JAVA introductory system tutorial (3) Basic syntax (1)

1. Keyword: a word given a specific meaning by the Java language (the letters that make up the keyword are all lowercase)

1) Use Keywords for defining data types

## 2 ) Used to determine the data type value keyword

true, false, null

3) Used to define process control Keywords

if, else, switch, case, default, while, do, for, break, continue, return

4) Use Keywords used to define access permission modifiers

private, protected, public

5) Used to define class, function, variable modifiers Keywords

abstract, final, static, synchronized

## 6) Keywords used to define the relationship between classes

extends, implements

7) Keywords used to define the creation of instances, reference instances, and determine instances

new, this, super, instanceof

8) Keywords used for exception handling

try, catch, finally, throw , throws

9) Keywords used for packages

package、import

10 ) Other modifier keywords

native, strictfp, transient, volatile, assert

##2. Identifier: for classes, excuses, Character sequence used when naming methods, variables, etc.

(ps: must be English uppercase and lowercase letters, numeric characters, ￥ and _)

Note: It cannot start with a number; it cannot be a keyword in java; Java is strictly case-sensitive

##In order to have a It is a good habit for beginners to write comments first and then code when writing programs.

4. Constant: Its value cannot be changed during the execution of the program

String constant Content enclosed in double quotes

Decimal constants All decimals Character constants Contents enclosed in single quotes
Boolean constants are more unique, only true and false
Empty constants Null (explained in the array part)

5. Variable: During the execution of the program, the amount that its value can change within a certain range

Data type variable name = initialization value;
Note: The format is fixed, remember the format, and adapt to changes

6. Data type: Java language is a strongly typed language. Each type of data has a clear and specific data type defined, and different sizes of memory space are allocated in the memory

Precautions for using variables:
Scope: In which level of braces the variable is defined, the scope of which brace is the scope of the variable. Two variables with the same name cannot be defined in the same scope.
Initialization value: Cannot be used directly without initialization value
It is recommended to define only one variable on a line, you can define multiple, but it is not recommended

Example question:

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”);

Observe the above output

#END

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