Java控制结构
作者：互联网
控制结构
- 程序流程控制介绍
顺序控制
分支控制if-else
单分支
案例演示
01:
import java.util.Scanner;
public class IfWorkDemo
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
System.out.println("input your age");
int age = myScanner.nextInt();
if (age > 18)
{
System.out.println("Your age beyond 18 ,you shoule be responsible to your action ");
}
}
}
流程图
双分支
案例演示
01:
import java.util.Scanner;
public class IfWorkDemo01
{
public static void main(String [] args)
{
Scanner myScanner = new Scanner(System.in);
int age = myScanner.nextInt();
System.out.println("input your age");
if (age > 18)
{
System.out.println("Your age beyond 18 ,you shoule be responsible to your action ")
}
else
{
System.out.println("You are not old no enough to let you off this time");
}
}
}
流程图
单分支和双分支练习题
01:
public class PracticeWorkDemo
{
public static void main(String [] args)
{
int x = 7;
int y = 4;
if (x > 5)
{
if (y > 5)
{
System.out.println(x+y);
}
System.out.println("okokok");
}
else
{
System.out.println("x is "+x);
}
}
}
OUTPUT:
okokok
02：
public class IfWorkDemo
{
public static void main(String[] args)
{
double d1 = 34.5;
double d2 = 2.6;
if (d1 > 10.0 && d2 < 20.0)
{
System.out.println("两个数之和 = "+(d1+d2);
}
}
}
03:
public class SumWorkDemo
{
public static void main(String[] args)
{
int num1 = 10;
int num2 = 5;
int sum = num1+num2;
if (sum % 3==0 && sum % 5==0)
{
System.out.println("ok");
}
else
{
System.out.println("no");
}
}
}
04:
import java.util.Scanner;
public class YearWorkDemo
{
public static void main(String[] args)
{
Scaneer myYear = new Scanner(System.in);
int year = myYear.nextInt();
if ((year %4==0 && year %100 !=0 )|| year %400 ==0)
{
System.out.println("ok");
}
else
{
System.out.println("no");
}
}
}
多分支
流程图
案例演示
import java.util.Scanner;
public class MaWorkDemo
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
int score = myScanner.nextInt();
if (score >= 1 && score <= 100)
{
if (score == 100)
{
System.out.println("信用极好");
}
else if (score > 80 && score <= 99)
{
System.out.println("信用优秀");
}
else if (score >=60 && score <= 80)
{
System.out.println("信用一般");
}
else
{
System.out.println("信用不及格");
}
}
else
{
System.out.println("Input is Error");
}
}
}
小练习
01:
boolean b = true;
if (b==false)
{
System.out.println("a");
}
else if (b)
{
System.out.println("b");
}
else if (!b)
{
System.out.println("c");
}
else
{
System.out.println("d");
}
//output = b
02:
boolean b = true;
if (b=false)
{
System.out.println("a");
}
else if (b)
{
System.out.println("b");
}
else if (!b)
{
System.out.println("c");
}
else
{
System.out.println("d");
}
//output = c
嵌套分支
应用案例
01:
import java.util.Scanner;
public class MatchWorkDemo
{
public static void main(String [] args)
{
Scanner myScanner = new Scanner(System.in);
System.out.println("Please input score");
double score = myScanner.nextDouble();
if (score > 8.0)
{
System.out.println("Please input gender");
char gender = myScanner.next().charAt(0);
if (gender == '男')
{
System.out.println("Enter the men's division");
}
else if (gender== '女')
{
System.out.println("Enter the women's division");
}
else
{
System.out.println("Input Error");
}
}
else
{
System.out.println("cup tie");
}
}
}
代码略。
switch分支结构
流程图
案例演示
01:
import java.util.Scanner;
public class SwitchWorkDemo
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
System.out.println("Please input a-g");
char c1 = myScanner.next().charAt(0);
switch(c1)
{
case 'a':
System.out.println("Today is monday");
break;
case 'b':
System.out.println("Today is Tuesday");
break;
case 'c':
System.out.println("Today is Wednesday");
break;
//......
