Table of contents
What is the JDK? What is it composed of?
Let's start to study and consolidate now. I just started learning Java last year, but I stopped learning halfway. This is a very irresponsible approach, not only irresponsible to oneself, but also irresponsible to the entire college career! Let us supervise and study together, even if it is a salted fish, we must be the saltiest fish, so that everyone dare not try it!
consolidate
Java does not run directly in the system, but in a virtual machine;
javac compilation tool
java runtime tool
jdb debugging tool
jhat memory analysis tool
Programming: Java programmers write .java code, C language writes C code, and Python programmers write .py code.
Compilation: The machine only understands the machine language of 0011, and the process of converting the code of .java.c.py for the machine to understand.
Run: Let the machine execute the compiled instructions.
What is the JDK? What is it composed of?
JDK is the Java Development Kit
JVM virtual machine: where Java programs run
Core class library: Java has written something, we can use it directly~
Development tools: javac java jdb jhat etc.
What is JRE?
JRE is the Java runtime environment
JVM core class library running tools
The inclusion relationship among JDK, JRK, and JVM?
JDK includes JRE
And the JRE contains the JVM
keywords
Keywords are English words that are endowed with specific meanings by Java.
Keywords are all lowercase
class : used to create or define a class; a class is the most basic unit; the class keyword indicates a class, followed by the class name.
Literal
Integer type: number without decimal point
DecimalType: Number with decimal point
String type: content enclosed in double quotes
Character type: Enclosed in single quotes, there can only be one
Boolean type: Boolean value, only two values, true and false, indicating true and false
empty type: null
The empty type cannot be printed directly. If it must be printed, it can only be printed in the form of a string, such as
System.out.println("null");
The literal value is to tell the programmer that the data is written in the program!
Special characters
\t Tab character (used most when printing tabular form)
When printing, make up the length of the previous string to 8, or an integer multiple of 8, at least one space and at most 8 spaces.
variable
The following is the basic usage of variables:
- Define variables and output
- Variables participate in the calculation
- Modify the value of a variable record
Note on variables
only one value can be stored
Variable names are not allowed to be defined repeatedly
One statement can define multiple variables
Variables must be assigned a value before being used
When defining a variable, the assignment cannot be written separately
variable scope
Variable summary:
Exercise: watch the following video
data storage in computer
Computer storage rules: In a computer, any data is stored in binary.
Any base to decimal
binary to decimal
Octal to Decimal
Decimal to Decimal
Decimal to other base
ASCII code table
