实心菱形：

算法如下：

public void print(int num) {for (int a = 1; a <= num; a++) {for (int b = 1; b <= num - a; b++) {System.out.print(" ");}for (int b = 1; b <= a; b++) {System.out.print("* ");}System.out.println();}for (int a = num - 1; a > 0; a--) {for (int b = 1; b <= num - a; b++) {System.out.print(" ");}for (int b = 1; b <= a; b++) {System.out.print("* ");}System.out.println();}
}

空心菱形：

算法如下：

public void print(int num) {for (int a = 1; a <= num; a++) {for (int b = 1; b <= num - a; b++) {System.out.print(" ");}System.out.print("*");if (a != 1) {for (int b = 1; b < 2 * a - 2; b++) {System.out.print(" ");}System.out.print("*");}System.out.println();}for (int a = num - 1; a > 0; a--) {for (int b = 1; b <= num - a; b++) {System.out.print(" ");}System.out.print("*");if (a != 1) {for (int b = 1; b < 2 * a - 2; b++) {System.out.print(" ");}System.out.print("*");}System.out.println();}
}