What is the difference between ArrayList and general array in java? How efficient is it?

The following column will introduce the difference between ArrayList and general arrays in the java Quick Start column.

What is ArrayList?

The implementation principle of ArrayList is actually an array (dynamic array).

What is the difference between dynamic arrays and general arrays?

Compared with arrays in Java, the capacity of ArrayList can grow dynamically.

How efficient is ArrayList?

ArrayList is not thread-safe, so it is more efficient, but it can only be used in a single-threaded environment.

Which classes does ArrayList mainly inherit and implement which interfaces?

ArrayList mainly inherits the AbstractList class and implements List, RandomAccess, Cloneable, SerializableInterface

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

RandomAccess means that it has the ability to access quickly. ArrayList can be accessed based on the subscript with a time complexity of O(1)[^1] element. Since the underlying structure of ArrayList is an array, it occupies a continuous memory space, and its length is the size of the array. Therefore, it also has the disadvantages of arrays and is not very efficient in space, but it also has its advantages, that is, fast query speed and time efficiency. Faster.

What are the constants and variables of ArrayList?

// 序列ID
private static final long serialVersionUID = 8683452581122892189L;

// ArrayList默认的初始容量大小
private static final int DEFAULT_CAPACITY = 10;

// 空对象数组，用于空实例的共享空数组实例
private static final Object[] EMPTY_ELEMENTDATA = {};

// 空对象数组，如果使用默认的构造函数创建，则默认对象内容是该值
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

// 存放当前数据，不参与序列化
transient Object[] elementData; // non-private to simplify nested class access

// list大小
private int size;

When the elements in the collection exceed the specified length of the array, the array will be expanded. The expansion operation is the reason why the ArrayList storage operation is slow, especially when the amount of data is large. Expansion will take more and more time.

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