JavaSE- basis

---

title: 基础
date: 2019-02-20 23:00:58
tags:
- JavaSE
categories:
- JavaSE
toc: true

Noun Definition

The JVM ( the Java Virtual Machine, the Java Virtual Machine )

Because of JVM, so the same Java program can be executed in three different operating systems. This realization of cross-platform Java programs. Also known as Java has good portability.

The JRE ( the Java Runtime Environment, the Java Runtime Environment )

Including the required Java Virtual Machine and Java core libraries and other programs, to develop good if you want to run a Java program, the computer only needs to install the JRE.

The JDK ( the Java Developme Kit, the Java Development Kit )

JDK is available to developers, which contains the Java development tools, including the JRE. So install the JDK, you do not have to install the JRE alone. Which development tools: compiler tools Javac.exe, packaging tools Jar.exe and so on.

In simple terms: JDK developed using Java programs, go to the JRE run by the JVM to ensure cross-platform.

Naming Rules

Named to try to make us "see to know the name meaning" in development, this is a good habit.

package

全部小写。
单级包：ahojcn
多级包：cn.ahoj

Class or interface

一个单词：单词的首字母必须大写（Student、Dog）
多个单词：每个单词的首字母必须大写（HelloWorld、StudentName）

Method or variable

一个单词：单词的首字母小写（main、age）
多个单词：从第二个单词开始，每个单词的首字母大写（studentAge、showAllClass()）

constant

一个单词：全部大写
多个单词：每个字母都大写，用_隔开

Representation of the data

2,8,10,16-ary

Binary: 0bthe beginning

Octal: 0the beginning

Decimal: The default is decimal integer

Hex: 0xbeginning

System.out.println(0b1001);	// 2
System.out.println(0100); // 8
System.out.println(1001); // 10
System.out.println(0xffff); // 16

type of data

Java is a strongly typed language, for each data has a clearly defined data type specific, allocated memory space of different sizes in memory.

Basic data types:

1. Numeric:

   • Integer type:byte(1),short(2),int(4),long(8，超过了int范围需要加L或l，建议使用L)

   • Floating-point type:float(4，单精度浮点数用f或F标记，建议使用F，不加默认是double类型),double(8)

   • Character:char(2)

   • Boolean:boolean(1)

2. Reference data types:

   • class:class

   • interface:interface

   • Array:[]

*** Variable Caution: scope, initialization value, only one line is recommended to define a variable. ***

Cast

Note: boolean type can not be converted to other data types

1. Default conversion

   • byte，short，char => int => long => float => double

   • byte, short, char does not convert each other, they are involved in computing is first converted to an int

2. Cast

   • Variable name = target type (target type) (converted data)

   E.g:byte c = (byte)(a+b);

Some special

1. Out of multiple loops

   class test {
   	public static void main(String[] args) {
   
   		OUT:    // 相当于给循环起了个名字
   		for (int i = 0; i < 10; i++) {
   			for (int j = 0; j < 10; j++) {
   				for (int k = 0; k < 10; k++) {
   					System.out.println("test");
   					break OUT;
   				}
   			}
   		}
          
   	}
   }
   

2. …

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