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Detailed introduction to java inheritance

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1. The concept of inheritance

1. The mechanism by which a parent class derives a subclass is called inheritance.

2. Inheritance is a powerful means of reusing program code. When the same properties and methods exist between multiple classes, the parent class can be abstracted from these classes.

3. We only need to define these properties and methods in the parent class, and the subclass does not need to redefine these properties and methods, and can directly inherit from the parent class.

4. Through inheritance, the subclass will automatically have the attributes and methods defined in the parent class.

5. If some of the members of two or more classes are the same, through the inheritance mechanism, existing classes can be used to create new classes.

6. Subclasses not only have members of the parent class, but can also define new members.

7. It can improve the reusability and scalability of software.

2. Parent class and subclass

1. Parent class: base class or super class

2. Child Class: Derived class or derived class

3. Implementation of inheritance

## Use the extends keyword.

4. Access control

The Java language sets access control rules to restrict the use of class members from external classes

##publicNo

##Access modifier

Self access

In-package access

Outside-package subclass access

Outside access

Yes Yes Yes

protected

是

Default

Yes

##private

Yes

##No

Note: Properties and methods modified with private in the parent class are transparent to the subclass and cannot be inherited.

5. Constructor methods in inheritance

1. The constructor method of the parent class cannot be inherited by the subclass

2. The subclass must call the parent -class structural method before performing its own structure. The call is divided into two types:

A. Display call: The first sentence in the constructor of the subclass uses the key to use the super key in the constructor method of the subclass Word to specify which constructor of the parent class is called

use use using using use use using using using use using ’ use use ’ through through using ’ out out through out through off out through through out through ’'s' itself'’’ ’ ‐ ‐‐ ‐‐ ‐‐‐‐ ‐ mark be d Construction method

B. Ciscroof call: If the call is not displayed, the parent class has no parameter constructing method, which is equivalent to displaying the call super (), which means that when the default structure method of the parent class is called You can omit Super ()

Special attention:

1. The call order of the sub -class structure method is to call the parent class structure method first, and call the constructor of the subclass

2. When there is a default constructor in the parent class, the subclass constructor can omit the explicit call to the parent class constructor

3. When the parent class only has a constructor with parameters, the subclass The super keyword must be used in the constructor to display the constructor of the parent class

6. Class hierarchy 1.Java The language does not support multiple inheritance, that is, a class can only directly inherit one class.

2. A class can have multiple indirect parent classes.

3. All Java classes directly or indirectly inherit the java.lang.Object class

4. The Object class is the ancestor of all Java classes, and all Java classes are defined in the Object class Objects all have the same behavior

7. Default inheritance 1. When defining a class, the extends keyword is not used, then This class directly inherits from the Object class

Chapter 8:

JavaPolymorphism

1. The concept of polymorphism Official definition: Polymorphism means that calling member methods of a class will form different implementation methods

Personal understanding: Parent class The reference points to the subclass object

Effectively improve the scalability of the program. Based on writing

Dynamic polymorphism is also called dynamic binding. Usually when we say polymorphism, we generally refer to dynamic polymorphism.

2. Method rewriting

If a method is defined in a subclass and its name, return value type and parameter list exactly match a method in the parent class, we can say that the subclass method overrides the parent class method.

special attention:

1. Method rewriting must have inheritance relationships. The methods of the parent class are the same.

#3. Polymorphic Application

When a parent class reference points to a subclass object and a method is called through the reference, the subclass reuse is always called. The method after writing

assigns the object of the subclass to the reference of the parent class. This behavior is called "upcasting", which can implicitly convertthe reference of the parent class back to the subclass. Quoting, this behavior is called "downward transformation", which needs to be compulsory conversion

## This special attention: When the cither of the parent class points to the sub -class object, the call of the method is dynamic analysis. That is, dynamic selection based on the actual type of the object is called dynamic binding

Object

Class The Object class is the parent class or indirect parent class of all Java classes. According to the polymorphic characteristics, we can use the reference of the Object class to point to any object Note: If you can determine the specific type of the object referenced by Object, you can perform downward transformation (forced conversion)

equals

Method The purpose of the equals method is to detect whether two objects are equal Provided by the Object class The implementation of the equals method is: detect whether the memory addresses of the two objects are in the same area (whether the memory addresses are equal) Usually we need to rewrite the equals method in order to redefine the rules for judgment Whether two objects are equal.

