programmingLanguages.add("Ruby");

        System.out.println("Initial List: " + programmingLanguages);

        // Remove the element at index 5
        // 删除索引为5的元素
        programmingLanguages.remove(5);
        System.out.println("After remove(5): " + programmingLanguages);

        // Remove the first occurrence of the given element from the ArrayList
        // 从ArrayList中删除给定元素的第一次出现的地方
        // (The remove() method returns false if the element does not exist in the ArrayList)
        // （如果ArrayList中不存在该元素，则remove()方法将返回false）
        boolean isRemoved = programmingLanguages.remove("Kotlin");
        System.out.println("After remove("Kotlin"): " + programmingLanguages);

        // Remove all the elements that exist in a given collection
        // 删除给定集合中存在的所有元素
        List<String> scriptingLanguages = new ArrayList<>();
        scriptingLanguages.add("Python");
        scriptingLanguages.add("Ruby");
        scriptingLanguages.add("Perl");

        programmingLanguages.removeAll(scriptingLanguages);
        System.out.println("After removeAll(scriptingLanguages): " + programmingLanguages);

        // Remove all the elements that satisfy the given predicate
        // 删除所有满足给定谓词的元素
        programmingLanguages.removeIf(new Predicate<String>() {
            @Override
            public boolean test(String s) {
                return s.startsWith("C");
            }
        });

        /*
            The above removeIf() call can also be written using lambda expression like this -
            programmingLanguages.removeIf(s -> s.startsWith("C"))
            上面的removeIf()调用也可以使用lambda表达式编写，
            programmingLanguages.removeIf（s-> s.startsWith（“ C”））
        */

        System.out.println("After Removing all elements that start with "C": " + programmingLanguages);

        // Remove all elements from the ArrayList
        // 从ArrayList中删除所有元素
        programmingLanguages.clear();
        System.out.println("After clear(): " + programmingLanguages);
    }
}

Output

Initial List: [C, C++, Java, Kotlin, Python, Perl, Ruby]
After remove(5): [C, C++, Java, Kotlin, Python, Ruby]
After remove("Kotlin"): [C, C++, Java, Python, Ruby]
After removeAll(scriptingLanguages): [C, C++, Java]
After Removing all elements that start with "C": [Java]
After clear(): []

Iterating over an ArrayList

遍历ArrayList

The following example shows how to iterate over an ArrayList using

1. Java 8 forEach and lambda expression.
2. iterator().
3. iterator() and Java 8 forEachRemaining() method.
4. listIterator().
5. Simple for-each loop.
6. for loop with index.

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

public class IterateOverArrayListExample {

    public static void main(String[] args) {

        List<String> tvShows = new ArrayList<>();
        tvShows.add("Breaking Bad");
        tvShows.add("Game Of Thrones");
        tvShows.add("Friends");
        tvShows.add("Prison break");

        // 使用Java 8 forEach和lambda进行迭代
        System.out.println("=== Iterate using Java 8 forEach and lambda ===");
        tvShows.forEach(tvShow -> {
            System.out.println(tvShow);
        });

        // 使用iterator()进行迭代
        System.out.println("
=== Iterate using an iterator() ===");
        Iterator<String> tvShowIterator = tvShows.iterator();
        while (tvShowIterator.hasNext()) {
            String tvShow = tvShowIterator.next();
            System.out.println(tvShow);
        }

        // 使用iterator()和Java 8 forEachRemaining()方法进行迭代
        System.out.println("
=== Iterate using an iterator() and Java 8 forEachRemaining() method ===");
        tvShowIterator = tvShows.iterator();
        tvShowIterator.forEachRemaining(tvShow -> {
            System.out.println(tvShow);
        });