default :
System.out.println("Input Error");
}
}
}
细节讨论
- 表达式数据类型，应和case后的常量类型一致，或者是可以自动转成相互比较的类型，比如输入的是字符，而常量是int
01:
public class SwitchWorkDemo
{
public static void main(String[] args)
{
char c = 'a';
switch(c)
{
case 'a':
System.out.println("ok1");
break;
//correct char -> int
case 20:
System.out.println("ok2");
break;
//error String 无法转成 char
case 'hello':
System.out.println("ok3");
break;
default:
System.out.println("ok4");
}
}
}
02:
public class SwitchWorkDemo
{
public static void main(String[] args)
{
String c = "a";
switch(c)
{
case "b":
System.out.println("ok1");
break;
case "20":
System.out.println("ok2");
break;
//error String 无法转成 int
case 20:
System.out.println("ok3");
break;
default:
System.out.println("ok4");
}
}
}
- switch(表达式)中表达式的返回值必须是(byte,short,int,char,enum[枚举],String)
01:
public class SwitchWorkDemo
{
public static void main(String[] args)
{
double c = 1.1;
switch(c)
{
//error switch(表达式)中表达式的返回值必须是(byte,short,int,char,enum[枚举],String)
case 1.1:
System.out.println("ok1");
break;
default:
System.out.println("ok2");
}
}
}
- case子句中的值必须是常量(1,‘a’)或者是常量表达式,而不能是变量
class SwitchWork
{
public static void main(String[] args)
{
char c = 'a';
char c2 = 'c';
switch(c)
{
//error case子句中的值必须是常量(1,'a')或者是常量表达式,而不能是变量
case c2:
System.out.println("ok");
break;
default:
System.out.println("ok2");
}
}
}
-
default是可选的，就是说，可以没有default。如果没有default子句，有没有匹配任何常量，则没有输出
-
break语句用来在执行完一个case分支后使程序跳出switch语句块，如果没有写break，程序会顺序执行到switch结尾
public class SwitchDemo {
public static void main(String[] args)
{
char c = 'b';
char c2 = 'c';
switch(c)
{
case 'b'://没有break，穿透了
System.out.println("ok1");
case 'c':
System.out.println("ok2");
default:
System.out.println("ok3");
}
}
}
结果如下:
小练习
01:
import java.util.Scanner;
public class ConvertWork
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
System.out.println("Please input a-e");
char c1 = myScanner.next().charAt(0);
switch(c1)
{
case 'a':
System.out.println("A");
break;
case 'b':
System.out.println("B");
break;
case 'c':
System.out.println("C");
break;
case 'd':
System.out.println("D");
break;
case 'e':
System.out.println("E");
break;
default :
System.out.println("Input Error");
}
}
}
02:
public class SwitchDemo
{
public static void main(String[] args)
{
double score = 88.5;
if (score >=0 && score <=100)
{
switch((int)(score/60))
{
case 0:
System.out.println("不合格");
break;
case 1:
System.out.println("合格");
break;
// default:
// System.out.println("Input Error");
}
}
else
{
System.out.println("Input Error");
}
}
}
03:
import java.util.Scanner;
public class SwitchDemo
{
public static void main(String [] args)
{
Scanner myScanner = new Scanner(System.in);
System.out.println("Input month");
int month = myScanner.nextInt();
switch(month)
{
case 3:
case 4:
case 5:
System.out.println("springtime");
break;
case 6:
case 7:
case 8:
System.out.println("summer");
break;
case 9:
case 10:
case 11:
System.out.println("autumn");
case 1:
case 2:
case 12:
System.out.println("winter");
default :
System.out.println("Input Error");
}
}
}
switch和if的比较
先死后活，化繁为简
for循环控制
流程图
使用细节
小练习
01:
public class ForExercise
{
public static void main(String[] args)
{
int cnt = 0;
int sum = 0;
int start = 1;
int end = 100;
int num = 9;
for (int i = start;i<=end;i++)
{
if (i % num==0)
{
System.out.println("i = "+i);
cnt++;
sum+=i;
}
}
System.out.println("cnt = "+cnt);