Personal understanding: The equals method is to customize the game rules and formulate the conditions for object equality according to our needs

Six,

toString

Method## The toString method returns a string describing the object (this must be rewritten when using the object directly for output) Method) Personal understanding: Simplify the process of outputting object information. After rewriting, you can directly output the objectSeven,

final

Keywords The final keyword can be used to modify classes, methods and variables. Final means immutable 1. When final modifies a class: it means that the class is not allowed to be inherited by other classes 2. When final modifies a method: it means that the method is not allowed to be overridden by subclasses 3. When When final modifies a variable: it means that the variable is a constant

Note: constants modified with final must be explicitly initialized

Chapter 9: Abstract Classes and Interfaces

1. Abstract Class Concept

Looking at the inheritance diagram of a class from bottom to top, we find that classes gradually become more general and more abstract.

Due to the general nature of class variables, we will not be able to create meaningful implementation processes for methods in classes.

Sometimes we need a parent class that only defines a general form that can be shared by all its subclasses, and lets each subclass supplement the implementation details. Such a class is called an abstract class .

Note: A class containing an abstract method must be an abstract class. An abstract class can contain attributes and non-abstract methods.

2. Abstract method

When defining a Java method, you can only define the declaration part of the method without writing the method body ( That is, the implementation details of the method), such a method is called an abstract method.

Note: Abstract methods must be modified with the abstract keyword class.

3. Special attention

èAbstract classes and abstract methods must be modified with the abstract keyword

èAbstract methods There is no method body, and there is no pair of braces. There must be a semicolon after the parameter list

èIf the class contains an abstract method, then the class must be an abstract class, but on the contrary, the abstract class does not Does not necessarily contain abstract methods.

èAbstract classes cannot be instantiated (that is, objects of abstract classes cannot be created)

èStatic methods can be declared in abstract classes

4. The use of abstract classes

Since abstract classes cannot create objects, we can only use abstract classes through inheritance. (Supports polymorphism)

Note:

1. If the subclass inherits an abstract class, then the subclass must override all abstract methods in the parent class

2. If an abstract class inherits another abstract class, then the subclass does not need to override the abstract method in the parent class

5. Summary of abstract classes

abstract class is mainly inherited by the subclass, and then the subclass is played. Then the role of the abstract class is mainly manifested in the following two aspects:

## 1. Code reuse (surface phenomenon )

2. Plan the behavior of its subclasses (key understanding)

Use one sentence to summarize abstract classes: Abstract classes are abstractions of classes

6. Interface concepts

## 不 Do not support more inheritance in Java, but provide strong interface support

# interface: The methods in the class are specified, and there is no need to consider the inheritance relationship between classes

A class can only directly inherit one parent class, but can implement multiple interfaces

An interface is a collection of abstract methods and static constants. In essence, an interface is a special abstract class. This abstract class can only contain the definition of abstract methods and static constants, but does not have variables and methods. Implementation

7. Pay special attention

Because the methods defined in the interface must be abstract methods, you can define abstract methods in the interface as Omit the public and abstract keywords

Because the variables defined in the interface must be static constants, you can omit the public, static, and final keywords when defining static constants in the interface

8. Use of interfaces

## Similar to inheritance, Java classes can implement interfaces, and implementation classes must override all abstract methods in the interface

An implementation class can implement multiple interfaces, each interface is separated by a comma. An interface can also be implemented by multiple classes

Interfaces also support polymorphic programming: interface references point to implementation class objects

that that support multiple inheritance are supported. Summary of interfaces: Interfaces are abstractions of behaviors (i.e. methods)

10. Comparison of interfaces and abstract classes

1. Similarities ：

dict t to Differences:

been can have Impact, but for interfaces, they should not be changed once created. 11. Predefined interface

Comparable

There is an Arrays class in the java.util package. The sort method of this class can sort an Sort the object array, but there is a prerequisite that the objects in the array must implement the Comparable interface

Chapter 1: Exception Handling

1. Exception Concept

Exception (Exception) refers to an event that occurs during program running, which can interrupt the normal execution of program instructions

In Java language , exceptions can be divided into two categories: Error (Error): refers to internal errors in the JVM system, resource exhaustion and other serious situations Exception: due to programming errors or accidental General problems caused by external factors, such as square root of negative numbers, null pointer access, trying to read non-existent files, and network connection interruption issues

