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"%1$#" in String.format in Java is a formatting flag which is used to specify the minimum width for the formatted string. The "1$" is the index of the argument that will be formatted and "#" is the flag that specifies that if the argument is a numeric type (e.g. Integer, Double), it should be formatted with a leading numerical sign, such as "+" or "-".
For example, if we have the following string:
String.format("%1$#+10d", 123);
The output will be:
"+ 123"
In this example, the "1$" specifies that the argument to be formatted is the first argument (123), the "#" flag specifies that the numeric value should be displayed with a leading plus or minus sign (in this case, a plus sign), and the "+10" specifies that the formatted string should be at least 10 characters wide, with any extra space added to the left.