2-1 Java language Basics

The contents of this chapter:

· Key words

· Identifier

· Comments

· Constants and variables

· Operator

· Statement

· Function

· Array

Key words:

· Keyword Overview

Words that are given a specific meaning by the Java language

· Keyword features

The letters that make up the keywords are all lowercase

· Keyword considerations

Goto and const exist as reserved words and are not currently used

Advanced Notepad like notepad++, with special color tags for keywords, very intuitive

Determine which of the following are keywords

Class,helloworld,public,static,void,main,string.system

Keywords to define the data type
Class	Interface	Byte	Short	Int
Long	Float	Double	Char	Boolean
void
Used to define a data type worth a keyword
True	False	Null
Keywords to define Process Control
If	Else	Switch	Case	Default
While	Do	For	Break	Continue
Return
Keywords used to define access rights modifiers
Private	Protected	Public
Keywords for defining classes, functions, variable modifiers
Abstract	Final	Static	Synchronized
Keywords used to define the relationship between classes and classes
Extends	Implements
Used to define instance and reference instances, and to determine the keywords
New	This	Super	instanceof
Keywords for exception handling
Try	Catch	Finally	Throw	Throws
Keywords for the package
Package	Import
Other modifier keywords
Native	Strictfp	Transient	Volatile	Assert

Identifier

· Identifiers Overview

Is the sequence of characters used to name classes, interfaces, variables, etc.

· Composition rules

English uppercase and lowercase letters

numeric characters

$ and _

· Precautions

Cannot start with a number

Cannot be a keyword in Java

Case sensitive

Attention:

1: From the name, no more casual, no rules, inadequate surrounding area

2: For precautions, examples show

3: Under those legal, those not valid:

helloworld,dataclass,_983, $bS 5_c7,class (x), dataclass# (x), 98.3 (x), Hello World (x)

Identifiers (common naming rules)

· Package (in fact, is the folder, to solve the same name problem)

Single and multi-level examples respectively

· Class or interface

An example of a word and multiple words respectively

· Methods and variables

An example of a word and multiple words respectively

· Constant

An example of a word and multiple words respectively

Comments:

Comments Overview

Text used to interpret the description program

Comment Classification format in Java

Single-line Comment

Format://Comment text

Multi-line comments

Format:/* Comment Text */

Document comments

Format:/** Note text */

Precautions:

A comment is a good programming habit that a programmer must have. Beginners can develop a habit of writing a program: Write a comment and then write the code.

Put your thoughts through the comments first, in the code to reflect. Because the code is just a form of reflection of thought.

Annotated version of HelloWorld:

Requirements: Write a program in the console output HelloWorld

Analysis:

Write a Java program that defines the class first.

To be able to be called by the JVM, a program must define the Main method.

In order for the program to have output, the output statement must be used.

Realize:

The class keyword is used for defining classes, followed by the class name

Main Method Basic Format

Output Statement Basic format

Simple features of annotations:

Explain the procedure, improve the reading of the program

can help us in the wrong way, we'll explain the more advanced troubleshooting

Constant:

Constants Overview

Its value cannot be changed during program execution

Constant classification in Java

Literal constants

Custom Constants (Object-oriented section)

The contents of a string constant enclosed in double quotation marks

integer constant all integers

12,23

Decimal constant All Decimals

12.34,56.78

The contents of a character constant enclosed in single quotation marks

' A ', ' a ', ' 0 '

Boolean constants are more specific, only true and false

Empty constant Null (part of the array explained)

Different binary:

Java provides 4 forms of expression for integer constants

Binary

Octal

Decimal

Hexadecimal

Overview of the binary system

In the system: is the carry system, is a kind of carry-out method stipulated by people. For any kind of binary--x, it means that the number operation at a certain position is every x in a bit. Binary is every two in one, eight into the system is every eight into a, decimal is every ten into one, Hex is 16 into one.

