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This article introduces the knowledge of "what are the reference data types in java". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

What are the reference data types in java

There are two data types in Java, of which there are mainly 8 basic data types and reference data types. Except for the 8 basic data types, all of them are reference data types, and the basic data types in 8 are byte,short,int,long,char,boolean,float,double respectively, as shown below:

1. Boolean: the data value is only true or false, which is suitable for logical calculation.

2. Char: chartype (character type) data occupies 2 bytes in memory. Chartype data is used to represent characters in the general sense, each character accounts for 2 bytes. Java characters are encoded by Unicode. The storage range of the first 128 bytes of ASCII-compatible characters is in the range of\ u00000 ~\ uFFFF. When defining character data, you should pay attention to adding'', for example,'1' represents the character'1' instead of the numeric value 1.

3. Byte: byte type (byte type) data occupies 1 byte in memory, indicating that the stored data range is-128 to 127.

4. Short: short integer data takes up 2 bytes in memory.

5. Int: int (integer) data takes up 4 bytes in memory.

6. Long: long (long integer) data takes up 8 bytes in memory.

7. Float: float (single-precision floating-point) data takes up 4 bytes in memory. (the precision of float is 7-8 bits)

8. Double: double-precision floating-point data takes up 8 bytes in memory.

Let's talk about the reference data types in java:

There are three types of reference data: classes, interfaces, and arrays.

1. Class-like reference

It can be created by us. I won't say much about it here, but I will mainly explain the classes in several java libraries.

Object:Object is a very important class, Object is the root class of the class hierarchy, each class uses Object as the superclass, and all objects (including arrays) implement the methods of this class. All classes can be defined with Object

Such as:

Object object= new Integer (1); to define an Interger class Integer I = (Integer) object; to cast this Object into an Interger class

The String:String class represents a string, and all string literals in the Java program, such as "abc", are implemented as instances of this class. Check the individual characters of the sequence, compare strings, search strings, extract substrings, create a copy of the string, in which all characters are converted to uppercase or lowercase.

Date:Date represents a specific moment, accurate to milliseconds. Date classes are now generally replaced by Calendar and GregorianCalendar

The Void:Void class is a placeholder class that cannot be instantiated and maintains a reference to the Class object that represents the Java keyword void.

At the same time, there is also a corresponding Class such as Integer Long Boolean Byte Character Double Float Short

II. Interface interface reference

It can be created by us. I won't say much about it here, but I will mainly explain the interface interface in several java libraries.

List: list, users of this interface can precisely control the insertion position of each element in the list. Users can access elements based on their integer index (position in the list) and search for elements in the list. The List interface provides two ways to search for specified objects. From a performance point of view, these methods should be used with care. In many implementations, they will perform costly linear searches. The List interface provides two ways to efficiently insert and remove multiple elements anywhere in the list.

Add (): inserts the specified element in the list.

Remove (): removes the element at the specified location in the list.

Get (int index): returns the element at the specified location in the list.

Map:

K-the type of key maintained by this mapping

V-the type of the mapped value maps the key to the object of the value. A mapping cannot contain duplicate keys; each key can be mapped to at most one value.

Put (K key,V value): associates the specified value with the specified key in this mapping (optional). If this mapping previously contains a mapping of the key, the old value is replaced with the specified value (if and only if true is returned, the mapping m contains the mapping of key k). Remove (Object key) if there is a mapping of a key, remove it from the mapping (optional). More specifically, if this mapping contains a key==null? The mapping relationship from key k to value v of k==null: key.equals (k) is removed. (the mapping can only contain at most one such mapping.) get (Object key): returns the value mapped by the specified key, or null if the mapping does not contain the mapping for that key.

Here we mainly use String List Map Object, which is the most commonly used Number ArrayList Arrays, etc.

3. Array reference

Array: a collection of elements of the same data type (reference data type) stored in a continuous block of memory.

Each data in an array is called an array element. The elements in the array are indexed to indicate where they are stored, and the index (subscript) starts at 0.

Definition of array

The first way: type [] array name, such as int [] nums

The second way: type array name []; such as int nums []

Most Java programmers prefer the first style because it separates the data type int [] from the variable name num.

Initialization of array

Arrays in Java must be initialized before they can be used.

Initialization allocates memory to array elements and assigns an initial value to each element.

There are two ways to initialize arrays:

-static initialization:

Syntax format: type [] array name = new array type [] {element 1, element 2, element 3. Element n}

Simplified syntax: type [] array name = {element 1, element 2, element 3. Element n}

-dynamic initialization:

If we do not know in advance what data is stored in the array, but only the number of data to be stored, we can use dynamic initialization.

Dynamic initialization: we specify the length of the array during initialization, and the system automatically assigns initial values to the array elements.

Format: type [] array name = new array type [array length]

Note: no matter how the array is initialized, once initialized, the length of the array is fixed and cannot be changed unless reinitialized. In other words, the array is of fixed length.

Why are there basic data types and reference data types in Java?

The reference type is in the heap and the basic type is in the stack.

The stack space is small and continuous and is often placed in the cache. The reference type cache miss has a high rate and requires one more dereference.

Object also needs to store one more object header, which is too high for basic data types.

That's all for "what are the reference data types in java?" Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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