Question

Assume that the double variable number holds the value \(0.179 .\) What format pattern would you use with a Decimalformat object to display the number as .18 ?

Short answer

Answer: We can display a decimal number without leading zero and rounded to two decimal places using a DecimalFormat object in Java by following these steps: 1. Import DecimalFormat and RoundingMode from the java.text package. 2. Create a DecimalFormat object with a format pattern (e.g. ".00") and set the RoundingMode (e.g., RoundingMode.HALF_UP). 3. Apply the DecimalFormat object to the given decimal number. 4. Display the formatted number. For example, to display the decimal number 0.179 as .18, create a DecimalFormat object with the format pattern ".00" and apply it to the number. The formatted number will be displayed as .18.

Step-by-step solution

Import DecimalFormat and RoundingMode

In order to use DecimalFormat to format a number, we need to import it along with the RoundingMode enum from the java.text package: ```java import java.text.DecimalFormat; import java.text.DecimalFormatSymbols; import java.util.Locale; import java.math.RoundingMode; ```

Create a DecimalFormat object with the correct format pattern

To format the number without a leading zero and to round it to two decimal places, we can use the format pattern "0.00". To achieve this, create a DecimalFormat object, set the rounding mode, and set the decimal format symbols with a new instance without the leading zero. Here's how: ```java DecimalFormat df = new DecimalFormat(".00"); df.setRoundingMode(RoundingMode.HALF_UP); DecimalFormatSymbols dfs = new DecimalFormatSymbols(Locale.getDefault()); dfs.setDecimalSeparator('.'); dfs.setGroupingSeparator(','); df.setDecimalFormatSymbols(dfs); ```

Apply the DecimalFormat object to the given number

Now that we have the DecimalFormat object set up with the correct format pattern, apply it to the given double variable "number" holding the value 0.179: ```java double number = 0.179; String formattedNumber = df.format(number); ```

Display the formatted number

Finally, display the formatted number as ".18" using the formattedNumber variable: ```java System.out.println(formattedNumber); // Output: .18 ```

Key concepts

java.text package

In Java, the java.text package is essential for text processing and includes classes and interfaces for handling text, dates, numbers, and messages in a way that is independent of natural languages. This means that developers can write programs that can be easily localized—or adapted for different languages and regions without engineering changes.

Specifically regarding number formatting, the java.text package provides the DecimalFormat class, which is a concrete subclass of NumberFormat. With DecimalFormat, you can format decimal numbers into strings with customized patterns, such as rounding numbers, showing commas for thousands, or displaying currency. It greatly simplifies the task of displaying numbers in a more human-readable form or as per a specific locale's standards.

RoundingMode

When formatting numbers, precise control over rounding is often necessary. In Java, the RoundingMode enumeration provides multiple rounding behaviors, such as CEILING, FLOOR, HALF_UP (which is similar to the common mathematical practice of rounding), HALF_DOWN, HALF_EVEN (also known as bankers' rounding), UP, DOWN, and UNNECESSARY (which throws an exception if rounding is needed).

Choosing the correct RoundingMode can be critical for certain applications, such as financial calculations where rounding might have significant effects. For example, HALF_UP mode, the most commonly used mode, will round up if the digit to the right is 5 or more. In the exercise, this mode ensures the output is .18 when the input number is 0.179.

Number formatting in Java

Number formatting in Java is made relatively straightforward using the classes provided within the java.text package. The DecimalFormat class is particularly useful as it allows for the customization of number formatting, such as the precision of decimals, or whether or not to show commas in large numbers.

To do so, we define a DecimalFormat pattern. For instance, '.00' defines a format with two decimal places and no leading digit. This pattern is then applied through a DecimalFormat object to the number we wish to format. In the exercise, the goal was to format the number 0.179 to .18, meaning the number should have two decimal places, rounding up if necessary. Thanks to the DecimalFormat and RoundingMode, we can control the output string of our formatted number to meet various requirements.