Java knowledge points-basics

Java Programming:

Java is divided into JAVASE (Standard Edition), J2EE (Enterprise Edition), JAVAME (Micro version)

Basic and advanced
Basic: syntax, semantics, keywords
Advanced: application scenarios: file reading, multi-threading, network, collection framework

　

　JDK
Java provides a set of API (interface),
32-bit operating system is not allowed to install 64, but 64-bit can be installed.
Installed in the default location, you can start the java editor without configuring environment variables.

Eclipse：

1. Workspace: Project storage location
2. Package explorer: Project management location
3 . src: Store source code
4. package: window folder, each point is a first-level directory

Variables:

　Comments
　1. Single-line comments // Most of them use single-line comments
　2. Multi-line comments/* */
　3. Documentation comments The javadoc tool generates a class declaration and method declaration

　Content
　1. Identifier, keyword
　2. Variable
　3. Operator

　Data type

　1. Numeric integer int with decimal number

　2. Character char single character, 'A'

　Identifier
Java rules that the entire identifier consists of 0-9 a-z A-Z _ $, numbers cannot begin

Programmer’s rules Each identifier should be as meaningful as possible, do not use meaningless abcd a1, a2.

Use multiple meaningful words to combine.

The type of the value itself

Operator
1. Assignment operator =: Assign the value on the right side of = to the left side of =.
　2. Arithmetic operators + - * / % first */ and then +-, use () to increase the priority.

　Java Keywords
There are meaningful words assigned by the system, which have been occupied by the system and are not allowed to be used as identifiers. Class name, package name, method name, variable name.

Variables must be declared first before use
Variables are not allowed to have duplicate names (Valid range in the life cycle)
Exceptions are divided into two types, compile-time exceptions and run-time exceptions (the program crashes during running)

　 Basic structure of variables
　Variable type variable name (identifier) ​​= variable value

　Naming convention
　//Method name, all words after the first letter of the variable name are lowercase Capital letters
// The first letter of each word in the class name and project name must be capitalized
// The package name must be all lowercase

Package

All classes in the system have their own packages. When using them, they need to be imported. Import their package names into import
All classes in java.lang do not need to be imported.

　Basic data types
Numeric type: Integer: short (double bytes) int (four bytes) long (eight bytes)
Decimal: float (four bytes) double (eight bytes)
Non-numeric characters: char (2 bytes are calculated according to the encoding format) (GBK, GB2312, UTF-8)
Logic: boolean (1/8 byte) true false 1 bit 1/8byte
Byte: byte (1 byte)

Reference data type All other non-basic types They are all reference types
　1. //Constant type 1 int type
　2. // 1l long type
　3. // 1.0 double type
　4. // 1.0f float type

　Basic data type conversion
　1. When converting integers, short>int>long can be converted at will, long>int>short is forced type conversion and may overflow.
　2. When converting an integer to a decimal, you can convert it at any time. When converting a decimal to an integer, forced type conversion is required and the decimal places are lost.
　3. When converting decimals, float>double can be converted at will, while double>float must be forced to type conversion.
　4. When converting non-numeric types, they will first be converted to the corresponding integer type. When converted back, forced type conversion is required. Type conversion.

　i++ and ++i
　1. i++: do the operation first, and then +1;
　2. ++ i: First do its own +1, then do the operation;

Character type: a=97 A=65 0=48

　　Relational operator
　< > == != >= <=
The relational operator ultimately returns boolean

Logical Operators

     

  break
## 　1. Jump out of the branch structure of switch 2. Break out of the loop structure (jump out of the inner loop)

#　
continue　1. End this cycle and continue the next cycle

Array

　

Data structure

Array, linked list, queue, stack, tree. . .

Definition Create a continuous space in memory

Once initialized (new), space cannot be added or deleted. # 　Declaration

　1. Data type[]
Identifier;　2. Data type identifier [];
## 　Allocate space

　

In addition to the basic data types in Java, which are stored directly on the stack, other types must use the new keyword to Make space in the pile.
#　

Define first, then use after initialization.

1. Array subscript out of bounds　2. java.lang.
#ArrayIndexOutOfBoundsException --> Array out of bounds## 　 Basic data types and reference data types

　

When the basic data types point to the same address, modifying one of the variables will not affect the other variables. (In the stack)

Reference data type When pointing to the same address, modifying one of the variables will also affect the other variable. (References are on the stack, objects are on the heap)

String exception. 　

Multidimensional array

Definition
Data type[][] Variable name = new data type[length][length];

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