您必须在System.out.println()调用中添加一些条件。
还有：

• 使用适当的变量/方法名称，避免使用单字母名称
• 您应该在0处进行索引;这更符合逻辑

public class Main {
    public static void main(String[] args) {
        printTriangles();
    }
    private static void printTriangles() {
        //Scanner sc = new Scanner(System.in);
        //System.out.print("Number of rows: ");
        int rows = 4; // sc.nextInt();
        for (int row = rows; row > 0; row--) {
            for (int col = 0; col < row; col++) {
                System.out.print(" * ");
           }
           if (row > 1) {
               System.out.println();
           }
        }
        for (int row = 0; row <= rows; row++) {
            for (int col = 0; col < row; col++) {
                System.out.print(" * ");
            }
            if (row < rows) {
               System.out.println();
           }
        }
    }
}

输出

其中输入= 4

*  *  *  * 
 *  *  * 
 *  * 
 * 
 * 
 *  * 
 *  *  * 
 *  *  *  *

试试这个，即没有-1的内部循环

for(int row=a; row>=1; row--){

    for (int col=1; col <=row; col++){
        System.out.print(" * ");
   }
   System.out.println();
}
for(int row=1; row<=a; row++){

    for (int col=1; col <=row; col++){
        System.out.print(" * ");
    }
       System.out.println();
}