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Common APIs

2022-07-03 07:06:00 【L gold p】

1.String

1、1 summary

java.lang.String： It's a string class , The bottom floor is a final Embellished char Array , therefore String Many features are the properties of arrays

For example, once the length is determined Can't change

Once the string is created , This string object cannot be changed

To improve string access and storage efficiency Java The virtual machine uses a caching mechanism , Save the strings to the string constant pool

In the process of execution , If you want to use a string a,String s1="a" First, search the character constant pool Is there a a, If not, create one If there is Sting s2="a"; No more creation , Take the existing one a return

So lead to String s1="a" String s2="a" here s1==s2 He is also true Because they point to the same string object Namely a

1、2 Basic use

1、3 Do not splice frequently

1、4 Construction method

1、5 Common methods

Study API

1 What is the function

2 What are input and output parameters

3 How to use it?

// 1 char charAt (int index ) : Returns... In the string , The character in the specified position

String s1 = "qwehihasd";

char c1 = s1.charAt(2);

// e

System.out.println(c1);

// 2 boolean endsWith(String suffix) : Determines whether the string ends with the specified string

// boolean startsWith(String prefix) : ditto , Judgment begins

System.out.println("Hello.java".endsWith(".java"));

// Note blank space , If there is a space, it won't match

System.out.println("Hello.java".endsWith(".java "));

// 3 boolean equalsIgnoreCase(String str) : Case insensitive comparison of two strings for equality

System.out.println("abc".equalsIgnoreCase("aBc"));

// 4 byte[] getBytes() : Converts a string to a byte array and returns

byte[] bytes = "abc".getBytes();

for (int i = 0; i < bytes.length; i++) {

System.out.println(bytes[i]);

}

// 5 char[] toCharArray() : Converts a string to a character array and returns

char[] chars = "abc".toCharArray();

for (int i = 0; i < chars.length; i++) {

System.out.println(chars[i]);

}

// 6 int indexOf(String str) : Gets the starting index of the specified string in the string , No return found -1

System.out.println("askdhqwbe".indexOf("kd")); // 2

System.out.println("askdhqwbe".indexOf("kda")); // -1

// 7 int indexOf(String str,int index) :

// Start at the specified location ( contain ), Gets the starting index of the specified string in the string , No return found -1

System.out.println("askdhaqwbe".indexOf("a", 5)); // -1

// 8 index lastIndexOf(String str) : ditto , The last index to appear No return found -1

System.out.println("askdhaqwbe".lastIndexOf("a")); // 5

// 9 int length() : Returns the length of the string

System.out.println("abc".length());

// 10 String replaceAll(String regex . String replacement);

// Replace specified characters , regular expression

// String replace (String str . String replacement); Regular expressions are not supported

// use 1 hold a Replaced and returned a new string

// regular expression , But I didn't write regular expressions There is no difference

System.out.println("hucgasdqweasd".replaceAll("a", "1"));

System.out.println("hucgasdqweasd".replace("a", "1"));

// because . In regular expressions , Arbitrary character

System.out.println("qwe.rty.yui.uio".replaceAll(".", ","));

// have access to \ escape

System.out.println("qwe.rty.yui.uio".replaceAll("\\.", ","));

System.out.println("qwe.rty.yui.uio".replace(".", ","));

// 11 String[] split(String regex) : Split string , Return string array , regular expression , Be careful .....

// spot , Need to escape

String s2 = "2022.1.14";

String[] arr = s2.split("\\.");

for (int i = 0; i < arr.length; i++) {

System.out.println(arr[i]);

}

// 12 String substring(int begin); In the string , Substring starting with a subscript ( contain )

System.out.println("abcdef".substring(2));// cdef

// 13 String substring(int begin, int end) :

// In the string , Start with a subscript ( contain ) Substring ending at a subscript ( It doesn't contain )

System.out.println("abcdef".substring(2, 4));// cd

// 14 String trim() : Remove the spaces on both sides of the string

System.out

.println(" a d sadasd ");

System.out

.println(" a d sadasd "

.trim());

// 15 String toUpperCase() : Turn capitalization

// String toLowerCase() : Turn lowercase

System.out.println("asd".toUpperCase());

// 16 static String valueOf(Object obj) :

// Calling the toString Method , If null, No more calls toString Instead, it returns a string null

String_05 s = null;

// When printing a reference type , Automatically called String Of valueOf therefore Automatically called toString Method

System.out.println(s);

1、6 Be careful

2、StringBuffer and StringBuilder

2、1 summary

2、2 Basic use

2、3 Common methods

3、 Packaging

3、1 summary

【 ask 】 Want to do more with basic type data , What do I do ？

【 answer 】 The most convenient way is to encapsulate it as an object . Because more properties and behaviors can be defined in the object description to operate on the basic data type . We don't need to encapsulate basic types by ourselves ,JDK It has been encapsulated for us .

【 Concept 】

1. Boxing is the automatic conversion of basic data types to wrapper types

2. Unpacking is automatically converting the wrapper type to the basic data type

3、2 Use

3.3 Integer

3.3.1 Basic use

3.3.2 transformation

3.3.3 Common methods

3.3.4 Automatic packing and unpacking

3.3.5、 Deep shaping constant pool

Judge the current value Whether in -128~127 Between

If it's not in range Just directly new object

3.3.6 summary

4.System

4.1 summary

System Class provides the public static long currentTimeMillis() Used to return the current time Between and 1970 year 1 month 1 Japan 0 when 0 branch 0 The time difference in milliseconds between seconds .

This method is suitable for calculating time difference .

System Class represents system , Many system level properties and control methods are placed inside the class . This class is located in java.lang package .

 Because the constructor of this class is private Of , So we can't create objects of this class , That is, it can't be realized Instantiate this class . Its internal member variables and member methods are static Of , So it can also be very convenient To call .

 Member variables

System Class contains in、out and err Three member variables , They represent the standard input stream

( Keyboard entry ), Standard output stream ( Monitor ) And standard error output stream ( Monitor ).

 Member method

native long currentTimeMillis()：

The function of this method is to return the current computer time , Time is expressed in the current computer

Between and GMT Time ( Greenwich mean time )1970 year 1 month 1 Number 0 when 0 branch 0 The number of milliseconds in seconds .

void exit(int status)：

The function of this method is to exit the program . among status The value of is 0 It means normal exit , Non zero means

Abnormal exit . Using this method, the exit function of program can be realized in GUI programming .

4.2 Common methods