Different binary data composition

Binary

Made up of 0, 1. Start with 0b

Octal

by 0,1,... 7 composition. Start with 0

Decimal

by 0,1,... 9 composition. Integers are decimal by default

Hexadecimal

by 0,1,... 9,a,b,c,d,e,f (both uppercase and lowercase). Start with 0x

Binary conversion

Other binary to Decimal

Introduction of the conclusion by decimal

Convert the 0b100,0100,0x100 into decimal

Decimal to another binary

Introduction of the conclusion by decimal

The 52 is represented as binary, octal, hexadecimal

Fast conversion of decimal and binary binary

8421 yards

100 Turn into binary

101101 turn into decimal

Binary and octal, how hexadecimal is converted

Using decimal as a bridge

Binary to octal 3-bit combination

100110

Binary to hexadecimal 4-bit combination

100110

Symbolic data representation

Within the computer, there are 3 notation numbers: The original code, the inverse code, and the complement. All data is performed in the complement.

Original code

is the binary fixed-point notation, that is, the highest bit is the sign bit, "0" means positive, "1" is negative, and the remaining bits represent the size of the value.

Anti-code

The inverse code of a positive number is the same as its original code, and the inverse of a negative number is a bitwise negation of its original code, except for the sign bit.

Complement

The complement of a positive number is the same as its original code, and the complement of a negative number is the minus 1 of its inverse code.

Variable

Variable overview

The amount of the value that can change within a range during the execution of a program

Understanding: Like the unknown in mathematics

Variable definition Format

Variable name of data type = initialization value;

Note: The format is fixed, remember the format, status quo

Data type:

The Java language is a strongly typed language that defines specific data types for each type of data, and allocates a different size of memory space in memory

Considerations for Using Variables:

Scope

The variable is defined in the curly braces, and the range of the curly braces is the scope of the variable. Two variables of the same name cannot be defined in the same scope.

Initialize value

No initialization value can be used directly

It is recommended to define only one variable on a single line

Multiple can be defined, but not recommended

+ is an operator, we should be able to understand, do the addition of data.

The Boolean type cannot be converted to another data type

Default conversion

Byte,short,char-int-long-float-double

Byte,short,char to each other, they participate in the operation first converted to the INT type

Forced conversions

Target type variable name = (target type) (data to be converted);

1: Case demonstration implicit conversion, and drawing explanation

byte B = 10;

int i = 100;

System.out.println (B+i);

Have a problem

byte BB = b + i;

System.out.println (BB);

No problem

Int J = B + i;

System.out.println (j);

2: Just above the example can not, but, we clearly know that it is within this range, I would like to assign value, swollen?

With cast

Target data type variable name = (target data type) (converted data);

3: So, what kind of conversion do we use?

Generally, it is best not to arbitrarily use forced type conversions, which can easily result in loss of data accuracy.

(1) When a large data type is converted to a small capacity data type, it is necessary to add a forced conversion character, but it may result in reduced precision or overflow; use with caution.

(2) When there are many types of data blending operations, the system first automatically converts all data into the data type with the largest capacity before it is calculated.

Interview questions

BYTE B1=3,b2=4,b;

B=B1+B2;

b=3+4;

Which is the compilation failed? Why is it?

Study Questions

byte B = 130; Is there a problem? What can I do if I want to make the assignment right? What is the result?

Exercise: byte B = 300;

Please write down the following program results

System.out.println (' a ');

System.out.println (' a ' + 1);

System.out.println ("Hello" + ' a ' + 1);

System.out.println (' A ' +1+ "Hello");

System.out.println ("5+5=" +5+5);

System.out.println (5+5+ "=5+5");

Summarize:

1: Keywords (master)

(1) Words that are given a specific meaning by the Java language

(2) Features:

All lowercase.

(3) Precautions:

A:goto and const exist as reserved words.

B: High-level notebooks like notepad++ have special color tags on keywords

2: identifier (master)

(1) is a sequence of characters that give names to classes, interfaces, methods, variables, etc.

(2) Composition rules:

A: English uppercase and lowercase letters

B: Digital

C:$ and _

(3) Precautions:

A: Cannot start with a number

B: cannot be a keyword in Java

C: Case Sensitive

(4) Common naming conventions (see Name and meaning)

A: Package All lowercase

Single-level package: lowercase

Example: liuyi,com

Multi-level package: lowercase, and separated by.