        // 使用listIterator()进行双向迭代
        System.out.println("
=== Iterate using a listIterator() to traverse in both directions ===");
        // Here, we start from the end of the list and traverse backwards.
        // 在这里，我们从列表的末尾开始并向后移动
        ListIterator<String> tvShowListIterator = tvShows.listIterator(tvShows.size());
        while (tvShowListIterator.hasPrevious()) {
            String tvShow = tvShowListIterator.previous();
            System.out.println(tvShow);
        }

        // 使用简单的for-each循环进行迭代
        System.out.println("
=== Iterate using simple for-each loop ===");
        for (String tvShow : tvShows) {
            System.out.println(tvShow);
        }

        // 使用带索引的for循环进行迭代
        System.out.println("
=== Iterate using for loop with index ===");
        for (int i = 0; i < tvShows.size(); i++) {
            System.out.println(tvShows.get(i));
        }
    }
}

Output

=== Iterate using Java 8 forEach and lambda ===
Breaking Bad
Game Of Thrones
Friends
Prison break

=== Iterate using an iterator() ===
Breaking Bad
Game Of Thrones
Friends
Prison break

=== Iterate using an iterator() and Java 8 forEachRemaining() method ===
Breaking Bad
Game Of Thrones
Friends
Prison break

=== Iterate using a listIterator() to traverse in both directions ===
Prison break
Friends
Game Of Thrones
Breaking Bad

=== Iterate using simple for-each loop ===
Breaking Bad
Game Of Thrones
Friends
Prison break

=== Iterate using for loop with index ===
Breaking Bad
Game Of Thrones
Friends
Prison break

The iterator() and listIterator() methods are useful when you need to modify the ArrayList while traversing.

当您需要在遍历时修改ArrayList时，iterator()和listIterator()方法很有用

Consider the following example, where we remove elements from the ArrayList using iterator.remove() method while traversing through it -

考虑以下示例，在该示例中，我们遍历数组时使用iterator.remove()方法从ArrayList中删除了元素

package com.callicoder.arraylist;

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

public class ArrayListIteratorRemoveExample {

    public static void main(String[] args) {

        List<Integer> numbers = new ArrayList<>();
        numbers.add(13);
        numbers.add(18);
        numbers.add(25);
        numbers.add(40);

        Iterator<Integer> numbersIterator = numbers.iterator();
        while (numbersIterator.hasNext()) {
            Integer num = numbersIterator.next();
            if (num % 2 != 0) {
                numbersIterator.remove();
            }
        }

        System.out.println(numbers);
    }
}

Output

[18, 40]

Searching for elements in an ArrayList

在ArrayList中搜索元素

The example below shows how to:

• Check if an ArrayList contains a given element | contains()

  检查ArrayList是否包含给定元素

• Find the index of the first occurrence of an element in an ArrayList | indexOf()

  查找ArrayList中第一次出现的元素的索引

• Find the index of the last occurrence of an element in an ArrayList | lastIndexOf()

  查找ArrayList中最后一次出现的元素的索引

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;

public class SearchElementsInArrayListExample {

    public static void main(String[] args) {

        List<String> names = new ArrayList<>();
        names.add("John");
        names.add("Alice");
        names.add("Bob");
        names.add("Steve");
        names.add("John");
        names.add("Steve");
        names.add("Maria");

        // Check if an ArrayList contains a given element
        // 检查ArrayList是否包含给定元素
        // 名称数组是否包含“鲍勃”？
        System.out.println("Does names array contain "Bob"? : " + names.contains("Bob"));

        // Find the index of the first occurrence of an element in an ArrayList
        // 查找ArrayList中第一次出现的元素的索引
        System.out.println("indexOf "Steve": " + names.indexOf("Steve"));
        System.out.println("indexOf "Mark": " + names.indexOf("Mark"));

        // Find the index of the last occurrence of an element in an ArrayList
        // 查找ArrayList中最后一次出现的元素的索引
        System.out.println("lastIndexOf "John" : " + names.lastIndexOf("John"));
        System.out.println("lastIndexOf "Bill" : " + names.lastIndexOf("Bill"));
    }
}

Output

indexOf "Steve": 3
indexOf "Mark": -1
lastIndexOf "John" : 4
lastIndexOf "Bill" : -1

ArrayList of user defined objects

用户自定义对象的ArrayList

Since ArrayList supports generics, you can create an ArrayList of any type. It can be of simple types like Integer, String, Double or complex types like an ArrayList of ArrayLists, or an ArrayList of HashMaps or an ArrayList of any user defined objects.

