How to Left-Pad Strings with Zeros in Java?

Left Zero-Padding Strings: A Comprehensive Solution

Left zero-padding strings is a common task in various programming contexts. This article explores a solution to left-pad a string with zeros using the String.format() function.

Left Padding Numeric Strings with Zeros

For strings that consist solely of numbers, a straightforward approach is to convert them to integers and then apply padding:

int num = Integer.parseInt("129018");
String padded = String.format("%010d", num);
System.out.println(padded); // Output: "0000129018"

Zero-Padding Non-Numeric Strings

If the string contains non-numeric characters, the above method will not work. In such cases, a more general approach is required:

StringBuilder sb = new StringBuilder();
int numZeros = 10 - "mystring".length();
for (int i = 0; i < numZeros; i++) {
    sb.append('0');
}
sb.append("mystring");
String padded = sb.toString();
System.out.println(padded);

This approach manually constructs a string by appending the desired number of zeros, followed by the original string.

Note: The total output length, as specified in the question (10), can be customized by adjusting the numZeros variable.

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