While循环

• while是最基本的循环,它的结构为:

while (布尔表达式){
    //循环内容
        }

• 只要布尔表达式为true,循环就会一直执行下去。
• 我们大多数情况是会让循环停止下来的,我们需要一个让表达式失效的方式来结束循环。
• 少部分情况需要循环一直执行,比如服务器的请求响应监听等。
• 循环条件一直为true就会造成无限循环【死循环】 ,我们正常的业务编程中应该尽量避免死循环。会影响程序性能或者造成程序卡死奔溃!

while代码一:

package com.kuang.struct;

public class WhileDemon01 {
    public static void main(String[] args) {

        //输出1~5
        int i = 0;

        while (i<5){
            i++;
            System.out.println(i);
        }
    }
}

运行结果一:

D:\environment\java\jdk1.8\bin\java.exe "-javaagent:D:\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=64102:D:\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath D:\environment\java\jdk1.8\jre\lib\charsets.jar;D:\environment\java\jdk1.8\jre\lib\deploy.jar;D:\environment\java\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\environment\java\jdk1.8\jre\lib\ext\cldrdata.jar;D:\environment\java\jdk1.8\jre\lib\ext\dnsns.jar;D:\environment\java\jdk1.8\jre\lib\ext\jaccess.jar;D:\environment\java\jdk1.8\jre\lib\ext\jfxrt.jar;D:\environment\java\jdk1.8\jre\lib\ext\localedata.jar;D:\environment\java\jdk1.8\jre\lib\ext\nashorn.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunec.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\environment\java\jdk1.8\jre\lib\ext\zipfs.jar;D:\environment\java\jdk1.8\jre\lib\javaws.jar;D:\environment\java\jdk1.8\jre\lib\jce.jar;D:\environment\java\jdk1.8\jre\lib\jfr.jar;D:\environment\java\jdk1.8\jre\lib\jfxswt.jar;D:\environment\java\jdk1.8\jre\lib\jsse.jar;D:\environment\java\jdk1.8\jre\lib\management-agent.jar;D:\environment\java\jdk1.8\jre\lib\plugin.jar;D:\environment\java\jdk1.8\jre\lib\resources.jar;D:\environment\java\jdk1.8\jre\lib\rt.jar;E:\桌面\java学习\production\基础语法 com.kuang.struct.WhileDemon01
1
2
3
4
5

Process finished with exit code 0

while的死循环代码二:

package com.kuang.struct;

public class WhileDemon02 {
    public static void main(String[] args) {
        //死循环  避免死循环
        while(true){
            //等待客户端链接
            //定时检查
            //.......
        }
    }
}

while代码三:

package com.kuang.struct;

public class WhileDemon03 {
    public static void main(String[] args) {
        //计算1+2+3+.....+100=?
        int i = 0;  //初始值
        int sum=0;  //和
        while (i<101){
            sum = sum +i;
            i++;

        }
        System.out.println("1+2+....+100="+sum);
    }
}

while运行结果三:

D:\environment\java\jdk1.8\bin\java.exe "-javaagent:D:\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=64176:D:\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath D:\environment\java\jdk1.8\jre\lib\charsets.jar;D:\environment\java\jdk1.8\jre\lib\deploy.jar;D:\environment\java\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\environment\java\jdk1.8\jre\lib\ext\cldrdata.jar;D:\environment\java\jdk1.8\jre\lib\ext\dnsns.jar;D:\environment\java\jdk1.8\jre\lib\ext\jaccess.jar;D:\environment\java\jdk1.8\jre\lib\ext\jfxrt.jar;D:\environment\java\jdk1.8\jre\lib\ext\localedata.jar;D:\environment\java\jdk1.8\jre\lib\ext\nashorn.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunec.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\environment\java\jdk1.8\jre\lib\ext\zipfs.jar;D:\environment\java\jdk1.8\jre\lib\javaws.jar;D:\environment\java\jdk1.8\jre\lib\jce.jar;D:\environment\java\jdk1.8\jre\lib\jfr.jar;D:\environment\java\jdk1.8\jre\lib\jfxswt.jar;D:\environment\java\jdk1.8\jre\lib\jsse.jar;D:\environment\java\jdk1.8\jre\lib\management-agent.jar;D:\environment\java\jdk1.8\jre\lib\plugin.jar;D:\environment\java\jdk1.8\jre\lib\resources.jar;D:\environment\java\jdk1.8\jre\lib\rt.jar;E:\桌面\java学习\production\基础语法 com.kuang.struct.WhileDemon03
1+2+....+100=5050