System.out.println("sum = "+sum);
}
}
02:
public class ForWorkDemo
{
public static void main(String [] args)
{
int n = 5;
for (int i = 0;i<=n;i++)
{
System.out.println(i+"+"+(n-i)+"="+n);
}
}
}
while循环控制
01:
public class WhileWorkDemo
{
public static void main(String[] args)
{
int i = 1;
while(i <= 10)
{
System.out.println("i = "+i);
i++;
}
}
}
流程图
注意事项和细节说明
小练习
01:
public class WhileWork
{
public static void main(String[] args)
{
int i = 1;
while(i <= 100)
{
if (i%3==0)
{
System.out.println(i);
//i++;//easy error
}
i++;
}
}
}
02:
public class WhileWork
{
public static void main(String[] args)
{
int i = 40;
while(i<=200)
{
if (i %2==0)
{
System.out.println(i);
}
i++;
}
}
}
do…while循环控制
流程图
01:
public class DoWhileWork
{
public static void main(String[] args)
{
int i = 1;
do
{
System.out.println(i);
i++;
}while(i <= 10);
System.out.println("continue");
}
}
注意事项和细节说明
小练习
01:
public class DoWhileWork
{
public static void main(String[] args)
{
int cnt = 0;
int i = 1;
do
{
if (i % 5==0 && i%3!=0)
{
System.out.println(i);
cnt++;
}
i++;
}while(i<=200);
System.out.println(cnt);
}
}
02:
import java.util.Scanner;
public class DoWhileWork
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
char answer = ' ';
do
{
System.out.println("老韩使用五连鞭");
System.out.println("老韩问:还钱吗?y/n")；
answer = myScanner.next().charAt(0);
System.out.println("His ansmer = "+answer);
}while(answer!='y');
System.out.println("李三还钱了");
}
}
多重循环控制
01:
public class ForWorkDemo
{
public static void main(String[] args)
{
for (int i = 0;i<2;i++)
for (int j = 0;j<3;j++)
{
System.out.println("i = "+i+" j = "+j);
}
}
}
结果如下:
小练习
01:
import java.util.Scanner;
public class ForWorkDemo
{
public static void main(String[] args)
{
Scanner myScanner = new Scanner(System.in);
double sumbig = 0;
for (int i = 1;i<=3;i++)
{
double sum = 0;
int cnt = 0;
for (int j = 1;j<=5;j++)
{
System.out.println("Please input 第"+i+"个班级的第"+j+"个学生");
double score = myScanner.nextDouble();
sum+=score;
if (score >= 60)
{
cnt++;
}
}
System.out.println("第"+i+"个班级及格的学生有"+cnt+"个");
System.out.println("第"+i+"个班级的平均分为:" + sum/5);
sumbig+=(sum/5);
}
System.out.println("bigsum = "+(sumbig/3));
}
}
02：
代码略。
大练习
化繁为简
01：
public class PrintWorkDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
System.out.println("*****");
// *****
// *****
// *****
// *****
// *****
}
}
}
02:
public class PrintWorkDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
for (int j = 1;j<=i;j++)
{
System.out.print("*");
}
System.out.println();
}
}
}
//*
//**
//***
//****
//*****
03:
public class PrintWorkDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
for (int k = 1;k<=5-i;k++)
{
System.out.print(" ");
}
for (int j = 1;j<=2*i-1;j++)
{
System.out.print("*");
}
System.out.println();
}
}
}
// *
// ***
// *****
// *******
//*********
04:
public class PrintWorkDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
for (int k = 1;k<=5-i;k++)
{
System.out.print(" ");
}
for (int j = 1;j<=2*i-1;j++)
{
if (i!=5 && j==1 || j==2*i-1)
System.out.print("*");
else if (i!=5)
System.out.print(" ");
else
System.out.print("*");
}
System.out.println();
}
}
}
// *
// * *
// * *
// * *
//*********
跳转控制语句break
- Math.random()
返回带正号的double值，该值大于等于0.0,且小于1.0
01:
public class RandomBreakWork {
public static void main(String[] agrs)
{
int cnt = 0;
while(true)
{
cnt++;
int flag = (int)(Math.random()*100)+1;