由于ArrayList支持泛型，因此您可以创建任何类型的ArrayList。它可以是简单类型，例如整数，字符串，双精度型，也可以是复杂类型，例如ArrayLists的ArrayList或HashMaps的ArrayList或任何用户自定义对象的ArrayList。

In the following example, you’ll learn how to create an ArrayList of user defined objects.

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;

class User {
    private String name;
    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

public class ArrayListUserDefinedObjectExample {

    public static void main(String[] args) {
        List<User> users = new ArrayList<>();
        users.add(new User("Rajeev", 25));
        users.add(new User("John", 34));
        users.add(new User("Steve",29));

        users.forEach(user -> {
            System.out.println("Name : " + user.getName() + " Age : " + user.getAge());
        });
    }
}

Output

Name : Rajeev Age : 25
Name : John Age : 34
Name : Steve Age : 29

Sorting an ArrayList

排序ArrayList

Sorting an ArrayList is a very common task that you will encounter in your programs. In this section, I’ll show you how to -

对ArrayList进行排序是您在程序中会遇到的非常常见的任务

• Sort an ArrayList using Collections.sort() method.

  使用Collections.sort()方法对ArrayList进行排序

• Sort an ArrayList using ArrayList.sort() method.

  使用ArrayList.sort()方法对ArrayList进行排序

• Sort an ArrayList of user defined objects with a custom comparator.

  使用自定义比较器对用户定义对象的ArrayList进行排序

1. Sort an ArrayList using Collections.sort() method

使用Collections.sort()方法对ArrayList进行排序

package com.callicoder.arraylist;

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

public class ArrayListCollectionsSortExample {

    public static void main(String[] args) {

        List<Integer> numbers = new ArrayList<>();
        numbers.add(13);
        numbers.add(7);
        numbers.add(18);
        numbers.add(5);
        numbers.add(2);

        System.out.println("Before : " + numbers);

        // Sorting an ArrayList using Collections.sort() method
        // 使用Collections.sort()方法对ArrayList进行排序
        Collections.sort(numbers);

        System.out.println("After : " + numbers);
    }
}

Output

Before : [13, 7, 18, 5, 2]
After : [2, 5, 7, 13, 18]

2. Sort an ArrayList using ArrayList.sort() method

使用ArrayList.sort()方法对ArrayList进行排序

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class ArrayListSortExample {

    public static void main(String[] args) {

        List<String> names = new ArrayList<>();
        names.add("Lisa");
        names.add("Jennifer");
        names.add("Mark");
        names.add("David");

        System.out.println("Names : " + names);

        // Sort an ArrayList using its sort() method. You must pass a Comparator to the ArrayList.sort() method.
        // 使用其sort()方法对ArrayList进行排序
        // 您必须将Comparator传递给ArrayList.sort()方法
        names.sort(new Comparator<String>() {
            @Override
            public int compare(String name1, String name2) {
                return name1.compareTo(name2);
            }
        });

        // The above sort() method call can also be written simply uisng lambda expressin
        // 上面的sort()方法调用也可以使用lambda表达式简单地编写
        names.sort((name1, name2) -> name1.compareTo(name2));

        // Following is an even more concise solution
        // 以下是更简洁的解决方案
        names.sort(Comparator.naturalOrder());

        System.out.println("Sorted Names : " + names);
    }
}

Output

Names : [Lisa, Jennifer, Mark, David]
Sorted Names : [David, Jennifer, Lisa, Mark]

