Java从零开始学八(循环结构)
一、循环结构
循环结构则是根据判断条件的成立与否，决定程序段落的执行次数，而这个程序段落就称为循环主体。
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二、While循环
while是循环语句，也是条件判断语句。当事先不知道循环该执行多少次的时，就要用到while循环。while循环的格式如下：
while (循环条件判断){
语句1 ;
语句2 ;
…
语句n ;
循环条件更改 ;
}
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" alt="" />
示例一、使用While进行100以内累加操作
package loop;
public class test1 {
public static void main(String[] args) {
// 使用While进行100以内累加操作
int sum=0; //定义变量保存累加的值
int i=1; //定义整型变量i
while(i<=100){
sum+=i;
i++;
}
System.out.println("100以内整数之和: "+sum);
}
}
结果：100以内整数之和: 5050
While是先判断条件是否成立，如果成立则进入循环，不成立则跳过循环
示例二、同上i的值变为101
package loop;
public class test {
public static void main(String[] args) {
// 使用While进行100以内累加操作
int sum=0; //定义变量保存累加的值
int i=101; //定义整型变量i
while(i<=100){
sum+=i;
i++;
}
System.out.println("100以内整数之和: "+sum);
}
}
结果：100以内整数之和: 0
三、do-while循环
do…while循环也是用于未知循环执行次数的时候，而while循环及do…while循环最大不同就是进入while循环前，while语句会先测试判断条件的真假，再决定是否执行循环主体，而do…while循环则是“先做再说”，每次都是先执行一次循环主体，然后再测试判断条件的真假，所以无论循环成立的条件是什么，使用do…while循环时，至少都会执行一次循环主体，语句格式：
do{
语句1 ;
语句2 ;
….
语句n ;
循环条件改变 ;
}while (循环条件判断);
aaarticlea/png;base64,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" alt="" />
示例三、使用do-While进行100以内累加操作
package loop;
public class test {
public static void main(String[] args) {
// 使用While进行100以内累加操作
int sum=0; //定义变量保存累加的值
int i=1; //定义整型变量i
do{
sum+=i;
i++;
}while(i<=100);
System.out.println("100以内整数之和: "+sum);
}
}
结果:100以内整数之和: 5050
四、for循环
对于while和do…while两种循环来讲，操作时并不一定非要明确的知道循环的次数，而如果开发者已经明确的知道了循环次数的话，那么就可以使用另外一种循环语句——for循环。 格式如下：
for (赋值初值；判断条件；赋值增减量){
语句1 ；
….
语句n ；
}
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" alt="" />
示例四、使用for进行100以内累加操作
package loop;
public class test2 {
public static void main(String[] args) {
// 使用for进行100以内累加操作
int sum=0; //定义变量保存累加的值
for(int i=1; i<=100;i++){
sum=sum+i;
}
System.out.println("100以内整数之和: "+sum);
}
}
结果:100以内整数之和: 5050
4.1循环嵌套
示例五、for循环打印99乘法表
package loop;
public class test3 {
public static void main(String[] args) {
// 使用for循环打印99乘法表
for(int i=1;i<=9;i++){
for(int j=1;j<=i;j++){
System.out.print(j+"*"+i+"="+(j*i)+"\t");
}
System.out.println();
}
}
}
结果:
1*1=1
1*2=2 2*2=4
1*3=3 2*3=6 3*3=9
1*4=4 2*4=8 3*4=12 4*4=16
1*5=5 2*5=10 3*5=15 4*5=20 5*5=25
1*6=6 2*6=12 3*6=18 4*6=24 5*6=30 6*6=36
1*7=7 2*7=14 3*7=21 4*7=28 5*7=35 6*7=42 7*7=49
1*8=8 2*8=16 3*8=24 4*8=32 5*8=40 6*8=48 7*8=56 8*8=64
1*9=9 2*9=18 3*9=27 4*9=36 5*9=45 6*9=54 7*9=63 8*9=72 9*9=81
五、break语句
break语句可以强迫程序中断循环，当程序执行到break语句时，即会离开循环，继续执行循环外的下一个语句，如果break语句出现在嵌套循环中的内层循环，则break 语句只会跳出当前层的循环。以下图的for循环为例，在循环主体中有break语句时，当程序执行到break，即会离开循环主体，而继续执行循环外层的语句。
示例六、实现输出1-10，遇到4时使用break;
package loop;
public class test4 {
public static void main(String[] args) {
// 实现输出1-10，遇到4时程序退出
for(int i=1;i<=10;i++){
if (i==4){
break;
}
System.out.print(i+" ");
}
System.out.println("循环结束");
}
}
结果:1 2 3 循环结束
六、continue语句
continue语句可以强迫程序跳到循环的起始处，当程序运行到continue语句时，即会停止运行剩余的循环主体，而是回到循环的开始处继续运行。以下图的for循环为例，在循环主体中有continue语句，当程序执行到continue，即会回到循环的起点，继续执行循环主体的部分语句。
continue是跳过当前循环进入下次循环
示例七、同上实现输出1-10，遇到4时使用continue;
package loop;
public class test4 {
public static void main(String[] args) {
// 实现输出1-10，遇到4时使用continue
for(int i=1;i<=10;i++){
if (i==4){
continue;
}
System.out.print(i+" ");
}
System.out.println("循环结束");
}
}
结果：1 2 3 5 6 7 8 9 10 循环结束
七、return语句
结束当前方法的执行并退出，返回调用该方法的语句处。
示例入、同上实现输出1-10，遇到4时使用return;
package loop;
public class test4 {
public static void main(String[] args) {
// 实现输出1-10，遇到4时使用return
for(int i=1;i<=10;i++){
if (i==4){
return;
}
System.out.print(i+" ");
}
System.out.println("循环结束");
}
}
结果：1 2 3
可以看到“循环结束”没有输出，当i=4时，满足执行return，结束整个方法的执行
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