Process finished with exit code 0

do…while循环

• 对于while语句而言,如果不满足条件,则不能进入循环。但有时候我们需要即使不满足条件,也至少执行一次。
• do…while循环和while循环相似,不同的是, do…while循环至少会执行一次。
• 语法

do {
    //代码语句
}while(布尔表达式);

While和do-While的区别:

• while先判断后执行。dowhile是先执行后判断!
• Do…while总是保证循环体会被至少执行一次!这是他们的主要差别。

DoWhile代码一:

package com.kuang.struct;

public class DoWhileDemon01 {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;
        do {
            sum = sum + i;
            i++;
        }while (i<=100);
        System.out.println(sum);
    }
}

DoWhile运行结果一:

D:\environment\java\jdk1.8\bin\java.exe "-javaagent:D:\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=64355:D:\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath D:\environment\java\jdk1.8\jre\lib\charsets.jar;D:\environment\java\jdk1.8\jre\lib\deploy.jar;D:\environment\java\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\environment\java\jdk1.8\jre\lib\ext\cldrdata.jar;D:\environment\java\jdk1.8\jre\lib\ext\dnsns.jar;D:\environment\java\jdk1.8\jre\lib\ext\jaccess.jar;D:\environment\java\jdk1.8\jre\lib\ext\jfxrt.jar;D:\environment\java\jdk1.8\jre\lib\ext\localedata.jar;D:\environment\java\jdk1.8\jre\lib\ext\nashorn.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunec.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\environment\java\jdk1.8\jre\lib\ext\zipfs.jar;D:\environment\java\jdk1.8\jre\lib\javaws.jar;D:\environment\java\jdk1.8\jre\lib\jce.jar;D:\environment\java\jdk1.8\jre\lib\jfr.jar;D:\environment\java\jdk1.8\jre\lib\jfxswt.jar;D:\environment\java\jdk1.8\jre\lib\jsse.jar;D:\environment\java\jdk1.8\jre\lib\management-agent.jar;D:\environment\java\jdk1.8\jre\lib\plugin.jar;D:\environment\java\jdk1.8\jre\lib\resources.jar;D:\environment\java\jdk1.8\jre\lib\rt.jar;E:\桌面\java学习\production\基础语法 com.kuang.struct.DoWhileDemon01
5050

Process finished with exit code 0

DoWhile代码二:

package com.kuang.struct;

public class DoWhileDemon02 {
    public static void main(String[] args) {
        int a = 0;
        while (a<0){
            System.out.println(a);
            a++;
        }
        System.out.println("=============================");
        do {
            System.out.println(a);
            a++;
        }while (a<0);
    }
}

DoWhile运行结果二:

D:\environment\java\jdk1.8\bin\java.exe "-javaagent:D:\IntelliJ IDEA 2020.3.2\lib\idea_rt.jar=64377:D:\IntelliJ IDEA 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath D:\environment\java\jdk1.8\jre\lib\charsets.jar;D:\environment\java\jdk1.8\jre\lib\deploy.jar;D:\environment\java\jdk1.8\jre\lib\ext\access-bridge-64.jar;D:\environment\java\jdk1.8\jre\lib\ext\cldrdata.jar;D:\environment\java\jdk1.8\jre\lib\ext\dnsns.jar;D:\environment\java\jdk1.8\jre\lib\ext\jaccess.jar;D:\environment\java\jdk1.8\jre\lib\ext\jfxrt.jar;D:\environment\java\jdk1.8\jre\lib\ext\localedata.jar;D:\environment\java\jdk1.8\jre\lib\ext\nashorn.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunec.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunjce_provider.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunmscapi.jar;D:\environment\java\jdk1.8\jre\lib\ext\sunpkcs11.jar;D:\environment\java\jdk1.8\jre\lib\ext\zipfs.jar;D:\environment\java\jdk1.8\jre\lib\javaws.jar;D:\environment\java\jdk1.8\jre\lib\jce.jar;D:\environment\java\jdk1.8\jre\lib\jfr.jar;D:\environment\java\jdk1.8\jre\lib\jfxswt.jar;D:\environment\java\jdk1.8\jre\lib\jsse.jar;D:\environment\java\jdk1.8\jre\lib\management-agent.jar;D:\environment\java\jdk1.8\jre\lib\plugin.jar;D:\environment\java\jdk1.8\jre\lib\resources.jar;D:\environment\java\jdk1.8\jre\lib\rt.jar;E:\桌面\java学习\production\基础语法 com.kuang.struct.DoWhileDemon02
=============================
0

Process finished with exit code 0