1. 三种结构
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三种结构：任何复杂的程序逻辑，都可以通过三种控制结构来实现 ▶ 顺序结构：从上到下依次线性执行每一条语句，没有跳转或分支的执行完程序 ▶ 分支结构：根据指定条件执行不同的代码块，并非每条语句都执行 ▶ 循环结构：以重复执行一段代码块，直到满足特定条件才停止语句的执行 |
2. 分支结构
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Java分支语句：Java中有 if 和 switch 语句 |
（1）if语句
语法：
if(boolean条件语句) {语句块}
- 格式
if (条件) {
// 满足条件时执行的代码块
}
// 不满足以上条件时执行的代码块
- 执行流程图
flowchat
st=>start: 开始
op=>operation: 程序主体
cond=>condition: if关系表达式
io=>inputoutput: 语句体
e=>end: 执行结束
st(right)->op(right)->cond
cond(yes)->io(bottom)->e
cond(no)->e
st@>op({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>cond({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>e({"stroke":"Red","stroke-width":6,"arrow-end":"classic-wide-long"})
st@>cond({"stroke":"Green","stroke-width":3,"arrow-end":"classic-wide-long"})@>io({"stroke":"#C0FF3E","stroke-width":6,"arrow-end":"classic-wide-long"})@>e({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})
- 代码演示
package bases;
public class Demo04_If {
/**
* 分支语句
*
* @param args
*/
public static void main(String[] args) {
/*
* if语句
*/
// 打折
double price = 300.0;
if (price >= 500) {
//满500则打八折
price *= 0.8;
}
System.out.println("最终消费金额为:" + price);
//判断成绩的合法性
int score = 80;
if (score >= 0 && score <= 100) {
System.out.println("成绩合法");
}
System.out.println("成绩不合法，请继续...");
}
}
（2）if……else 语句
语法：
if(boolean条件语句){语句块1} else{语句块2}
- 格式
if (条件) {
// 条件为真时执行的代码块
} else {
// 条件为假时执行的代码块
}
- 执行流程图
flowchat
st=>start: 开始
op=>operation: 程序主体
cond=>condition: if关系表达式
io=>inputoutput: 语句体1
io1=>inputoutput: 语句体2
sub1=>subroutine: else语句
e=>end: 执行结束
st(right)->op(right)->cond
cond(yes)->io(bottom)->e
cond(no)->sub1->io1->e
st@>op({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>cond({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>e({"stroke":"Red","stroke-width":6,"arrow-end":"classic-wide-long"})
st@>cond({"stroke":"Green","stroke-width":3,"arrow-end":"classic-wide-long"})@>io({"stroke":"#C0FF3E","stroke-width":6,"arrow-end":"classic-wide-long"})@>e({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})
st@>cond({"stroke":"Green","stroke-width":3,"arrow-end":"classic-wide-long"})@>sub1({"stroke":"red","stroke-width":6,"arrow-end":"classic-wide-long"})@>io1({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})@>e({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})
- 代码演示
package bases;
public class Demo04_If {
/**
* 分支语句
*
* @param args
*/
public static void main(String[] args) {
/*
* if……else语句
*/
//满500打8折，不满500打9折:
double price = 300.0;
if (price >= 500) {
price *= 0.8;
} else {
price *= 0.9;
}
System.out.println("最终消费金额为:" + price);
//判断成绩合法还是不合法
int score = 95;
if (score >= 0 && score <= 100) {
System.out.println(score + " 合法成绩");
} else {
System.out.println(score + " 不合法成绩");
}
}
}
（3）if……else…if 语句
语法：
if(boolean条件语句1){语句块1} else if(boolean条件语句2){语句块2} else{语句块3}
- 格式
if (条件1) {
// 如果条件1为真，则执行这里的代码
} else if (条件2) {
// 如果条件2为真，则执行这里的代码
} else {