if (flag==97) break;
}
System.out.println(cnt);
}
}
基本介绍和流程图
注意事项和细节说明
01:
public class RandomBreakWork {
public static void main(String[] agrs)
{
label1:
for (int j = 0;j<4;j++)
{
label2:
for (int i = 0;i<10;i++)
{
if (i == 2) break;
System.out.println("i = "+i);
}
}
}
}
结果如下:
02:
public class RandomBreakWork {
public static void main(String[] agrs)
{
label1:
for (int j = 0;j<4;j++)
{
label2:
for (int i = 0;i<10;i++)
{
if (i == 2) break label1;
System.out.println("i = "+i);
}
}
}
}
结果如下:
小练习
01:
public class RandomBreakWork {
public static void main(String[] agrs)
{
int sum = 0;
for (int i = 1;i<=100;i++)
{
sum+=i;
if (sum > 20)
{
System.out.println(i);
break;
}
}
}
}
//------------------------------------------
public class RandomBreakWork {
public static void main(String[] agrs)
{
int flag = 0;
int sum = 0;
for (int i = 1;i<=100;i++)
{
sum+=i;
if (sum > 20)
{
flag = i;
break;
}
}
System.out.println(flag);
}
}
02:
tip:
- 字符串的内容比较使用的方法equals
public class StringTestDemo
{
public static void main(String[] args)
{
String name = "Tom";
System.out.println(name.equals("Jack"));
System.out.println("jack".equals(name));//[推荐,可以避免空指针]
}
}
import java.util.Scanner;
public class RandomBreakWork {
public static void main(String[] agrs)
{
Scanner myScanner = new Scanner(System.in);
String name = "";
String passwd = "";
int chance = 3;
for (int i = 1;i<=3;i++)
{
System.out.println("Please input name");
name = myScanner.next();
System.out.println("Please input passwd");
passwd = myScanner.next();
//compare
if("丁真".equals(name) && "666".equals(passwd))
{
System.out.println("ok");
break;
}
chance--;
System.out.println("你还有"+chance+"次机会");
}
}
}
跳转控制语句continue
流程图
01:
public class RandomBreakWork {
public static void main(String[] agrs)
{
int i = 1;
while(i <= 4)
{
i++;
if (i==2)
{
continue;
}
System.out.println("i = "+i);
// i = 3
// i = 4
// i = 5
}
}
}
案例说明
01:
public class RandomBreakWork {
public static void main(String[] agrs)
{
label1:
for (int j = 0;j<2;j++)
{
label2:
for (int i = 0;i<10;i++)
{
if (i==2)
{
continue;
}
System.out.println("i = "+i);
}
}
}
}
结果如下:
i = 0
i = 1
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
i = 9
i = 0
i = 1
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
i = 9
02:
public class RandomBreakWork {
public static void main(String[] agrs)
{
label1:
for (int j = 0;j<2;j++)
{
label2:
for (int i = 0;i<10;i++)
{
if (i==2)
{
continue label1;
}
System.out.println("i = "+i);
}
}
}
}
结果如下:
i = 0
i = 1
i = 0
i = 1
跳转控制语句return
01:
public class ReturnDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
if (i==3)
{
System.out.println(i);
break ;
}
System.out.println("hello world");
}
System.out.println("go on...");
}
}
结果如下:
hello world
hello world
3
go on…
02:
public class ReturnDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
if (i==3)
{
System.out.println(i);
continue;
}
System.out.println("hello world");
}
System.out.println("go on...");
}
}
结果如下:
hello world
hello world
3
hello world
hello world
go on…
03:
public class ReturnDemo {
public static void main(String[] args)
{
for (int i = 1;i<=5;i++)
{
if (i==3)
{
System.out.println(i);
return;
}
System.out.println("hello world");
}
System.out.println("go on...");
}
}
结果如下:
hello world
hello world
3
标签：Java,String,int,System,控制结构,println,public,out 来源： https://blog.csdn.net/m0_51955470/article/details/120834313