Java key terms

Simplicity, object-oriented, distributed, robustness, security, architecture neutrality, portability, interpreted, high performance, multithreading, dynamic

The difference between JDK and JRE

JDK: Java Development Kit (java development kit)
JRE: Java Runtime Environment (java runtime environment)
JRE is a java runtime environment, including java virtual machine, java basic class library. It is the software environment required for running programs written in java language, and is provided to users who want to run java programs.
JDK is a java development kit, a development kit required by programmers to write java programs in java language, and is provided for programmers to use. JDK includes JRE, as well as javac, a compiler for compiling java source code, as well as many tools for debugging and analyzing java programs: jconsole, jvisualvm and other tools, as well as documentation and demo example programs required for writing java programs. .

HelloWorld

public class HelloWorld {
    
    
    /**public  访问修饰符，用于控制程序的其他部分对这段代码的访问级别。
     * class 类
     * HelloWorld  类名
     * main是程序的入口
     * */
    public static void main(String[] args) {
    
    
        /*java注释*/
        //java注释
        /*
         *java 多行注释
         **/
        System.out.println("Hello World!");
    }
}

Eight basic types of java

int 4 bytes
short 2 bytes
long 8 bytes
byte 1 byte
float 4 bytes
double 8 bytes
char The literal value of char type should be enclosed in single quotes
boolean Boolean type only has true and false
1 byte (Byte) = 8 bits (bit)
eight basic data types: byte, short, int, long, float, double, boolean, char.

Operator

Operators are used to connect values.
Arithmetic operators +-* /% take remainder
. The general form of ternary operation is: expression 1? expression 2: expression 3;
for example, min=(a<b)?a:b;
Insert picture description here

Insert picture description here
Bit operation (shift, and &, or |, exclusive or ^, not ~)
shift is divided into left shift <<, right shift >>, unsigned left shift <<<, unsigned right shift >>>

Mathematical function

Math.exp(x) //Returns the arithmetic constant E to the power of x

Math.sqrt(x) //return the square root of x

Math.ceil(x) //Math.ceil(10.1) =>11(advance one)

Math.floor(x) // Math.floor(15.6) =>15(去尾)

Math.rint(x) //Round to the nearest integer of x, return an even integer if the distance is equal

Math.round(x) //Round to integer

Math.min/max Math.abs //returns the minimum and maximum of two numbers/returns the absolute value

Math.pow(a,b) //return a to the b power

Introduction to java core technology 01