Common auxiliary classes

Java Summary of knowledge points ： You can enter from here if you want to see

7.1 Number class

​ abstract class Number yes BigDecimal、BigInteger、Byte、Double、Float、Integer、Long、Short The parent of a class . They are the corresponding byte、double And other basic data types are encapsulated into classes ,Java Call it the wrapper class

7.1.1 Integer class

Integer and int The difference between ：

int yes 8 One of the basic data in ,Integer yes java Is the basic data type int Provided encapsulation class . They have different characteristics and usages .
1、 size ：int In memory stack , Occupy 4 byte , Fast storage .Integer On the memory heap , Large space and slow speed .
2、 The default value is ：int by 0,Integer by null, You can distinguish between an unassigned value and a value of 0 The difference between , and int You can't .
3、Integer Is the class provided , Therefore, some methods and properties are provided internally .
4、int Mainly used in calculation , and Integer Used in type conversion 、 Access data to a collection . Methods provided 

If you create an integer using an encapsulated class . Values in -128~127 Between time , No new objects will be created , Instead, it directly refers to the created objects in the constant pool .

Integer i1 = 100, i2 = 100, i3=200, i4=200;
i1 and i2 Share one object ,i3 and i4 Use different objects 

7.1.2 Boolean value

7.1.3 floating-point

7.1.4 Character

7.1.5 Automatic unpacking and packing

Automatic unpacking and packing

• Packing ： Change the basic data type into a wrapper class object .

• Unpacking ： Take out the basic data type of the package in the wrapper class .

• When autoboxing, the compiler calls valueOf Convert the original type value to an object , At the same time, when the case is opened automatically , The compiler calls something like intValue()、
  doubleValue() This kind of method converts the object to the original type value .

  

7.2 Math class

​ Contains properties and methods for performing basic mathematical operations , Such as the primary index 、 logarithm 、 Square root and trigonometric function .Math All methods are defined as static form , adopt Math Class can be called directly in the main function .

math class

​ Math.round(11.5) and Math.round(-11.5) Value

​ answer ： round To round off ,Math.round(11.5)=12 and Math.round(-11.5)=-11

7.3 random class

Common methods ：

7.4 BigInteger

7.5、 BigDecimal

7.6 Object

7.7、String Related classes

1. String： String class , By final The value of the modifier cannot be changed . Realized Serializable and Comparable Interface , Serialization and size comparison are supported ; Save a reference to an address after its definition , Every time the value is changed, a new memory address assignment is reallocated , Instead of modifying in the original address .

2. StringBuilder： String class , Its value can be modified , Thread unsafe , Fast .

3. StringBuffder： String class , Its value can be modified , Thread safety , Speed is relative to StringBuilder slow .

   StringBuilder and StringBuffder At the bottom of the is a character array , The expandable length after creation is 16 Characters , When the modification scope exceeds 16 character , The capacity of the underlying array will be expanded . Create a new array with the size of the original 2 Doubling 2, Point the pointer to the new common array address .

   StringBuilder and StringBuffder Common methods ：
   1、.append("").append("").....;： String concatenation 
   2、.delete(int start,int end)： Delete the contents of the specified range ,[start,end)
   3、.replace(int start, int end, String str)： Replace the contents of the specified range [start,end)
   4、.insert(int offset, xxx)： Insert the specified content in the specified position 
   5、.reverse() ： Reverse the current character sequence 
   6、public int indexOf(String str) :  Returns the index of the first occurrence of a specified substring in the current string 
   7、public String substring(int start,int end) ： Returns a substring of the specified range [start,end)
   8、public int length() :  Returns the length of the string 
   9、public char charAt(int n ) :  Gets the character at the specified index 
   10、public void setCharAt(int n ,char ch) :  Set the character at the specified index 
   

Next, we can intuitively compare the use of String and StringBuilder The difference in speed

Calculate the additional 100,000,0 The time taken for the second character

1. Use String

   public static void main(String[] args) {
         
       long start = System.currentTimeMillis();
       String str = "";
       for(int i=0; i<1000000; i++){
         
           str += 1;
       }
       long end = System.currentTimeMillis();
       System.out.printf(" Use String when ：%d",end-start);
   }
   

2. Switch to StringBuilder

   public static void main(String[] args) {
         
       long start = System.currentTimeMillis();
       StringBuilder builder = new StringBuilder();
       for(int i=0; i<1000000; i++){
         
           builder.append(1);
       }
       long end = System.currentTimeMillis();
       System.out.printf(" Use StringBuilder when ：%d",end-start);
   }
   

3. Use StringBuffder

   public static void main(String[] args) {
         
       long start = System.currentTimeMillis();
       StringBuffer buffer = new StringBuffer();
       for(int i=0; i<1000000; i++){
         
           buffer.append(1);
       }
       long end = System.currentTimeMillis();
       System.out.printf(" Use StringBuffer when ：%d",end-start);
   }
   

It is obvious from the above that String When compared with the latter two, the speed is not a little slower , But many times the gap , and StringBuilder Comparison StringBuffder Faster, too .

7.8、 Time processing