Java控制结构
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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 :
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