输出三角形

class ForForTest{

public static void main(String[] args){

for(int i = 1; i<= 5; i++){ //控制行数

for(int j = 1; j <= i; j++){ //控制列数

System.out.print("*");

}

System.out.println();

}

for(int i = 1; i<= 4; i++){

for(int j = 1; j<= 5-i; j++){

System.out.print("*");

}

System.out.println();

}

}

}

结果:

*

**

***

****

*****

****

***

**

*

九九乘法表

class NineNineTable{

public static void main(String[] args){

for(int i = 1; i <= 9; i++){

for(int j = 1; j <= i; j++){

System.out.print( i + " * " + j + " = " + i*j + " ");

}

System.out.println();

}

}

}

结果:

1 * 1 = 1

2 * 1 = 2 2 * 2 = 4

3 * 1 = 3 3 * 2 = 6 3 * 3 = 9

4 * 1 = 4 4 * 2 = 8 4 * 3 = 12 4 * 4 = 16

5 * 1 = 5 5 * 2 = 10 5 * 3 = 15 5 * 4 = 20 5 * 5 = 25

6 * 1 = 6 6 * 2 = 12 6 * 3 = 18 6 * 4 = 24 6 * 5 = 30 6 * 6 = 36

7 * 1 = 7 7 * 2 = 14 7 * 3 = 21 7 * 4 = 28 7 * 5 = 35 7 * 6 = 42 7 * 7 = 49

8 * 1 = 8 8 * 2 = 16 8 * 3 = 24 8 * 4 = 32 8 * 5 = 40 8 * 6 = 48 8 * 7 = 56 8 * 8 = 64

9 * 1 = 9 9 * 2 = 18 9 * 3 = 27 9 * 4 = 36 9 * 5 = 45 9 * 6 = 54 9 * 7 = 63 9 * 8 = 72 9 * 9 = 81

求100以内的质数

class PrimeNumberTest{

public static void main(String[] args){

boolean isFlag = true; //标识i是否被j除尽，一旦除尽，修改其值

for(int i = 2; i <= 100; i++){ //遍历100以内的自然数

for(int j =2; j < i; j++){

if(i % j == 0){

isFlag = false;

break;

}

}

if(isFlag == true){

System.out.println(i);

}

//重置 isFlag

isFlag = true;

}

}

}

结果:

2

3

5

7

11

13

17

19

23

29

31

37

41

43

47

53

59

61

67

71

73

79

83

89

97

求100000以内的质数个数

class PrimeNumberTest2{

public static void main(String[] args){

boolean isFlag = true; //标识i是否被j除尽，一旦除尽，修改其值

int count = 0; //质数的个数

//获取当前时间距离 1970-01-01 00:00:00 的毫秒数

long start = System.currentTimeMillis();

for(int i = 2; i <= 100000; i++){ //遍历100000以内的自然数

for(int j =2; j <= Math.sqrt(i); j++){

if(i % j == 0){

isFlag = false;

break;

}

}

if(isFlag == true){

//System.out.println(i);

count++;

}

//重置 isFlag

isFlag = true;

}

//获取当前时间距离 1970-01-01 00:00:00 的毫秒数

long end = System.currentTimeMillis();

System.out.println("质数的个数为: " + count);

System.out.println("所花费的时间为: " + (end - start));

}

}

class PrimeNumberTest3{

public static void main(String[] args){

int count = 0; //质数的个数

//获取当前时间距离 1970-01-01 00:00:00 的毫秒数

long start = System.currentTimeMillis();

klvchen:for(int i = 2; i <= 100000; i++){ //遍历100000以内的自然数

for(int j =2; j <= Math.sqrt(i); j++){

if(i % j == 0){

continue klvchen;

}

}

//能执行到此步骤的，都是质数

count++;

}

//获取当前时间距离 1970-01-01 00:00:00 的毫秒数

long end = System.currentTimeMillis();

System.out.println("质数的个数为: " + count);

System.out.println("所花费的时间为: " + (end - start));

}

}