Java language provides a complete set of abnormal processing mechanisms. The correct use of this mechanism will help improve the robustness of the program

注意: Exception and error have a common parent Throwable

##2. Processing Mechanism

If an exception occurs during the execution of a Java program, the system will automatically detect and generate a corresponding exception class object, and then Give it to the runtime system
The runtime system then looks for the corresponding code to handle the exception. If the runtime system cannot find any code that can handle the exception, the runtime system will terminate accordingly. The Java program will also exit
Programmers do not have the ability to handle Errors and can only handle Exceptions under normal circumstances

3. Exception handling characteristics

# classification of abnormal conditions, use the Java class to indicate abnormalities, have scalability and reusability
code separation of code and normal process code separation. Improved the readability of the program
   with the With with with the ability to catch exceptions and handle them immediately, or to throw them to the upper layer for processing, are flexible
4. ExceptionClass

Exception is divided into two branches
1. Runtime Exceptions: such as type conversion exceptions, array index out-of-bounds exceptions, null pointer exceptions, etc.
        2. Compile-time exceptions: SQL statement execution exceptions, exceptions in reading non-existent files, etc.
5. Common exception types

##
Arithmetic exception##ArrayIndexOutOfBoundsExceptionNullPointerExceptionClassNotFoundException

6. The purpose of exception handling
èReturn to a safe and known state
è Ability to allow the user to execute other commands
èSave all work if possible
èExit if necessary to avoid further harm
èExceptions can only be used Due to abnormal situations, exceptions cannot be used to control the normal flow of the program
                                                                                                                                                                                                                                 may be too large.
7. Use exception handling
Java provides try-catch-finally statement to support exception handling
try block : Code that may generate exceptions
Catch block: Handling code when an exception occurs
Finally block: Unconditional execution code (usually used to release resources)

           Note: Usually when the program generates an exception and enters the catch statement block, the programmer can use the printStackTrace method to output the exception information to the console
8. Throw statement Exception
Declaring that throwing an exception is the second way of Java exception handling. If the code in a method may generate some kind of exception during runtime, it is not necessary in this method, or When you are not sure how to handle such exceptions, you can use the throws keyword to declare an exception. At this time, such exceptions will not be handled in the method, but will be handled by the caller of the method
Chapter 3: Collection Framework
1. Collection Overview
The length of the array must be specified when creating a Java array , once the array is created, its length cannot be changed
In many practical situations, the length of the array is not fixed, so the Java language provides a more advanced container, the collection framework, to solve related problems
‐ , etc., having A whole composed of "data" with similar or identical properties
Systemically speaking, the basic collections in Java can be divided into the following three types:
List (List): List collection to distinguish the order of elements, and allow repetitive elements
(SET):
# SET collection to not distinguish the order of elements, no repetitive elements
Mapping (Map):
The Map collection saves pairs of "key-value" information. Keys are not allowed to be repeated. Each key can only map one value at most
3. Elements in the collection
           Java collections can only store reference data type data (i.e. objects). In fact, the collection stores references to objects, not the objects themselves. The elements in the collection are equivalent to reference type variables

4.Collection##Interface
The java.util.Collection interface is the parent interface that describes the Set and List collection types, which defines universal methods related to collection operations
Commonly used methods:
boolean add(Object o)
        boolean remove(Object o)
          int size()
            boolean isEmpty()
        boolean contains(Object) o)
void clear()
Iterator iterator()
Object[] toArray()
##5.ListInterface List interface is a sub-interface of Collection interface
List interface regulations Users can precisely control the insertion position of list elements, and have added functions such as accessing elements based on indexes. Corresponding methods have been added to the interface.
                                                                                                      void add(int index,Object element)
Object get(int index)
Object set(int index,Object element)
int indexOf(Object o)
Object remove(int index)
6. ArrayList## The java.util.ArrayList class implements the List interface for Expressing an array list with variable length
             ArrayList allows the value to be null, and the default capacity is 10. In addition to implementing all the functions defined by the List interface, it also provides some methods to manipulate the size of the ArrayList capacity

