入门练习，打印一个菱形，效果如下  

     *
    * *
  *  *  *
*  *  *  *

  *  *  *

    * *

     *

public class diamond {
    public static void main(String[] args) {
        for (int i=1;i<=4;i++){
            for (int j=1;j<=4-i;j++){
                System.out.print(" ");
            }
            for (int k=1;k<i;k++){
                System.out.print("* ");
            }
            System.out.println("*");
        }
        for(int i=1;i<4;i++){
            for (int j=1;j<=i;j++){
                System.out.print(" ");
            }
            for (int j=1;j<=4-i;j++){ 
                System.out.print("* ");
            }
            System.out.println();
        }

    }
}