安卓动态字符串 : %1d vs %1$d

Question

包含 $ 的格式字符串有区别吗？

%1d 对比 %1$d

我注意到在应用程序代码中我们同时拥有两者，而且两者似乎都可以正常工作。

或者顺便说一句，使用 %1d to %2d vs %1$d to %2$d

Answer 1

%d - just display number

int first = 10;
System.out.printf("%d", first);
// output:
// 10

---

%1d - display number with information how much space should be "reserved" (at least one)

int first = 10;
System.out.printf("->%1d<-", first);
// output:
// ->10<-

%9d - reserve "9" chars (if number will be shorter - put spaces there) and align to right

int first = 10;
System.out.printf("->%9d<-", first);
// output:
// ->       10<-
//   123456789

%-9d - same as above but align to LEFT (this minus is important)

int first = 10;
System.out.printf("->%-9d<-", first);
// output:
// ->10       <-
//   123456789

---

%1$d - use (in format) position of the element from varargs (so you can REUSE elements instead of passing them two times)

int first = 10;
int second = 20;
System.out.printf("%1$d %2$d %1$d", first, second);
// output:
// 10 20 10

---

%1$9d - mix two of them. Get first parameter (1$) and reserve nine chars (%9d)

int first = 10;
System.out.printf("->%1$9d<-", first);
// output:
// ->       10<-
//   123456789