// 如果以上条件都不满足，则执行这里的代码
}
- 执行流程图
flowchat
st=>start: 开始
op=>operation: 程序主体
cond=>condition: if关系表达式1
io=>inputoutput: 语句体1
cond1=>condition: else if关系表达式2
io1=>inputoutput: 语句体2
sub1=>subroutine: else语句
io2=>inputoutput: 语句体3
e=>end: 执行结束
st(right)->op(right)->cond
cond(yes)->io(bottom)->e
cond(no)->cond1
cond1(yes)->io1->e
cond1(no)->sub1->io2->e
st@>op({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>cond({"stroke":"cyan","stroke-width":3,"arrow-end":"classic-wide-long"})@>e({"stroke":"Red","stroke-width":6,"arrow-end":"classic-wide-long"})
st@>cond({"stroke":"red","stroke-width":3,"arrow-end":"classic-wide-long"})@>cond1({"stroke":"red","stroke-width":6,"arrow-end":"classic-wide-long"})@>sub1({"stroke":"red","stroke-width":6,"arrow-end":"classic-wide-long"})@>io2({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})@>e({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})
st@>cond({"stroke":"Green","stroke-width":3,"arrow-end":"classic-wide-long"})@>io({"stroke":"green","stroke-width":6,"arrow-end":"classic-wide-long"})@>e({"stroke":"#66CD00","stroke-width":3,"arrow-end":"classic-wide-long"})
st@>cond1({"stroke":"green","stroke-width":6,"arrow-end":"classic-wide-long"})@>io1({"stroke":"green","stroke-width":6,"arrow-end":"classic-wide-long"})
- 代码演示
package bases;
public class Demo04_If {
/**
* 分支语句
*
* @param args
*/
public static void main(String[] args) {
/*
* if……else…if语句
*/
//满2000打5折，满1000不满2000打7折，满500不满1000打8折，不满500打9折:
double price = 6000.0;
if (price >= 2000) {
price *= 0.5;
} else if (price >= 1000) {
price *= 0.7;
} else if (price >= 500) {
price *= 0.8;
} else {
price *= 0.9;
}
System.out.println("最终消费金额为:" + price);
}
}
3. switch分支语句
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Switch语句：一种用于多分支条件判断的控制流语句（根据一个表达式的值，匹配相应的分支来执行相应的代码块） ▶ 优点：效率高，结构清晰，代码易读 |
- 格式
switch (整数、字符、枚举或字符串类型等) {
case 值1:
// 执行语句块1
break;
case 值2:
// 执行语句块2
break;
...
case 值n:
// 执行语句块n
break;
default:
// 默认语句块
break;
}
注：==如果某个case分支的代码块没有break语句，将会继续执行下一个case分支的代码，称为"case穿透"，如果所有case都不匹配，则会执行default语句块（可选）==
- 执行流程图
graph LR
A[开始] --> B{Switch条件}
B -- case 1 --> C[执行语句块1]
B -- case 2 --> D[执行语句块2]
B -- case 3 --> E[执行语句块3]
B -- default --> F[默认语句块]
C --> G[结束]
D --> G
E --> G
F --> G
- 代码演示
package bases;
import java.util.Scanner;
public class Demo04_Switch {
public static void main(String[] args) {
/*
* switch分支语句
*/
Scanner scanner = new Scanner(System.in);
System.out.print("请输入用户命令：");
int command = scanner.nextInt();
System.out.println("用户指令：" + command);
// 案例：输入数字转换为星期
System.out.print("请输入需要转换的数字：");
int day = scanner.nextInt();
String dayName;
switch (day) {
case 1:
dayName = "星期一";
break;
case 2:
dayName = "星期二";
break;
case 3:
dayName = "星期三";
break;
case 4:
dayName = "星期四";
break;
case 5:
dayName = "星期五";
break;
case 6:
dayName = "星期六";
break;
case 7:
dayName = "星期日";
break;
default:
dayName = "无效的日期";
break;
}
System.out.println("您输入的数字为：" + day + " 转换为星期数为：" + dayName);
}
}
