Formatting numeric values to a specific precision is a common requirement in Java applications. Several technique are available to constrain a floating-point value to exactly four decimal digits.

Using DecimalFormat

The DecimalFormat class, part of java.text, creates configurable pattern-based formatters. A pattern such as "0.0000" ensures four digits appear after the decimal separator.

import java.text.DecimalFormat;

public class DecimalFormatter {
    public static void main(String[] args) {
        double rawValue = 7.123456;
        DecimalFormat formatter = new DecimalFormat("0.0000");
        String output = formatter.format(rawValue);
        System.out.println(output);  // 7.1235
    }
}

The pattern "0.0000" forces zeros in empty places, whereas "#.####" would avoid them. The result is influenced by the RoundingMode configured on the formatter instance.

Using String.format

Static formatting with String.format or System.out.printf provides a quick way to limit decimal digits through a format specifier.

public class StringBasedFormat {
    public static void main(String[] args) {
        double measurement = 2.9876543;
        String truncated = String.format("%.4f", measurement);
        System.out.println(truncated);  // 2.9877
    }
}

The specifier %.4f handles rounding using HALF_UP by default. It is concise for log messages or display text, but06 the underlying value remains a full-precsiion double.

Using BigDecimal

When exact decimal representation andor controlled rounding are mandatory, java.math.BigDecimal is the preferred choice. It stores an unscaled integer value coupled with a scale.

import java.math.BigDecimal;
import java.math.RoundingMode;

public class BigDecimalScaling {
    public static void main(String[] args) {
        String literal = "5.678901";
        BigDecimal exactVal = new BigDecimal(literal);
        BigDecimal scaledVal = exactVal.setScale(
            4,
            RoundingMode.HALF_EVEN
        );
        System.out.println(scaledVal.toPlainString());  // 5.6789
    }
}

setScale requires a precision value and a rounding mode. Encoding numbers via string constructor avoids floating-point representation errors.use this approach for monetary amounts or when accumulation errors cannot be tolerated.

Each technique serves a different context: DecimalFormat for flexible locale-specific patterns, String.format for inline text decoration, and BigDecimal for strict arithmetic control. Selecting appropriately keeps code both correct and intention-revealing.