2022-07-28 study notes of group 4 self-cultivation class (every day)

 API, Application programming interface .java API It's a programmer's Dictionary , yes JDK The description document of the class provided to us in , We can check API The way , To learn java Class provided , And learn how to use them .

API Use steps :

Open help document .
Click to display , Find the index , See the input box .
Who are you looking for ？ Enter... In the input box , And then go back .
Look at the bag .java.lang The following classes don't need a guide package , Other needs .
Look at the explanation and description of the class .
Learn how to construct .
Use the member method .

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Scanner class

Scanner Use steps

1、 view classes

java.util.Scanner: This class needs import Use... After import

2、 Look at the construction method

public Scanner: Construct a new Scanner, The value it generates is scanned from the specified input stream .

3、 View member methods

public int nextInt(): Scan the next tag of the input information as a int value , Use Scanner class , Complete the operation of receiving keyboard input data

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Random 

Examples of this class are used to generate pseudo-random numbers

Random Use steps

view classes ：

java.util.Random: This class needs to be used after bootstrapping .

Look at the construction method public Random(): Create a random array

View member methods public int nextInt(int n): Returns a pseudo-random number , The scope is 0（ Include ） and Specify the value n( barring ) Between int value

Create a Random object , Every time you call nextInt() Method , Will generate a random number  

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 Arraylist class

java.util.ArrayList Is the implementation of variable size array , Data stored inside is called an element , This class provides some methods to manipulate internal storage elements .ArrayList Elements can be added continuously , It also grows in size

ArrayList The bottom layer of the collection adopts the data structure of array

ArrayList Collection is not thread safe

ArrayList Set initialization capacity 10

ArrayList The bottom of the set is Object An array of types Object[]

1、ArrayList Class is located java.util In bag , You need to quote it before using , The syntax is as follows ：

import java.util.ArrayList;// quote ArrayList class

ArrayList<E> objectName=new ArrayList<>();// initialization

E: Generic data types , Used for setting up objectName Data type of , Only data types can be referenced

 2、ArrayList It's an array queue , Provides related additions 、 Delete 、 modify 、 Ergodic function

        2.1、 Add elements to ArrayList have access to add() Method

        2.2、 visit ArrayList Elements in can be used get() Method ：

        2.3、 modify ArrayList Elements in can be used set() Method ：

        2.4、 Delete ArrayList Elements in can be used remove() Method

        2.5、 Calculation ArrayList The number of elements in can be used size() Method

2.6、 use for Loop through the elements in the array list

2.7for-each To traverse  

3、 Other reference data types

 4、ArrayList Sort

Collections class It is also a very useful class , be located java.util In bag , Provided sort() Method to sort a list of characters or numbers

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String class

 java.lang.String Class represents string . This class does not need to be imported

1、String Class creation object

use String Class creation objects usually have two forms ：

        1.1、String str="cxy";

        2.2、String str =new String("cxy");

2、String Common methods of class

        1.1、 Get string length ：

         Variable name .length();

        

        2.1、 Get the position of the character in the string

        Variable name .indexOf (String str);

         Search from the beginning str First occurrence in string ;

        Variable name .indexOf(String str,int fromIndex);

         From the subscript fromIndex Start searching at str First occurrence in string

        Variable name .lastIndexOf(String str);

         Start at the tail and find str Last occurrence in string

        Variable name .lastIndexOf(String str,int fromIndex);

         From the subscript fromIndex Start searching at str Last occurrence in string

 2.2、 Get the character at a certain position of the string

Variable name .charAt(int index);

The index of the search string is Index String of positions

2.3、 Intercepting string

Variable name .substring(fromindex,endlndex); 

from fromIndex Start intercepting at endIndex At the end of , Does not contain the subscript endIndex The characters of ;

Variable name .substring(fromIndex);

from fromIndex From the beginning to the end ;

2.4、 Split the string into an array

        Variable name 1.split(String str2);

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static keyword

1、 Class variables ：

When static When decorating member variables , This variable is called a class variable

give an example ： static int num;

2、 Static methods

When static When decorating member methods , This method is called class method

Definition ：

Modifier static return type Method name （ parameter list ）{

}

3、 Considerations for static method calls

         3.1、 Static methods can directly access class variables and static methods

        3.2、 Static methods cannot directly access ordinary member variables or member methods . Member methods can directly access class variables and static methods

        3.3、 In the static method , Out of commission this keyword

Static methods can only access static members

4、 Static code block

        4.1、 Location ： Class

        4.2、 perform ： It is executed as the class is loaded and once , Prior to the main Execution of methods and construction methods

Format ：

public class Name{
    static{
       Execute statement   
    }
}

  effect ： Initialize and assign values to class variables

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Arrays class

java.util.Arrays This class contains various methods of manipulating arrays

1、 How to operate the array

        public static String toString(int[] a) ： Returns a string representation of the contents of a specified array

        public static void sort(int[] a) ： For the specified int Type arrays are sorted in numerical ascending order

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Math class

 java.lang.Math Class contains methods for performing basic mathematical operations , Such as the primary index 、 logarithm 、 Square root and trigonometric function

1、public static double abs(double a) ： return double The absolute value of value

2、public static double ceil(double a) ： Returns the smallest integer greater than or equal to the parameter

3、public static double floor(double a) ： Returns an integer less than or equal to the maximum parameter

4、public static long round(double a) ： Returns the closest parameter long( It's equivalent to rounding )       

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Object class

java.lang.Object Class is Java The root class in language , When an object is instantiated , The ultimate parent is Object

If a class has no specially specified parent , Then the default is to inherit Object class

public String toString(): Returns the string representation of the object         

toString Method returns the memory address , In development , It is often necessary to get the corresponding string representation according to the properties of the object , So we have to rewrite

        Rewriting covers

          

public boolean equals(Object obj): Indicates whether some other object is associated with this object “ equal ”

To compare two strings, you must use equals Method , Out of commission "=="

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Date time class

1、Date class

java.util.Date class Represents a specific moment , Accurate to milliseconds

Common methods