’ ’ ’ s java. The .Iterator interface describes a tool for traversing various collection elements in a unified manner, called an iterator
boolean hasNext()
Object next()
8. Generics## The generic (Gernerics) mechanism was introduced since JavaSE 5.0. It actually parameterizes the originally determined and unchanged types
Benefits of generics:
1. As an expansion of the original java type system, the use of generics can improve the type safety, maintainability and reliability of java applications
2. Can solve the problem of data modeling in collections
How to use:
When creating a collection container, specify the element types it is allowed to save, and then the compiler is responsible for adding the type validity check of the elements. There is no need to perform styling processing when accessing the collection elements.
Summary: Data type parameterization
9. HashSetClass
## The java. Ensure the order of elements in the set
HashSet is allowed to contain null value elements, but there can only be one null element
10.
TreeSetClass The java.util.TreeSet class also implements the java.util.Set interface, which describes the Set A variant of the collection that can implement sorting functions
When an object element is added to the TreeSet collection, it will automatically be inserted into the ordered object sequence according to certain comparison rules to ensure that the TreeSet The object sequence composed of collection elements is always arranged in "ascending order"
The TreeSet class supports two sorting methods: natural sorting (default) and custom sorting
When using natural sorting, the elements in the collection The Comparable interface must be implemented. TreeSet calls the compareTo method of the object to sort the elements.
There are some classes in the JDK class library that implement the Comparable interface, such as: Integer, Double, String, etc.

Eleven,
List##set and SetTraversal of collections List type collections: You can use ordinary for loops, super for loops and iterators to traverse Set type collections : You can use super for loops and iterators to traverse

Note: Since the Set type collection has no index, you cannot use ordinary for loops to traverse
Twelve,
Map（Map） Map is a key A collection of objects for mapping, each element of which contains two objects (key objects and value objects) Typical Map type collections include HashMap and TreeMap
Add element method: put( Object key, Object value)
down ’ps together togetherps together out’s together's together's together's together's together's together's together's' together's'ps' right together out's' way's'ps' right together'ce’s rightOUT right clean right way cleanceumblececerce togetherce right together right clean right together right clean right together out right clean out, so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so so...
Thirteen, TreeMap##Category
The TreeMap class implements the SortedMap interface, so the TreeMap can be sorted according to key objects, but the prerequisite is that the stored key objects must implement the Comparable interface
Fourteen ,MapCollection traversal
Since the key objects cannot be repeated, all the key objects can be The key object is regarded as a Set type collection
Get this Set type collection through the keySet() method of the Map collection
Traverse the Set collection composed of all key objects

In the process of traversing, you can get a value object every time you get a key object
## 第 Chapter 4:
## o #1. Overview
Most programming work involves data transfer control (input/output) Java IO is implemented in a stream manner, that is, the transfer of data is regarded as the flow between the purpose and source of data
The IO class library is located in the java.io package and supports various common input and output streams. Abstracted
2. Classification of streams
Based on the difference in IO processing data, it is divided into: byte stream and character stream                                                                                                                                                                According to different functions: data flow and operation flow
                                            According to different implementation methods, divided into: bottom-level flow and high-level flow
3. Bytes Stream
Treats bytes as the minimum transmission unit Composed of corresponding data streams and operation streams
Byte input stream abstract parent class: InputStream
Byte Output Stream Abstract Father Class: OUTPUTSTREAM
Data Stream (Underwater Stream): Responsible for building a channel for byte data transmission, responsible for the transmission of byte data, Only basic byte data access methods are provided. InputStream and OutputStream are the parent classes of all byte stream classes. They are both abstract classes and cannot be instantiated. Several methods are defined to standardize the behavior of subclasses, such as: the IO stream must be closed after the end.
InputStream /OutputStream
FileInputStream/FileOutputStream
Operation stream (high-level stream): does not read or write data from IO devices, only reads or writes data from other streams, provides Ordinary streams do not provide methods to simplify programming or improve IO efficiency. The operation stream takes advantage of the "decorator" design pattern, inherits the InputStream class itself, and adds a large number of methods to "decorate" other data flow classes, avoiding the creation of a large number of new functional classes.
BufferedInputStream/BufferedOutputStream
4. Character stream
Character input stream abstract parent class: Reader Character output stream abstract parent class: Writer
Data stream (underlying stream):
Reader/Writer
FileReader/FileWriter
Operation flow (high-level flow):
BufferedReader/BufferedWriter
##5 , Byte character bridge stream
InputStreamReader: Use the adapter design pattern to convert the InputStream type to the Reader type
OutputStreamWriter: Use the adapter design pattern to convert the OutputStream type to Writer type
6.FileBasic concepts of classes
File class: represents an abstract representation of file and directory path names.
The File class can create, delete, rename files, obtain paths, create time, etc. It is the only operation class related to the file itself.
Chapter 6: Thread
Thread is the sequential control flow within a program
1. Threads and processes
1. Each process has independent code and data space, and process switching is expensive
2. Threads are lightweight processes. Threads of the same type share code and data space. Each thread has an independent running stack and program counter. Thread switching overhead is small
3. Multi-process: It can run simultaneously in the operating system Run multiple tasks (programs)
4. Multi-threading: Multiple sequence flows are executed simultaneously in the same application
2. Create threads
Java threads are implemented through the java.lang.Thread class. Each thread completes its operation through the run method corresponding to a specific Thread object. The run method is called thread body.
       There are two ways to create a thread in Java language:
                                                                                                                                                                                                    .Rewrite the run method in the Runnable interface
3. Create an object of the implementation class
4. Create a thread object and pass the implementation class object into
5. Call Start method startup thread
# B. Inherit the Thread class
1. Define a class and directly inherit the Thread class creation thread
## 2. Rewill the Run method in the Thread class
##                                                                                                                                                                                                                                                  3. Create a thread object
                      4. Call the start method to start the thread
# è Use the Runnable interface to create a thread
to separate the CPU, code, and data. Form a clear model
The implementation class can also inherit some useful properties and methods from other classes
It is helpful to maintain the consistency of the program style
è Directly inherit the Thread class creation Thread

