java.String.format &Formatter

o(∩_∩)o 

String result = String.format("The format method is %s!", "great");
// result now equals  "The format method is great!".

You don't need to use numbers to indicate positioning. By default, the position of the argument is simply the order in which it appears in the string.

You will always use a % followed by some other characters to let the method know how it should display the string. %s is probably the most common, and it just means that the argument should be treated as a string.

// we can specify the # of decimals we want to show for a floating point:
String result = String.format("10 / 3 = %.2f", 10.0 / 3.0);
// result now equals  "10 / 3 = 3.33"

// we can add commas to long numbers:
result = String.format("Today we processed %,d transactions.", 1000000);
// result now equals  "Today we processed 1,000,000 transactions."

String.format just uses a java.util.Formatter, so for a full description of the options you can see the Formatter javadocs.

Instead of looking at the source code, you should read the javadoc String.format() and Formatter syntax

String aString = "world";
int aInt = 20;
String.format("Hello, %s on line %d",  aString, aInt );

To do what you tried ( use an argument index ) you use: n$

String.format("Line:%2$d. Value:%1$s. Result: Hello %1$s at line %2$d", aString, aInt );