How to Left-Pad a String with Zeros in Java?

Left Padding a String with Zeros in Java

Similar inquiries have been discussed in the past, but there remains some ambiguity regarding the technique for left padding a string with zeros. Specifically, let's aim to achieve the following transformation:

Input: "129018"
Output: "0000129018"

The final output should consist of exactly ten characters.

Numeric Strings

For strings containing only numeric characters, a simple approach is to convert the string to an integer and then use the String.format method to apply left padding:

String input = "129018";
int number = Integer.parseInt(input);
String padded = String.format("%010d", number);

Non-Numeric Strings

For non-numeric strings, alternative approaches are required:

String Concatenation

You can concatenate the required number of zeros to the beginning of the string:

String input = "abc";
StringBuilder padded = new StringBuilder();
int paddingLength = 10 - input.length();
for (int i = 0; i < paddingLength; i++) {
    padded.append('0');
}
padded.append(input);

Substring Replacement

You can replace a substring of the appropriate length at the beginning of the string with zeros:

String input = "abc";
String padded = String.format("%10s", input).replaceFirst(" ", "0");

StringBuilder Transformation

You can create a StringBuilder from the existing string and insert zeros at the beginning:

String input = "abc";
StringBuilder padded = new StringBuilder(input);
for (int i = 0; i < 10 - input.length(); i++) {
    padded.insert(0, '0');
}

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