##4. Other concepts
1. Background processing (Background Processing): Give way to other programs, while the CPU is free, when there is no Tasks that can only be run when higher priority and more urgent tasks need to be processed, such as: printing document operations, etc.
2. Background Thread/Daemon Thread: Runs in the background, Usually some secondary threads that provide services to other threads, such as: garbage collection thread in JVM 3. User thread (User Thread): Thread created by the user or created by the system for the user
4. Main Thread (Main Thread): The main thread for application running, which is a type of user thread
5. Sub Thread (Sub Thread): Created in a running thread New thread
## 五五新新新新
This provided by class:
public final boolean isDaemon() // Determine whether a thread is a background thread When all user threads have stopped running, whether it exits normally or terminates early with an error, the JVM will exit. It can be understood that the background thread is not taken seriously and serves the user thread 6. Thread life cycle
New state: Newly created thread object
Ready state: Enters the ready state after calling the start method
Running status: control from the thread scheduling of JVM to control
blocking status: When the incident that causes threads occurs, adjust the JVM thread scheduler to the blocking state, such as in the thread Perform IO operations or wait for events such as data input from the keyboard Termination state: When all programs in the run method are executed, the thread will enter the termination state
Special Note: It is forbidden to start a terminated thread again, and a thread is not allowed to be started multiple times
7. Thread priority
The priority of a Java thread is represented by a number, ranging from 1 to 10
The default priority of the main thread is 5, and the priority of the child thread is the same as the priority of its parent thread by default
Relevant methods provided by the Thread class
Public final int getPriority()//Get the priority of the thread
Public final void setPriority(int newPriority)//Set the priority of the thread
Relevant static integer constants:
Thread.MIN_PRIORITY=1
Thread.MAX_PRIORITY=10
Thread.NORM_PRIORITY=5
   Execute the currently running thread to enter the blocking state, and then wake up and transfer to the ready state after the specified "delay time"
Related methods provided by the Thread class: public static void sleep(long millis) Public static void sleep(long millis,int nanos)
Note: 1 second = 1000 milliseconds, 1 millisecond = 1 million nanoseconds
9. Thread concession
Let the running thread actively give up the CPU processing opportunity currently obtained, but instead of blocking the thread, it will redirect it Enter the ready state
Relevant methods provided by the Thread class: public static void yield()
10. Thread suspension and recovery
: Specify the threads in the current operation so that it can be transferred to the blocking state, and it will not automatically recover.
Relevant methods provided by the Thread class: public final void suspend()//Thread suspension
##11. Thread synchronization and deadlock
## A resource is shared data among multiple threads.
2. Thread synchronization: To solve the problem of data sharing, synchronization must be used. The so-called synchronization means that only one thread of multiple threads can execute the specified code in the same time period, and other threads must wait for this thread to complete. before you can continue execution.
                                                                                                                        Operations to be synchronized
}
(2) Synchronization method
’ ’ s ’ s ’ ’ ’ ’ ’ minate There are some guidelines that can be followed that go a long way in avoiding the risk of deadlocks and performance hazards: Move preprocessing and postprocessing that do not change with threads out of synchronized blocks.
(2) Do not block. Such as user input and other operations.
(3) Do not call the synchronization method for other objects when holding a lock.
4. Thread deadlock: Too much synchronization may cause deadlock. Deadlock operations generally only occur when the program is running.
                                                                                                                                                                                              can be shared in multiple threads, requires synchronization, but too much synchronization may cause deadlock.

