As a front-end programmer, its development prospects are far less than that of back-end programmers. Therefore, only by understanding the back end can we make ourselves more core competitive. This series of tutorials will quickly learn JAVA from the perspective of a front-end programmer.
New Project
To develop a JAVA program, our first step is to use IDEA to create a new project. When creating a new project, we can customize the project name (name), the storage location of the project (loaction) and select the JDK version .
After we use IDEA to create a project project, its default file structure is like this
The src folder is where we write the code. This folder has a Main.java file, and its default code is as follows
public class Main {
public static void main(String[] args) {
System.out.printf("Hello and welcome!");
for (int i = 1; i <= 5; i++) {
System.out.println("i = " + i);
}
}
}
The above code contains a public class named Main. In the Main class, there is a public static method called main, which is the main entry point of the JAVA program.
The System.out.printf method in java is similar to the front-end console.log() method. Run the above code, and the result is easy to think of
Note: JAVA programs are strictly case-sensitive; after the code runs, there will be an additional out compilation directory
basic grammar
variable
We create a BASE package under src to write some of our basic learning codes.
We create a BL class in the BASE package
We define a name variable. Different from the front end, JAVA needs to specify its data type before defining variables
package BASE;
public class BL {
public static void main(String[] args) {
// TODO 变量
// 1.变量的声明
// 数据类型 变量名称
String name;
// 2.变量的赋值
name = "shixiaoshi";
// 3.变量的使用
System.out.println(name);
// 【变量初始化】变量在使用前，必须先赋值
}
}
identifier
In Java, an identifier is a name used to name variables, methods, classes, and other entities (such as the name name of the above code). Identifiers need to follow certain naming rules:
- Identifiers can contain letters, numbers, and the underscore character (_).
- The first character of an identifier cannot be a number.
- Identifiers are case-sensitive, so myVariable and myvariable are two different identifiers.
- Identifiers cannot be Java keywords (eg, int, for, class, etc.).
type of data
When defining a variable, we need to specify the data type of the variable. There are mainly two types of basic data types and reference data types in Java , which are used to store different types of data.
basic data type
-
Integer data type (integer):
- byte: 1 byte, ranging from -128 to 127
- short: 2 bytes, range -32,768 to 32,767
- int: 4 bytes, range -2,147,483,648 to 2,147,483,647
- long: 8 bytes, ranging from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
-
Float data type (decimal):
- float: 4 bytes, the range is ±3.40282347E+38F (effective digits are 6-7 digits)
- double: 8 bytes, the range is ±1.7976931348623157E+308 (the effective number of digits is 15)
-
Character data type:
- char: 2 bytes, representing a Unicode character
-
Boolean data type:
- boolean: Indicates a logical value, which can only be true or false
Note: float data; the type needs to end with F, such as float num = 1.0f
reference data type
- String: Represents a string
- Array: represents an array
- Class: Indicates the class type
- Interface: Indicates the interface type
- Enum: Represents an enumeration type
data type conversion
Data conversion in Java can be divided into two types: implicit conversion and explicit conversion.
implicit conversion
Implicit conversion refers to the automatic conversion of a smaller data type to a larger data type without explicitly specifying the conversion operation. For example, if you assign an integer to a floating-point variable, Java will automatically convert the integer to a floating-point number.
explicit conversion
Explicit conversion refers to converting a larger data type to a smaller data type, requiring the use of cast operators. For example, to assign a floating-point number to an integer variable, you need to use a cast to convert the floating-point number to an integer.
package BASE;
public class SJZH {
public static void main(String[] args) {
// 隐式转换
byte b = 1;
short s = b;
int i = s;
long l = i;
float f = l;
double d =f;
// 显示转换,使用()进行强制转换
int h = (int) d;
}
}
operator
The operators of JAVA and JS are basically the same
arithmetic operator
Used to perform basic mathematical operations, including addition (+), subtraction (-), multiplication (*), division (/), and remainder (%).
relational operator
Used to compare the magnitude relationship of two values, such as equal to (==), not equal to (!=), greater than (>), less than (<), greater than or equal to (>=), and less than or equal to (<=).
Logical Operators
Used to combine multiple conditions, such as logical AND (&&), logical OR (||), and logical NOT (!).
bitwise operator
Used to perform bit-level operations on binary numbers, such as bitwise AND (&), bitwise OR (|), bitwise inversion (~), bitwise exclusive OR (^), and shift operators (<<, >> , >>>).
assignment operator
Used to assign a value to a variable, such as equal sign (=), plus equal to (+=), minus equal to (-=), etc.
Conditional operator (ternary operator)
Used to select one of two actions based on the true or false of a condition, eg condition? expression1 : expression2.
other operators
Including increment (++), decrement (–), member access (.), array access ([]), etc.
flow control statement
The flow control statement in JAVA is basically the same as the flow control statement in JS
Conditional statement if-else:
int x = 10;
if (x > 5) {
System.out.println("x大于5");
} else {
System.out.println("x小于等于5");
}
switch statement:
int day = 4;
String dayName;
switch (day) {
case 1:
dayName = "星期一";
break;
case 2:
dayName = "星期二";
break;
case 3:
dayName = "星期三";
break;
case 4:
dayName = "星期四";
break;
case 5:
dayName = "星期五";
break;
default:
dayName = "周末";
}
System.out.println("今天是" + dayName);
Loop statement for :
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
Loop statement while:
int i = 0;
while (i < 5) {
System.out.println(i);
i++;
}
Loop statement do-while:
int i = 0;
do {
System.out.println(i);
i++;
} while (i < 5);
Jump statement break:
for (int i = 0; i < 5; i++) {
if (i == 3) {
break;
}
System.out.println(i);
}
Jump statement continue:
for (int i = 0; i < 5; i++) {
if (i == 3) {
continue;
}
System.out.println(i);
}
