API: Application programming Interface.

There are many APIs for users to use in Java. Scanner and Random are one of them. The API is actually a class. The scanner class and Random class have been encapsulated. We just need to write them according to their syntax without knowing their basic source code

Class Scanner:

1. To use the scanner class, you need to import its package, import java.util.Scanner or import java.util. * (the former is to import the scanner class in util, and the latter is to import all classes in util)

2. Create object Scanner object name = new Scanner(System.in) / / System.in represents that the source is keyboard (in most cases)

3. Use the object and call its method object name. nextxx() / / call different methods according to the acceptance type

 1 import java.util.Scanner;
 2 //Input three numbers from the keyboard and output the maximum value
 3 public class ScannerDemo {
 4     public static void main(String[] args) {
 5         Scanner s=new Scanner(System.in);
 6         int num=s.nextInt();
 7         System.out.println(num);
 8         String str=s.next();
 9         System.out.println(str);
10         int a=s.nextInt();
11         int b=s.nextInt();
12         int c=s.nextInt();
13         int max=a>b?a:b;
14         int endmax=c>max?c:max;
15         System.out.println(endmax);
16 
17     }
18 }

Integer -- nextInt(), string -- next(), floating point -- nextFloat()

Class Random

1. import java.util.Random or import java.util. * (the former is the random class in import util, and the latter is all classes in import util)

2. Create Random r=new Random()

3. use

 1 import java.util.Random;
 2 import java.util.Scanner;
 3 //Guess random numbers, only five chances
 4 public class DemoRandom {
 5     public static void main(String[] args) {
 6         Random r=new Random();
 7         Scanner s=new Scanner(System.in);
 8         int res=r.nextInt(100);//[0,100)
 9         System.out.println(res);
10         int i=0;
11         while (i<5){
12             System.out.println("Please input the guess number, we will help you judge");
13             int num=s.nextInt();
14             if(num>res){
15                 System.out.println("Oh, big.");
16                 i++;
17                 continue;}
18             else if(num<res){
19                 System.out.println("Small");
20                 i++;
21                 continue;
22             }
23             else {
24                 System.out.println("Guess right.");
25                 i++;
26                 break;
27             }
28         }
29         if(i==5)
30             System.out.println("You've run out of times");
31         else
32             System.out.println("Congratulations, you used it"+i+"second");
33 
34     }
35 }

Object name. Method () / / r.nextInt(), i.e. randomly generate a number with an integer range -------- r.next(n) / / randomly generate a value between [0,n) (left closed and right open)

Anonymous object: that is, it can only be used once without naming the object. The next time it is used, it is a new anonymous object, which can be used as the parameter of the function and the return value of the function (new class name ())

 1 import java.util.Scanner;
 2 
 3 /*Anonymous object as parameter, return value
 4 */
 5 public class DemoAnonymous {
 6     public static void main(String[] args) {
 7         meth(new Scanner(System.in));
 8         Scanner s=meth2();
 9         int num=s.nextInt();
10         System.out.println(num);
11     }
12     public static void meth(Scanner sc){
13         int num=sc.nextInt();
14         System.out.println(num);
15     }
16     public static Scanner meth2(){
17         return new Scanner(System.in);
18     }
19 }

Line 7 takes an anonymous object as a parameter, and line 17 returns an anonymous object.