3. Sort an ArrayList of Objects using custom Comparator

使用自定义比较器对对象的ArrayList进行排序

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

class Person {
    private String name;
    private Integer age;

    public Person(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name=‘" + name + ‘‘‘ +
                ", age=" + age +
                ‘}‘;
    }
}

public class ArrayListObjectSortExample {

    public static void main(String[] args) {

        List<Person> people = new ArrayList<>();
        people.add(new Person("Sachin", 47));
        people.add(new Person("Chris", 34));
        people.add(new Person("Rajeev", 25));
        people.add(new Person("David", 31));

        System.out.println("Person List : " + people);

        // Sort People by their Age
        // 按年龄对人进行排序
        people.sort((person1, person2) -> {
            return person1.getAge() - person2.getAge();
        });

        // A more concise way of writing the above sorting function
        // 编写上述排序功能的更简洁方法
        people.sort(Comparator.comparingInt(Person::getAge));

        // 按年龄排序的列表
        System.out.println("Sorted Person List by Age : " + people);

        // You can also sort using Collections.sort() method by passing the custom Comparator
        // 您还可以通过传递自定义Comparator使用Collections.sort()方法进行排序
        Collections.sort(people, Comparator.comparing(Person::getName));
        // 按姓名排序的列表
        System.out.println("Sorted Person List by Name : " + people);
    }
}

Output

Person List : 
[Person{name=‘Sachin‘, age=47}, Person{name=‘Chris‘, age=34}, Person{name=‘Rajeev‘, age=25}, Person{name=‘David‘, age=31}]
Sorted Person List by Age : 
[Person{name=‘Rajeev‘, age=25}, Person{name=‘David‘, age=31}, Person{name=‘Chris‘, age=34}, Person{name=‘Sachin‘, age=47}]
Sorted Person List by Name :
[Person{name=‘Chris‘, age=34}, Person{name=‘David‘, age=31}, Person{name=‘Rajeev‘, age=25}, Person{name=‘Sachin‘, age=47}]

Synchronizing Access to an ArrayList

同步访问ArrayList

The ArrayList class is not synchronized. If multiple threads try to modify an ArrayList at the same time then the final result becomes not-deterministic because one thread might override the changes done by another thread.

ArrayList类不同步。如果多个线程尝试同时修改ArrayList，则最终结果将变得不确定，因为一个线程可能会覆盖另一线程所做的更改。

Example Demonstrating ArrayList’s unpredictable behavior in multi-threaded environments

演示ArrayList在多线程环境中的不可预测行为的示例

The following example shows what happens when multiple threads try to modify an ArrayList at the same time.

下面的示例显示了多个线程尝试同时修改ArrayList时发生的情况

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class UnsafeArrayListExample {

    public static void main(String[] args) throws InterruptedException {

        List<Integer> unsafeArrayList = new ArrayList<>();
        unsafeArrayList.add(1);
        unsafeArrayList.add(2);
        unsafeArrayList.add(3);

        // Create a thread pool of size 10
        // 创建大小为10的线程池
        ExecutorService executorService = Executors.newFixedThreadPool(10);

        // Create a Runnable task that increments each element of the ArrayList by one
        // 创建一个Runnable任务，将ArrayList的每个元素加1
        Runnable task = () -> {
            incrementArrayList(unsafeArrayList);
        };

        // Submit the task to the executor service 100 times.
        // 将任务提交给执行者服务100次
        // All the tasks will modify the ArrayList concurrently
        // 所有任务将同时修改ArrayList
        for(int i = 0; i < 100; i++) {
            executorService.submit(task);
        }

        executorService.shutdown();
        executorService.awaitTermination(60, TimeUnit.SECONDS);

        System.out.println(unsafeArrayList);
    }

    // Increment all the values in the ArrayList by one
    // 将ArrayList中的所有值加1
    private static void incrementArrayList(List<Integer> unsafeArrayList) {
        for(int i = 0; i < unsafeArrayList.size(); i++) {
            Integer value = unsafeArrayList.get(i);
            unsafeArrayList.set(i, value + 1);
        }
    }
}

Output

[99, 100, 100]

Try running the above program multiple times and see how it produces different outputs. To learn more about such issues in multi-threaded programs, check out my article on Java Concurrency Issues and Thread Synchronization.

