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Java编程速查表：从基础到高级概念

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"Java Cheat Sheet 是一份全面的Java编程指南，涵盖了从基础到高级概念的关键知识点。这份文档旨在帮助学习者快速理解和掌握Java语言，包括但不限于术语、基本语法、变量和数据类型、关键字、注释、访问修饰符、运算符、标识符、控制流程、包、方法、多态性、继承、数学类、抽象类与接口、数组、字符串处理、正则表达式、异常处理、命令行操作、集合框架、泛型以及多线程。Java作为一种高级编程语言，以其稳定性、面向对象特性、安全性以及易学性而闻名，创始人James Gosling因其在Java项目中的贡献被誉为"Java之父"。早期名为Oak的Java在商业上已经有一定知名度，为了区分与原有的Oak，团队将编程语言改名为Java。通过这份Cheat Sheet，开发者可以系统地了解并实践Java的核心特性，无论是初学者还是进阶开发者都能从中获益匪浅。"

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Java Cheat Sheet

3.Variables in Java

In Java, a variable is a data container that stores data values during the execution of

a Java program. A data type is allocated to each variable, indicating the type and

quantity of data it may store. It is a program’s fundamental storage unit. All variables

in Java must be defined before they may be used.

Syntax of declaring a variable:-

datatype variable_name;

Here, datatype refers to the type of data that can be stored in the variable.

variable_name refers to the name of the variable being declared.

In the above image, we can see that a variable named ‘age’ has been declared of type

‘int’ and has been initialised with the value ‘20’.

Types of variables:-

Page 15

Java Cheat Sheet

Local variable

- A local variable is a variable defined within a block, method, or

constructor. These variables have a scope that is limited to the block in which

they are defined. i.e., these variables are only accessible within that block.

Instance variable

- Variables that are declared inside a class without the ‘static’

keyword are referred to as instance variables. Instance variables are replicated

for every object of the class being created.

Static variable

- Variables that are declared inside a class with the ‘static’

keyword are referred to as static variables. Static variables are shared by all the

objects of the class and are not replicated. Static variables are created at the

start of program execution and automatically destroyed when the program is

finished. It is not necessary to initialise a static variable. It has a value of 0 by

default.

4.Datatypes in Java

The diﬀerent sizes and values that can be stored in the variable are defined by data

types. In Java, there are two types of data types:

Page 16

Java Cheat Sheet

Primitive Data Types in Java:-

Primitive data types are the building blocks of data manipulation in the Java

programming language. These are the most fundamental data types in the Java

programming language. Following are the diﬀerent primitive data types available in

Java:-

Page 17

Java Cheat Sheet

Data Type

Default Size Description

boolean 1 bit

Only two potential values

are stored in the Boolean

data type: true and false.

Simple flags that track

true/false circumstances

are stored in this data

type.

char 2 byte

A single 16-bit Unicode

character is represented

by the char data type. It

has a value range of

‘\u0000’ (or 0) to ‘\uﬀﬀ’ (or

65,535 inclusive).

byte 1 byte

It’s an 8-bit two-s

complement signed

integer. It has a value

range of -128 to 127.

(inclusive). It has a value of

0 by default. The byte data

type is used to preserve

memory in huge arrays

where space is at a

premium. Because a byte

is four times smaller than

an integer, it saves space.

short 2 bytes

A 16-bit signed two’s

complement integer is the

short data type. It has a

value range of -32,768 to

32,767. (inclusive). It has a

value of 0 by default.

Page 18

Java Cheat Sheet

Non Primitive data types in Java:-

The non-primitive data types in Java include classes, interfaces and arrays. We will

discuss more on them in the upcoming topics.

5.Java Keywords

a) Understanding the this keyword in Java:

The this keyword in java can be used in multiple ways. The following are the use

cases of the this keyword:

this: to refer to the current instance variable of the class

The this keyword can be used to refer to the current instance variable of a class. If

there is any ambiguity between the instance variables and arguments, this keyword

clears things out.

Example -

class

Sample{

int a;

Sample(int a){

this.a=a;

}

void display(){

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

}

class Test{

public static void main(String args[]){

Sample s1=new Sample(10);

s1.display();

}

Output -

a =

Page 19

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