Chapter 7:
Socket
Network Programming

1. What is a computer network? system, so that more computers can easily transfer information to each other and share resources such as hardware, software and data2.

Socket

Programming

## For "Socket" (Socket) Socket is usually used to implement Client-Server connection Two classes Socket and ServerSocket are defined in the java.net package, which are used to implement bidirectional Connected client and server3.
Socket

Basic steps of programming

1. Establish a network connection 2. Open the input/output stream connected to the Socket 3. Read data through the opened IO stream/ Writing operation 4. Close the opened IO stream and Socket objectSupplementary content 1: Graphical user interface
Graphical user interface (GUI -graphics user interface) provides a more humane and convenient operation method for visual interaction between users and programs. Therefore, designing a graphical user interface application must have its characteristic elements, such as menus, toolbars, controls, etc.
Java provides a rapid application development method based on GUI components for designing graphical user interface applications, and provides a large number of available components to support event-driven development.

1,

Swing

and

AWT

AWT: Early Java included an Abstract Window Toolkit (AWT) class library for basic GUI programming, such as Button, Checkbox, List, etc. These classes are all subclasses of the Component class

AWT components are considered heavyweight components because they rely on the local system to support drawing and display. Each AWT component has a related component in the local window system

Swing component It is written in pure Java code and is called a lightweight component. It does not rely on local components to support its display, but is completely drawn and displayed by Java code, so it is much more efficient than the usual AWT component.

Swing: The Swing library is newer. In order to avoid duplication of names, all Swing component classes start with the capital letter J. Most of the Swing components are sub -categories of JCOMPONENENENENENENENENENENENENENENENENENENENENT class

## 二 2. Window container ## JFRAME (window): top application Program window

JDialog (Dialog): Modal and modeless dialog boxes that collect user input

JPanel (Panel): A small container that accommodates part of the interface

Supplementary content two:
JDBC

1. Overview Most development languages provide database Visit

# JDBC is a set of interfaces and classes provided by Sun Company. It is encapsulated in the Java.SQL package and used to access the third -party driver and class provided by the database

# database manufacturer package, you can access a specific database

2.JDBC## Connectivity) Java database connection, supports Java applications to connect to the database
Basic functions:
1. Supports basic SQL statements, implements database operation functions in Java programs and simplifies the operation process



1. Do a good job of preparing for a certain database

Load the JDBC driver identification data Yuan 2. Establish a connection channel with the database

allocated a Connection object

3. Start request to show what you want to do

Allocate a Statement object

Use the Statement object to execute sql statements


Turn off the relevant objects, release resources

## 四, related categories and interfaces

## Java.sql.driverManager class
Driver management provides management functions for different database drivers java.sql.Driver interface
Database driver, software driver
java.sql. Connection interface
Connection object connected to a specific database
java.sql.Statement interface
Object that operates the database
java.sql.ResultSet interface
1 An interface program provided by a kind of hardware or software
Database driver: A collection of codes required by an application to operate a certain database. It can be understood as a software module that introduces mutual understanding between the application and the database
Enumeration:
Enumeration is to allow the value of a certain type of variable to be only one of several fixed values, otherwise the compiler will report an error. Enumeration allows the compiler to compile You can control the illegal values assigned by the source program. This goal cannot be achieved during the development stage by using ordinary variables.
After JDK1.5, the keyword enum is used to define a new type, called an enumeration type.
The enumeration class defined by enum keywords is actually equivalent to defining a class. Such inherits the enum class.

##Exception classes

Description

##ArithmeticException

##Array index out of bounds exception

##NullPointerException

Class not found exception

##IOException

Input and output exception

##FileNotFoundException

File not found exception

SQLException

SQL statement execution abnormal

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