1.巢狀迴圈：將一個迴圈結構巢狀在另一個迴圈結構中。例如A在B中迴圈

2.外層迴圈：B

內層迴圈: A

3.假設外層迴圈了M次，內層迴圈了N次，一共迴圈了M*N次

4.外層控制行數，內層控制個數

package struct;

public class demo21 {
    public static void main(String[] args) {
        //輸出五行星，每行四個
        for (int i = 1; i <= 5; i++) {
            for (int i1 = 1; i1 <=4; i1++) {

                System.out.print(
 
"*");
            }


            System.out.println();

        }
    }
}

package struct;

public class demo22 {
    public static void main(String[] args) {
        //輸出一個菱形
        for (int i = 1; i <5; i++) {
            for (int j = 1; j <=6-i; j++) {
                System.out.print(" ");
            }
            
 
for (int i1 = 1; i1 <i; i1++) {
                System.out.print("*");
            }
            for (int i1 = 1; i1 <=i; i1++) {
                System.out.print("*");
            }

            System.out.println();
        }
        for (int i = 1; i <=5; i++) {
            for (int i1 =1 ; i1 <=i; i1++) {
                System.out.print(
 
" ");
            }
            for (int i1 = 1; i1 < 6-i; i1++) {
                System.out.print("*");
            }
            for (int i1 = 1; i1 <=6-i; i1++) {
                System.out.print("*");
            }
            System.out.println();
            }

        }


}