System.out.printf() usage

Question

I am new to Java. While learning the printf method I came across the below question:

What would be the output of following program?

System.out.printf("%1$d + %b", 456, false);
System.out.println();
System.out.printf("%1$d + %b", 456);

The answer is:

456 + true
456 + true

Can someone help me to understand how true is getting printed without me passing it?

Answer 1

1$ is called Explicit indexing, the successive format of %1$d will not lead to the increment of index, so it will also use456 to format %b, and according to the doc:

If the argument arg is null, then the result is "false". If arg is a boolean or Boolean, then the result is the string returned by String.valueOf(arg). Otherwise, the result is "true".

that's why you always get true.

To get false:

System.out.printf("%1$d + %b", null); // null + false

or remove explicit indexing:

System.out.printf("%d + %b", 456, null); // 456 + false

Check the doc of java.uti.Formatter for more.