尝试多次运行上面的程序，看看它如何产生不同的输出。要了解有关多线程程序中此类问题的更多信息，请查看我有关Java并发问题和线程同步的文章

Example demonstrating how to synchronize concurrent modifications to an ArrayList

演示如何将并发修改同步到ArrayList的示例

All right! Now let’s see how we can synchronize access to the ArrayList in multi-threaded environments.

现在，让我看一下如何在多线程环境中同步对ArrayList的访问

The following example shows the synchronized version of the previous example. Unlike the previous program, the output of this program is deterministic and will always be the same.

以下示例显示了上一示例的同步版本。与以前的程序不同，该程序的输出是确定性的，并且将始终相同

package com.callicoder.arraylist;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class SynchronizedArrayListExample {

    public static void main(String[] args)  throws InterruptedException {

        List<Integer> safeArrayList = Collections.synchronizedList(new ArrayList<>());
        safeArrayList.add(1);
        safeArrayList.add(2);
        safeArrayList.add(3);

        // Create a thread pool of size 10
        // 创建大小为10的线程池
        ExecutorService executorService = Executors.newFixedThreadPool(10);

        // Create a Runnable task that increments each element of the ArrayList by one
        // 创建一个Runnable任务，将ArrayList的每个元素加1
        Runnable task = () -> {
            incrementArrayList(safeArrayList);
        };

        // Submit the task to the executor service 100 times.
        // 将任务提交给执行者服务100次。
        // All the tasks will modify the ArrayList concurrently
        // 所有任务将同时修改ArrayList
        for (int i = 0; i < 100; i++) {
            executorService.submit(task);
        }

        executorService.shutdown();
        executorService.awaitTermination(60, TimeUnit.SECONDS);

        System.out.println(safeArrayList);
    }

    // Increment all the values in the ArrayList by one
    // 将ArrayList中的所有值加1
    private static void incrementArrayList(List<Integer> safeArrayList) {

        synchronized (safeArrayList) {

            for (int i = 0; i < safeArrayList.size(); i++) {
                Integer value = safeArrayList.get(i);
                safeArrayList.set(i, value + 1);
            }
        }
    }
}

Output

[101, 102, 103]

The above example uses Collections.synchronizedList() method to get a synchronized view of the ArrayList.

上面的示例使用Collections.synchronizedList()方法获取ArrayList的同步视图

Moreover, the modifications to the ArrayList inside the incrementArrayList() method is wrapped inside a synchronized block. This ensures that no two threads can increment ArrayList elements at the same time.

此外，对incrementArrayList()方法中对ArrayList的修改被包装在一个同步块中。这样可以确保没有两个线程可以同时增加ArrayList元素

You can also use a CopyOnWriteArrayList if you need thread safety. It is a thread-safe version of the ArrayList class. It implements all the mutating operations by making a fresh copy of the ArrayList.

如果需要线程安全，也可以使用CopyOnWriteArrayList。它是ArrayList类的线程安全版本。它通过制作ArrayList的新副本来实现所有的变异操作

Conclusion

In this article, you learned what is an ArrayList, how to create an ArrayList, how to add, modify and remove elements from an ArrayList, how to iterate over an ArrayList, how to sort an ArrayList, and how to synchronize access to an ArrayList.

在本文中，您学习了什么是ArrayList，如何创建ArrayList，如何在ArrayList中添加，修改和删除元素，如何在ArrayList上进行迭代，如何对ArrayList进行排序以及如何同步对ArrayList的访问

Thank you for reading. See you in the next post.