Java SE02 Java language Fundamentals: keywords, identifiers, annotations

• Keywords: Some words that have been given Java meaning,class public static void

  Keyword that defines the access permission modifier

  Private protected public

  Defining classes, functions, variable modifiers

  Abstract final static synchronized

  Keywords that define relationships between classes and classes

  Extends implements

Define an instance, reference an instance, and judge an instance

New This super instanceof

Handling Exceptions

Try Catch finally throw throws

Keywords for the package

Package Import

Keywords for other modifiers

Native STRICTFP transient volatile assert

• Identifiers: Some of the names that you customize in your program, such as defining classes. Try to make it meaningful and enhance the reading of the program

26 English letter case, 0-9 digits, _$ two symbols. But cannot start with a number and cannot use the keyword

• Notes for easy reading, interpretation of program language

Single-line note the Yellow River into the ocean in the daytime

/*

Multi-line comments, can also be used to narrow the scope of the troubleshooting!

The day depends on the mountain

The Yellow River into the ocean

*/

/**

Document annotations, which can be extracted using Java Javadoc

Used as a program manual

*/

• Constants and variables

Constants are values that cannot be changed (integers, decimals, Boolean Boolean constants, character constants ' strings "" null)

A variable is a storage area in memory that has its own name and type

1. byte short int long//integer type
   

2. float Double//floating-point type
   

3. char//character type
   

4. Boolean//Boolean type

5. Class//Classes

6. interface//interface
   

[]//array

Class vardemo{public static void Main (string[] args) {byte b=126;//character test int x=327899;//test int length    System.out.println (b);    SYSTEM.OUT.PRINTLN (x); }}

Execution results

D:\lab\java>javac 0908.java
D:\lab\java>java Vardemo
126
327899

Lass vardemo2//automatic type promotion with coercion type conversion {public static void main (string[] args)/* {byte x=9;     int y=65535; Y=x+y;    Add x y to System.out.println (y);    } */{byte b=100;      B= (Byte) (b+30);//coercion type conversion System.out.println (b); }}

D:\lab\java>javac 090802.java
D:\lab\java>java VarDemo2
-126

The ASCII code and the GBK code of a single letter or kanji will also be corresponding to the code table in memory to produce corresponding values

{System.out.println (' I ' +0); }

D:\lab\java>javac 090802.java
D:\lab\java>java VarDemo2
25105

An interesting case.

{byte b=4;      BYTE b1=3;      BYTE b2=5;      B=B1+B2;          System.out.println (b); }}

D:\lab\java>javac 090802.java
090802.JAVA:20: Error: Possible loss of precision
B=B1+B2;
^
Required: Byte
Found in: int
1 errors

Cause: B1 B2 is a variable, although the above defines a value, but not always fixed, such as the next time to assign a value b1=127? If the byte load range is exceeded, the precision is lost. Here is a concept that lets us understand the variables.

B=B1+B2//Right is not fixed, there is no way to judge

But int does not have this problem (only 32-bit cast high-level full discard is reserved)

• Operator

• Statement

• Function

• Array

This article is from "The Old artisan" blog, declined reprint!

Java SE02 Java language Fundamentals: keywords, identifiers, annotations