Example: Cn.itcast,com.baidu

B: Class or interface

One word: Capitalize the first letter

Example: Student,demo

Multiple words: Capitalize the first letter of each word

Example: Helloworld,studentname

C: Method or Variable

One word: First letter lowercase

Example: Name,main

Multiple words: Start with the second word, capitalize the first letter of each word

Example: Studentage,showallnames ()

D: constant

All caps

One word: Uppercase

Example: PI

Multiple words: Uppercase and separated by _

Example: Student_max_age

3: note (master)

(1) The text that explains the program

(2) Classification:

A: Single-line comment//

B: Multiline Comment/**/

C: Documentation Comments (later)/** */

(3) wrote a annotated version of the HelloWorld case.

We're going to write a procedure in the back.

Demand:

Analysis:

Realize:

The code reflects:

(4) The role of annotations

A: Explain the procedure, improve the reading of the code.

B: Can help us debug the program.

Later, we will explain a more advanced debugging tool

4: Constant (Master)

(1) The amount of the value does not change during program execution

(2) Classification:

A: Literal constants

B: Custom Constants (later)

(3) Literal constants

A: string constant "Hello"

B: integer constant 12,23

C: decimal constant 12.345

D: Character constant ' a ', ' a ', ' 0 '

E: Boolean constant True,false

F: Empty constant Null (said later)

(4) provides four representations of integer constants in Java

A: Binary consists of 0, 1. Start with 0b.

B: Octal system by 0, 1, ... 7 composition. Start with 0.

C: Decimal by 0, 1, ... 9 composition. Integers are decimal by default.

D: Hex by 0, 1, ... 9,a,b,c,d,e,f (both uppercase and lowercase) are composed. Start with 0x.

5: Binary conversion (Learn)

(1) Other binary to decimal

Coefficient: is the value on each bit

Cardinality: X-binary cardinality is X

Right: The data on each digit, from the right, and numbering starting from 0, the corresponding number is the right to the data.

Results: The sum of the coefficients * cardinality ^ power power.

(2) Decimal to other binary

In addition to the base fetch, the remainder is reversed until the quotient is 0.

(3) Fast conversion method of the binary conversion

A: Conversion between decimal and binary

8421 yards.

B: Binary to octal, hexadecimal conversion

6: Variable (master)

(1) The amount of the value that can change within a certain range during the execution of a program

(2) Define the format of the variable:

A: Data type variable name = initialization value;

B: Data type variable name;

Variable name = initialization value;

7: Data type (master)

(1) Java is a strongly typed language that provides the corresponding data types for each type of data.

(2) Classification:

A: Basic Data type: 4 classes of 8

B: Reference data type: Class, interface, array.

(3) Basic data types

A: integer consumption of bytes

BYTE 1

Short 2

int 4

Long 8

B: Floating point

Float 4

Double 8

C: Character

Char 2

D: Boolean

Boolean 1

Attention:

integers are type int by default, and floating-point numbers are double by default.

Long integers to be added L or L.

Single-precision floating-point numbers are added F or f.

8: Data type conversion (mastering)

(1) Boolean type does not participate in conversions

(2) Default conversion

A: From small to large

B:byte,short,char--INT--long--float--double

C:byte,short,char do not convert each other, directly into the int type participates in the operation.

(3) Forced conversion

A: from big to small

B: There may be a loss of precision, generally not recommended for such use.

C: Format:

Target data type variable name = (target data type) (converted data);

(4) Study Questions and interview questions:

A: Are there any differences in the following two ways?

float f1 = 12.345f;

float F2 = (float) 12.345;

B: Is there a problem with the program below, and if so, where?

BYTE B1 = 3;

byte b2 = 4;

BYTE B3 = b1 + b2;

byte B4 = 3 + 4;

C: What is the result of the following operation?

byte B = (byte) 130;

D: Character participation operation

is to find the value inside the ASCII

' A ' 97

' A ' 65

' 0 ' 48

System.out.println (' a ');

System.out.println (' a ' + 1);

E: String participation operation

This is actually a string connection.

System.out.println ("Hello" + ' a ' + 1);

System.out.println (' A ' +1+ "Hello");

System.out.println ("5+5=" +5+5);

System.out.println (5+5+ "=5+5");

Appendix:

characters commonly used characters and ASCII code Comparison table

2-1 Java language Basics