Java. lang

Java. lang
1. Common basic jar packages

As a java language user, we can feel the advantages of the java language (platform independence, object-oriented, multithreading, efficient and scalable ), in addition, it has many implemented class libraries that can be directly used by us. These class libraries are provided in the form of jar packages and can also be used as Java APIs, it implements a variety of common operations for programmers and brings a lot of convenience for programmers to compile java program code.

As a beginner, I think it is necessary to master and apply the java basic library. Therefore, I summarized some basic knowledge points when I was a beginner in java. The main APIs provided by the Java2 standard are shown in the following table.

Package name	Overview
Java. applet	Provides the classes required to create an applet.
Java. awt	Contains all the classes used to create user interfaces and draw images.
Java. io	Provides input and output-related classes.
Java. beans	Contains classes related to developing javaBeans
Java. lang	Provides basic classes for java programming.
Java.net	Provides classes related to network operations.
Java. nio	Class that provides a buffer for Input and Output
Java. text	Provides classes and interfaces for processing text, dates, numbers, and messages.
Java. util	Provides various tools such as processing date, time, and random number generation.
Javax.net	Provides network application classes and network application extensions.
Java. swing	Provides a set of java-only component classes with the same features as AWT.

2. java. lang Package

Java. lang package is the basis of all other class libraries in the java language system. It has been embedded in the java Virtual Machine and created as an object. Therefore, we are using java. you can directly use java. all classes in the lang Package and directly reference the open, variable, and Operation Method in a class.

Class Name	Function
Boolean	It encapsulates values of the boolean Type and some methods to operate on this type.
Byte	Encapsulates the value of the byte type and some methods to operate on this type.
Character	It encapsulates values of the char type and some methods for this type.
Double	It encapsulates values of the double type and some methods for this type.
Float	It encapsulates the float type value and some methods to operate on this type.
Integer	It encapsulates the int type value and some methods to operate on this type.
Long	Encapsulates the value of the long type and some methods to operate on this type.
Short	It encapsulates values of the short Type and some methods to operate on this type.
String	Encapsulates the operation methods related to the string type.
Void	Indicates the declaration of the void keyword in Java. This class cannot be used as an instance.
Class	Describes the status of classes and interfaces in a running java application.
ClassLoader	Objects used to load classes
Enum	Used to define enumeration types
Math	Used for basic mathematical operations
Number	Abstract class is the parent class of the basic data type class
Object	Is the root class of all java classes
Package	Encapsulates java package implementation and standard version information
Runtime	The Runtime Class Object connects java applications to their Runtime environments.
StrictMath	Used to implement basic mathematical operations
StringBuffer	Variable string operations
StringBuilder	Create variable string objects
System	Encapsulated some methods related to the Java Virtual Machine System
Thread	Create and control threads
ThreadGroup	Create and control thread groups
Throwable	Defines the parent class of all errors or exceptions in java
Process	Define a process object and start the process object using the exec method in the Runtime class.

The main interfaces defined in the java. lang Package and their corresponding functions are described as follows:

Interface	Function
Appendable	Used to append a string
Cloneable	Used to copy class objects
Runnable	Used to implement the thread function of class objects
Comparable	Used to sort class objects

3. Object Class description

The Object class is the parent class of all classes in the java system. That is, the class objects created in the java program are derived from the object class, all implement or inherit the methods in the object class.

The Jar package contains constants, variables, and methods. When a class object is created, constants and variables in the object can be directly referenced, when calling a method in an object, you must specify the input parameters of the method and provide the data and return values that conform to the parameter type for the method. You must also know whether the method will generate an exception during calling, if an exception may occur in a method, use the try-catch statement structure to capture and handle the exception, or use the java keyword throws to throw the exception.

4. Basic Java Data Types

The basic data types specified in Java include boolean, byte, character, double, float, integer, long, and short. You can declare constants and variables of the basic data type. However, java is a pure object-oriented programming language. lang defines the corresponding classes for these basic data types (encapsulates the variable types of basic data types, constants related to variables, and related operation methods ), in this way, these basic types of values or variables can be processed as objects.

The java. lang package also defines the Number abstract data type, specifically for operations on pure numeric objects. Operations on pure numeric objects in basic data types include integer and floating point types. It can be seen that the Number class is a direct parent class of the integer and floating point types. In the number class, it defines operation methods and abstract methods suitable for pure numbers of different data types.

(Note: The character class encapsulates the values of the char type and related operation methods. All the characters are encoded in Unicode standard)

5. String & StringBuffer

Java. lang also provides a String class to process strings. The String class is used to process "immutable" strings. The StringBuffer class is also used to process "mutable" strings. Stirng and StringBuffer classes are declared as final types, so they cannot be used as parent classes and are inherited.

(1) String class

The String class is used to create a String object. Its value is treated as a constant after it is created. The String class also contains String-type values and related operation methods. All operation methods in the String class are for the created String object, and the String object is created through the String class constructor.

(2) StringBuffer class

The StringBuffer class is a string buffer class used to create variable-length string objects. The variable-length class indicates that the length and content of a string can be changed by calling some methods, for example, a New String object is formed by appending a new string sequence to the original string, or inserting a new character sequence to a position in the original string.

The StringBuffer class creates string objects based on the implementation of the new buffer zone. It stores string character sequences in the buffer zone. Because the buffer zone has a certain capacity, when the string length changes, if the buffer capacity is exceeded, that is, when the buffer overflow occurs, the java running system automatically expands the buffer capacity created by the StringBuffer object, ensuring the security of string operations.

6. Math class

Math class encapsulates various data types of arithmetic operations, including exponent, logarithm, square root, trigonometric function, and so on. All constants and methods in the math class are defined as static, so all members can be directly referenced.

7. Runtime class

The Runtime class encapsulates some methods related to the java Virtual Machine. When the java Virtual Machine starts every java application, it creates a runtime object, which ensures the connection between the java application and its Runtime environment, after a runtime object is created, you can directly call the methods defined in the runtime class in a java application (for example, operations such as obtaining the running information of the current program, exiting the program, and disabling the java Virtual Machine ).

8. System class

The System class encapsulates some constants related to the Computer Input/Output System and the operation methods related to the Java Virtual Machine. After the java Virtual Machine is started, the system object is created. The system object cannot be created in java applications. All constants and methods in the System class are defined as static. Therefore, you can directly call the constants and methods defined in the system class in java applications.

9. Summary

There are many available class libraries provided by the java language, which bring a lot of convenience to programmers. You need to view the help documentation and instructions for specific use and determine the function. The focus is on the input parameters, return values, and exceptions of an operation method.