1. Foundation type

1.1 INTRODUCTION

1.2 exercise 1: test maximum and minimum

Create a Basic project
Create cn.qile.basic package
Create test1 scope.java

package cn.qile.basic;

//This class is used to test the value range
public class Test1_Scope {
    public static void main(String[] args) {
        //Test integer data range byte short int long
        //Create variable save byte value -- variable type variable name = variable value
        byte min = Byte.MIN_VALUE;
        byte max = Byte.MAX_VALUE;
        System.out.println(min);//-128, print value of variable
        System.out.println(max);//127

        short smin = Short.MIN_VALUE;
        short smax = Short.MAX_VALUE;
        System.out.println(smin);//-32768
        System.out.println(smax);//32767

        //long -- Long
        long lmin = Long.MIN_VALUE;
        long lmax = Long.MAX_VALUE;
        System.out.println(lmin);//-9223372036854775808
        System.out.println(lmax);//9223372036854775807


        //TODO int -- Integer
        int imin = Integer.MIN_VALUE;
        int imax = Integer.MAX_VALUE;
        System.out.println(imin);//-2147483648
        System.out.println(imax);//2147483647


        //TODO float -- float double -- double
        float fmin = Float.MIN_VALUE;
        float fmax = Float.MAX_VALUE;
        System.out.println(fmin);//1.4E-45
        System.out.println(fmax);//3.4028235E38

        double dmin = Double.MIN_VALUE;
        double dmax = Double.MAX_VALUE;
        System.out.println(dmin);//4.9E-324
        System.out.println(dmax);//1.7976931348623157E308


        //Boolean type
        boolean b = true;//True, usually 1 for true
        boolean b2 = false;//False, usually 0 for false


        //Character type
        char c = 'a';
        char c2 = 'in';//char type can store a Chinese character
        char c3 = 97;
        System.out.println(c);
        System.out.println(c2);


        //char type data can save numbers, but when it is used, it is not the numbers themselves, but,
//Take the numbers to check the ascii code table, take out the characters corresponding to the numbers and use them
        System.out.println(c3);//a. Because the number 97 of ascii code table corresponds to a character

    }
}

1.3 exercise 2: circular area

The area of a circle: π rr
Circumference of circle: 2 π r

import java.util.Scanner;//Automatic guide package

//Find the area of circle
public class Test2_Area {
    
    public static void main(String[] args) {
        //1. Define variable record radius
//           double r = 10.1; / / dead
        
        //Dynamically receive the value of keyboard input
        double r = new Scanner(System.in).nextDouble();
        
        //2. Bring in formula calculation result
        double result = 3.14*r*r;
        
        //Dynamic concatenation string:+
        System.out.println("The area of a circle is"+result);
        
    }
    
}

1.4 exercise 3: variable exchange

Receive values a and b entered by the user.
Suppose that a = 1 and b = 2 exchange the values of a and b.

package cn.qile.basic;

import java.util.Scanner;

//Test variable exchange
public class Test3_Exchange {

    public static void main(String[] args) {
        //1. Two integers a and b for keyboard input
        System.out.print("a = ");
        int a = new Scanner(System.in).nextInt();
        System.out.print("b = ");
        int b = new Scanner(System.in).nextInt();

        //2. Exchange
        int c = a ;//Define c and record the value of a
        a = b ;//The value of b is given to a
        b = c ;//Give the value of c to b

        //3. Print values for a and b
        System.out.println("a = "+a);
        System.out.println("b = "+b);

    }
}

2. Literal value of base type

2.1 integer face value is int type

int a = 999999999999999; //int out of range

2.2 three integers smaller than int, byte, short and char, can be assigned directly with the values in the range

byte b = 127;//Yes, because 127 is within the value range of byte
byte b2 = 128; //Error, because the literal value of 128 is int type, 4 bytes, byte is 1 byte, cannot be put

2.3 the literal value of a floating-point number is of type double

double b = 1.5;

2.4 suffix LFD of face value

long a = 999999999999L;// 9999999999 literal value is of type int, but if it exceeds the value range of int, an error will be reported. You can only expand the data range, add suffix L, and change from int to long
float b = 3.0F;//3.0 literal value is of double type, with suffix L, from double to float
double d = 3D;//3. The literal value is of type int, with suffix D, from int to double

2.5 base prefix

0b-2 system, for example: 0b0010
 0-8 base, for example: 076
 For example: 0x0986
 \U - char type, hexadecimal as follows:

3. Type conversion of basic type

3.1 small to large / implicit conversion

3.2 large to small / display conversion

Cast required

package cn.qile.basic;

public class Test {
    
    public static void main(String[] args) {
        //Small to large -- implicit transformation
        short a = 10;
//Turn small to large, a on the right is small type, b on the left is large type. For details, take small type to large type, and you can assign values directly
        long b = a;
        
        //Large to small -- explicit conversion -- cast -- (small type)
        double m = 10.8;
        //m on the right is a large type, n on the left is a small type. Take the large type and assign values to the small type directly -- no way
        int n = (int)m;
        
    }
}

3.3 convert floating-point number to integer and discard decimal number directly

Either 0.1 or 0.9 are abandoned.