[Super invincible and detailed Han Shunping's java notes] From beginner to proficient - data types, character encoding table

1. Each integer type in Java has a fixed range and field length, which is not affected by the specific os [operating system] to ensure the portability of Java programs.

2. Java's integer constants (specific values) default to int type. When declaring a long constant, you must add 'l' or 'L' after it.

(The compiler first reads 1L and finds that there are 8 bytes. It puts it into int and finds that it cannot fit, so it reports an error.)

Code:

public class IntDetail {
    //编写一个 main 方法
    public static void main(String[] args) {
    //Java 的整型常量（具体值）默认为 int 型，声明 long 型常量须后加‘l’或‘L’
        int n1 = 1;//4 个字节
        //int n2 = 1L;//对不对?不对
        long n3 = 1L;//对
    }
}

3. Variables in Java programs are declared as int type. Long is used unless it is not large enough to represent a large number.

4.bit: The smallest storage unit in a computer

byte: the basic storage unit in a computer

1byte = 8 bit

3. Floating point type

1.Basic introduction

Java 's floating point type can represent a decimal, such as 123.4 , 7.8 , 0.12, etc.

2. Classification of floating point types

Floating point number = sign bit + exponent bit + mantissa bit
The mantissa part may be lost, resulting in loss of accuracy ( decimal values are approximate ) .

3. Usage details of floating point types

1. Each Java floating point type has a fixed range and field length, which is not affected by the specific OS [operating system linux, mac,] to ensure the portability of Java programs

2. Java's floating-point constants (specific values) default to double type. When declaring a float-type constant, you must add 'f' or 'F' after it.

(float=1.1 is an error. Floating-point values default to double type, so 1.1 is double type, which is 8 bytes. If you put it into float and find that it cannot fit, an error will be reported)

public class FloatDetail {
//编写一个 main 方法
public static void main(String[] args) {
//Java 的浮点型常量(具体值)默认为 double 型，声明 float 型常量，须后加‘f’或‘F' //float num1 = 1.1; //对不对?错误
float num2 = 1.1F; //对的
double num3 = 1.1; //对
double num4 = 1.1f; //对

4. Floating-point constants have two representation forms

Decimal number format: such as: 5.12 512.0f . 512 (must have decimal point)

Scientific notation form: 5.12e2 [5.12*10 to the power 2 = 512.0]

5.12E-2 [5.12/10 to the 2nd power or 5.12*10 to the 2nd power = 0.0512]

public class FloatDetail {
    //编写一个 main 方法
    public static void main(String[] args) {
    //十进制数形式：如：5.12 512.0f .512 (必须有小数点）
        double num5 = .123; //等价 0.123
        System.out.println(num5);
        //科学计数法形式:如：5.12e2 [5.12 * 10 的 2 次方 ] 5.12E-2 []
        System.out.println(5.12e2);//512.0
        System.out.println(5.12E-2);//0.0512
    }
}

5. Normally, the double type should be used because it is more precise than the float type.

double num9 = 2.1234567851;

float num10 = 2.1234567851F;

public class FloatDetail {
    //编写一个 main 方法
    public static void main(String[] args) {
    //通常情况下，应该使用 double 型，因为它比 float 型更精确。
    //[举例说明]double num9 = 2.1234567851;float num10 = 2.1234567851F;
        double num9 = 2.1234567851;
        float num10 = 2.1234567851F;
            System.out.println(num9);
            System.out.println(num10)；
    }
}

6. Traps in using floating point numbers: 2.7 and 8.1 / 3 comparison

public class FloatDetail {
    //编写一个 main 方法
    public static void main(String[] args) {double num11 = 2.7;
        double num12 = 8.1 / 3; //2.7
            System.out.println(num11);//2.7
            System.out.println(num12);//接近 2.7 的一个小数，而不是 2.7
    }
}

When we judge the equality of an operation result that is a decimal, we use the absolute value of the difference between the two numbers to judge within a certain precision range.

public class FloatDetail {
    //编写一个 main 方法
    public static void main(String[] args) {double num11 = 2.7;
        double num12 = 8.1 / 3; //2.7
        System.out.println(num11);//2.7
        System.out.println(num12);//接近 2.7 的一个小数，而不是 2.7
//得到一个重要的使用点: 当我们对运算结果是小数的进行相等判断是，要小心
//应该是以两个数的差值的绝对值，在某个精度范围类判断
    if( num11 == num12) {
        System.out.println("num11 == num12 相等");
}
//不输出这句话
//正确的写法 , ctrl + / 注释快捷键, 再次输入就取消注释
    if(Math.abs(num11 - num12) < 0.000001 ) {
        System.out.println("差值非常小，到我的规定精度，认为相等...");
        System.out.println(Math.abs(num11 - num12));
//细节:计算num11-num12的绝对值的差值，如果是直接查询得的的小数或者直接赋值，是可以判断相等
//通过Java API来看
        }
}

4. Character type (char)

1. Basic introduction

The character type can represent a single character . The character type is char . char is two bytes ( can store Chinese characters ) . For multiple characters, we use the string String

2. Code Char01.java

public class Char01{
	public static void main(String []args){
		char c1 = 'a';
		char c2 = 97;
		char c3 = 'a';
		char c7 = 'a' + 1;

		char c4 = '韩';
		char c6 = 38889;



		System.out.println(c1);//a
		System.out.println((int)c1);//97
		System.out.println(c1+1);//98
		System.out.println((char)(c1+1));//b
		System.out.println(c2);//a

		System.out.println(c7);//b
		System.out.println((int)c7);//98

		System.out.println(c4);//韩
		System.out.println((int)c4);//38889
		System.out.println(c6);//韩

	}
}

run:

3 character type usage details

1. A character constant is a single character enclosed in single quotes (' ').

For example: char c1 = 'a'; char c2 = 'Chen'; char c3 = '9';

~~char c3 = '99'; error report~~

Character constants enclosed in single quotes can only contain one character, and the char type can only store one character. Therefore, char c1='9' works fine because it contains only one character '9'. And char c1='99' will cause a compiler error because it contains two characters and cannot be stored in a char type variable.

2. Use the escape character '\' to convert characters into special character constants

For example: char c3 = '\n'; // '\n' represents a newline character

3. In Java , the essence of char is an integer. In the default output, it is the character corresponding to the unicode code;

To output the corresponding number, you can use (int) characters

URL: Unicode encoding conversion-Webmaster Tools

4. The char type can be operated on, which is equivalent to an integer, because it all corresponds to Unicode code .

unicode URL: The most complete unicode encoding_unicode code_LemonWatermelon's blog-CSDN blog

ascii website: ascii code comparison table - the most complete ASCII encoding table

4.Character encoding table

4.1 ASCII encoding

4.2 Unicode encoding

4.3 UTF-8 encoding

5. Boolean type: boolean

Code:

public class Boolean01 {
//编写一个 main 方法
public static void main(String[] args) {
//演示判断成绩是否通过的案例
//定义一个布尔变量
boolean isPass = true;//
if(isPass == true) {
System.out.println("考试通过，恭喜");
} else {
System.out.println("考试没有通过，下次努力");
}
}
}