Java 练习(输出三角形，九九乘法表, 100以内的质数)

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));
	}
	
}