public class Test {
    public static void main(String[] args) {
        char a = '*';
        int num = 6;
        
        for (int i = 0; i < num; i ++){
            for (int j = i; j > 0 ; j --) {
                System.out.print(' ');
            }
            for (int k = num - i; k > 0 ; k --){
                System.out.print(a);
            }
            System.out.println();
        } 
        
        System.out.println();
        
        for (int i = num - 1; i >= 0; i --){
            for (int j = i; j > 0 ; j --) {
                System.out.print(' ');
            }
            for (int k = num - i; k  > 0 ; k --){
                System.out.print(a);
            }
            System.out.println();
        } 
    }
}

------解决方案--------------------
Java code

public class Trangle2 {

    public static void main(String[] args) {
        String b[]=new String[6];
        String s[]=new String[6];
        String blank="";
        String star="*";
        for(int i=0;i<6;i++){
            b[i]=blank;
            blank+=" ";
            s[i]=star;
            star+="*";            
        }
        for(int i=0;i<6;i++){
            System.out.println(b[i]+s[5-i]);
        }
    }

}