1, 用for循环打印一个4*5的矩形

public class Test {	public static void main(String[] args){	for (int i = 1; i < 5; i++) {			for (int j = 1; j < 6; j++) {				System.out.print("*");//print不换行；			}			System.out.println();//println自动换行；		}	}}

2, 1-100以内前5个能被3整除的整数

public class Test {	public static void main(String[] args) {		int counter = 0;//counter是计数器 		for (int i = 1; i <= 100; i++) {			if (i % 3 == 0) {				System.out.println(i);//能被3整除打印一次记一次				counter++;			}			if (counter == 5) {				break;//记录5次中断循环			}		}	}}

 3, 101-200以内的质数(质数只能被1和自身整除)

public class Test {	public static void main(String[] args) {		for (int i = 101; i <= 200; i++) {			int t = 0;			for (int j = 2; j < i; j++) {				if (i % j == 0) {					t = 1;					break;				}			}			if (t == 0) {				System.out.println(i);			}                  }	}}