Class notes on June 12, 2021
Command line parameters
- Sometimes you want to pass a message to a program when it runs. This is achieved by passing command line parameters to the main() function.
package com.kuang.method;
public class Demo03 {
public static void main(String[] args) {
//args.length array length
for (int i = 0; i < args.length; i++) {
System.out.println("args["+i+"]:"+args[i]);
}
}
}
cd... / back up
There is no need to write the package name for compilation, and the package name must be written for execution
Variable parameter / indefinite parameter
- JDK1. Starting from 5, java supports passing variable parameters of the same type to a method
- In the method declaration, add an ellipsis (...) after specifying the parameter type
- Only one variable parameter can be specified in a method. It must be the last parameter of the method. Any ordinary parameter must be declared before it
package com.kuang.method;
public class Demo04 {
public static void main(String[] args) {
Demo04 demo04 = new Demo04();
demo04.test(1,2,3,4,5,6);
}
public void test(int... i){
System.out.println(i[0]);
System.out.println(i[1]);
System.out.println(i[2]);
System.out.println(i[3]);
System.out.println(i[4]);
System.out.println(i[5]);
}
}
printMax(…numbers:34,3,3,2,56.5); Always report errors!!!
Variable parameter call example
package com.kuang.method;
public class Demo041 {
public static void main(String args[]) {
//Call methods with variable parameters
printMax(new double[]{1,2,3});
}
public static void printMax(double... numbers){
if(numbers.length == 0){
System.out.println("No argument passed");
return;
}
double result = numbers[0];
//Take the maximum value
for (int i = 1; i < numbers.length; i++) {
if (numbers[i] > result){
result = numbers[i];
}
}
System.out.println("The max value is " + result);
}
}
Recursion (key and difficult points)
- Method A calls method B, which is easy to understand
- Recursion means that method A calls method A and calls itself
- Using recursion, we can solve some complex problems with simple programs. It usually transforms a large and complex problem layer by layer into a smaller problem similar to the original problem to solve. The recursive strategy only needs a small amount of program to describe the multiple repeated calculations required in the problem-solving process, which greatly reduces the amount of code of the program. The ability of recursion is to define an infinite set of objects with limited statements
- Recursive structure
- Recursive header: when not to call its own method. If there is no head, it will fall into a dead cycle
- Recursive body: when do I need to call my own method
Recursive example
package com.kuang.method;
public class Demo06 {
public static void main(String[] args) {
System.out.println(f(5));
}
//1!=1
//2!=2*1
//5!=5*4*3*2*1
//Boundary conditions, such as f(1)
//Pre stage
//Return phase
//Stack mechanism
//Small amount of calculation can use recursion, considering the computational power!
public static int f(int n){
if(n==1){
return 1;
}else{
return n*f(n-1);
}
}
}
Method call example
package com.kuang.method;
public class Demo041 {
public static void main(String args[]) {
//Call methods with variable parameters
printMax(new double[]{1,2,3});
}
public static void printMax(double... numbers){
if(numbers.length == 0){
System.out.println("No argument passed");
return;
}
double result = numbers[0];
//Take the maximum value
for (int i = 1; i < numbers.length; i++) {
if (numbers[i] > result){
result = numbers[i];
}
}
System.out.println("The max value is " + result);
}
}
Simple calculator (addition, subtraction, multiplication and division of two numbers)
package com.kuang.method;
import java.util.Scanner;
public class Calculator {
public static void main(String[] args) {
//User interactive keyboard input
Scanner scanner = new Scanner(System.in);
double sum =0;
System.out.println("Please enter the first number:");
double num1 = scanner.nextDouble();
System.out.println("Please enter the operation symbol:");
String operator = s canner.next();
System.out.println("Please enter the second number:");
double num2 = scanner.nextDouble();
//Defining variables: operation symbols
//The addition, subtraction, multiplication and division are respectively corresponding to the operation process
switch (operator){
case "+":
sum = num1 + num2;
break;
case "-":
sum = num1 - num2;
break;
case"*":
sum = num1 * num2;
break;
case"/":
sum = num1 / num2;
break;
}
System.out.println(sum);
}
}