Lernen Rounding and Parsing with DecimalFormat | Controlling Numeric Output with DecimalFormat

When you use DecimalFormat in Java, controlling how numbers are rounded is crucial for both the accuracy and presentation of your output. The DecimalFormat class allows you to set a specific rounding mode, which determines how it handles values that fall exactly between two possible rounded results. Rounding modes can affect financial calculations, scientific data, and any scenario where precision is important. Some common rounding modes include RoundingMode.HALF_UP (rounds towards the nearest neighbor, but rounds up if equidistant), RoundingMode.DOWN (always rounds towards zero), and RoundingMode.UP (always rounds away from zero). Choosing the right rounding mode ensures your application behaves as expected, especially in edge cases where values are right on the rounding boundary.

Main.java


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package com.example;

import java.text.DecimalFormat;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        double value = 2.675;

        DecimalFormat dfHalfUp = new DecimalFormat("#.##");
        dfHalfUp.setRoundingMode(RoundingMode.HALF_UP);

        DecimalFormat dfDown = new DecimalFormat("#.##");
        dfDown.setRoundingMode(RoundingMode.DOWN);

        DecimalFormat dfUp = new DecimalFormat("#.##");
        dfUp.setRoundingMode(RoundingMode.UP);

        System.out.println("Original value: " + value);
        System.out.println("HALF_UP: " + dfHalfUp.format(value));
        System.out.println("DOWN: " + dfDown.format(value));
        System.out.println("UP: " + dfUp.format(value));
    }
}

Beyond formatting, DecimalFormat can also convert formatted strings back into numbers using the parse() method. This is useful when you receive user input or data from external sources in a formatted string, and you need to work with it as a number in your application. The parse() method attempts to interpret the string according to the pattern and symbols defined in your DecimalFormat instance. If the string matches the pattern, you get a Number object; if not, a ParseException is thrown. Proper error handling is important to ensure your program can gracefully handle unexpected or invalid input.

Main.java


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package com.example;

import java.text.DecimalFormat;
import java.text.ParseException;

public class Main {
    public static void main(String[] args) {
        String formatted = "1,234.56";
        DecimalFormat df = new DecimalFormat("#,##0.00");

        try {
            Number number = df.parse(formatted);
            System.out.println("Parsed number: " + number.doubleValue());
        } catch (ParseException e) {
            System.out.println("Failed to parse: " + formatted);
        }
    }
}

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Abschnitt 2. Kapitel 3

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Suggested prompts:

Can you explain how to set a specific rounding mode with `DecimalFormat`?

What are some best practices for choosing a rounding mode in financial applications?

Can you give examples of how different rounding modes affect the output?

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Main.java


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package com.example;

import java.text.DecimalFormat;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        double value = 2.675;

        DecimalFormat dfHalfUp = new DecimalFormat("#.##");
        dfHalfUp.setRoundingMode(RoundingMode.HALF_UP);

        DecimalFormat dfDown = new DecimalFormat("#.##");
        dfDown.setRoundingMode(RoundingMode.DOWN);

        DecimalFormat dfUp = new DecimalFormat("#.##");
        dfUp.setRoundingMode(RoundingMode.UP);

        System.out.println("Original value: " + value);
        System.out.println("HALF_UP: " + dfHalfUp.format(value));
        System.out.println("DOWN: " + dfDown.format(value));
        System.out.println("UP: " + dfUp.format(value));
    }
}

Main.java


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package com.example;

import java.text.DecimalFormat;
import java.text.ParseException;

public class Main {
    public static void main(String[] args) {
        String formatted = "1,234.56";
        DecimalFormat df = new DecimalFormat("#,##0.00");

        try {
            Number number = df.parse(formatted);
            System.out.println("Parsed number: " + number.doubleValue());
        } catch (ParseException e) {
            System.out.println("Failed to parse: " + formatted);
        }
    }
}

War alles klar?

Danke für Ihr Feedback!

Abschnitt 2. Kapitel 3

Rounding and Parsing with DecimalFormat

1. What happens if you try to parse a string that doesn't match the DecimalFormat pattern?

2. Which method is used to set the rounding mode in DecimalFormat?

3. What is the result of parsing "1,234.56" with a DecimalFormat expecting two decimal places?

Rounding and Parsing with DecimalFormat

1. What happens if you try to parse a string that doesn't match the DecimalFormat pattern?

2. Which method is used to set the rounding mode in DecimalFormat?

3. What is the result of parsing "1,234.56" with a DecimalFormat expecting two decimal places?