table of Contents
1. Identifier keywords
Identifiers are composed of numbers (0~9) and letters (A~Z and a~z), dollar sign ($), underscore (_), and all symbols in the Unicode character set whose symbols are greater than 0xC0 (no spaces between the symbols) ).
The first symbol of the identifier is a letter, underscore, and dollar sign, and any letter, number, dollar sign, or underscore can follow.
Legal identifiers: date, $2011, _date, D_$date, etc.
Illegal identifiers: 123.com, 2com, for, if, etc.
The Java language currently defines 51 keywords. These keywords cannot be used as variable names, class names, and method names. These keywords are classified below.
Data types: boolean, int, long, short, byte, float, double, char, class, interface.
流程控制：if、else、do、while、for、switch、case、default、break、continue、return、try、catch、finally。
修饰符：public、protected、private、final、void、static、strict、abstract、transient、synchronized、volatile、native。
动作：package、import、throw、throws、extends、implements、this、supper、instanceof、new。
Reserved words: true, false, null, goto, const.
Second, the constant
Constant names are usually named in uppercase letters
After the constant final modification, the value cannot be modified
Constants can modify not only constants of data types, but also methods and classes
Modified final defined method can be inherited but cannot be overridden
Modified classes cannot be inherited (extends)
Constants are divided into 3 types
1. Static constant
需要 public static 来修饰，外部类可正常访问
public static final double PI = 3.14;2. Member constants
成员常量，类内部使用
final double PI = 3.14;3. Local constant
局部常量 ，方法内部使用
public static void main(String[] args) {
// 声明局部常量
final double x = 3.3;
}
Three, variables
Variables have member variables and local variables
1. Member variables
A. Global variables (instance variables)
No static modification
Life cycle: As long as the object is treated as a reference, the instance variable will exist
B. Static variables (class variables)
Decorate with static
Life cycle: depends on the life cycle of the class. The class will be destroyed only when it is completely recycled by the garbage collection mechanism
2. Local variables
A. Method local variables
从定义这个变量开始到方法结束这一段时间内有效。
public static void main(String[] args) {
int a = 7;
if (5 > 3) {
int s = 3; // 声明一个 int 类型的局部变量
System.out.println("s=" + s);
System.out.println("a=" + a);
}
System.out.println("a=" + a);
}
B. Method parameter variables
public static void testFun(int n) {
System.out.println("n=" + n);// n 方法局部变量
}C, code block local variables
public static void test() {
try {
System.out.println("Hello!Exception!");
} catch (Exception e) { // 异常处理块，参数为 Exception 类型
e.printStackTrace();
}
}Four, direct quantity
There are 3 types of direct quantity, basic data type, string type, null type
1) Direct quantity of int type
The integer value directly given in the program can be divided into four types: binary, decimal, octal and hexadecimal. Binary needs to start with 0B or 0b, octal needs to start with 0, and hexadecimal needs to start with 0x or Start with 0X. For example, 123, 012 (corresponding to decimal 10), 0x12 (corresponding to decimal 18) and so on.
2) Direct quantity of long type
After adding l or L after the integer value, it becomes the direct quantity of the long type. For example, 3L, 0x12L (corresponding to 18L in decimal).
3) Direct quantity of float type
Adding f or F after a floating-point number becomes a direct quantity of type float. This floating-point number can be in standard decimal form or in scientific notation form. For example, 5.34F, 3.14E5f.
4) Direct quantity of double type
Directly giving a floating-point number in standard decimal form or scientific notation form is the direct quantity of the double type. For example, 5.34, 3.14E5.
5) Direct quantity of boolean type
The only direct variables of this type are true and false.
6) Direct amount of char type
There are three forms of the direct quantity of the char type, which are characters enclosed in single quotes, escape characters, and characters represented by Unicode values. For example,'a','\n' and'\u0061'.
7) Direct quantity of String type
A sequence of characters enclosed in double quotes is a direct quantity of the String type. In most other languages, including C/C++, strings are implemented as arrays of characters. However, this is not the case in Java. In Java, strings are actually object types. As you will see later in the tutorial, because Java implements strings as objects, it has a wide range of string processing capabilities and is both powerful and easy to use.
8) Direct quantity of null type
The direct quantity of this type has only one value, which is null.
int a = 5;
char c = 'a';
boolean b = true;
float f = 5.12f;
double d = 4.12;
String name = "CJ点";
String url = "https://mp.csdn.net/postedit/102516903";
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