Java - convert String to double

In Java, we can convert String to double in couple of different ways.

Short solutions:

// solution 1
double num1 = Double.parseDouble("123.45");

// solution 2
Double num2 = Double.valueOf("123.45");

// solution 3
Double num3 = new Double("123.45");

1. Using Double.parseDouble()

public class Example1 {

    public static void main(String[] args) {
        String str = "123.45";
        double num = Double.parseDouble(str);
        System.out.println(num); // 123.45
    }
}

Output:

123.45

When we use Double.parseDouble() we can use +, - as prefix. Also we can use d or D as postfix. Below we have practical example.

public class Example1DifferentPatterns {

    public static void main(String[] args) {
        // +, - as prefix
        System.out.println(Double.parseDouble("+123.45")); // 123.45
        System.out.println(Double.parseDouble("-123.45")); // -123.45
        
        // d, D - as postfix
        System.out.println(Double.parseDouble("123.45d")); // 123.45
        System.out.println(Double.parseDouble("123.45D")); // 123.45
        
        // other examples
        System.out.println(Double.parseDouble("+123.45d")); // 123.45
        System.out.println(Double.parseDouble("+123.45000111000d")); // 123.45000111
    }
}

2. Using Double.valueOf()

public class Example2 {

    public static void main(String[] args) {
        String str = "123.45";
        double num = Double.valueOf(str);
        System.out.println(num); // 123.45
    }
}

Output:

123.45

3. Using constructor - new Double(String s)

public class Example3 {

    public static void main(String[] args) {
        String str = "123.45";
        double num = new Double(str);
        System.out.println(num); // 123.45
    }
}

Output:

123.45

4. Using DecimalFormat parse method

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

public class Example4 {

    public static void main(String[] args) throws ParseException {
        String str = "123.45";
        double num = DecimalFormat.getNumberInstance().parse(str).doubleValue();
        System.out.println(num); // 123.45
    }
}

Output:

123.45

5. Using sun.misc.FloatingDecimal.parseDouble()

public class Example5 {

    public static void main(String[] args) {
        String str = "123.45";
        double num = sun.misc.FloatingDecimal.parseDouble(str);
        System.out.println(num); // 123.45
    }
}

 Output:

123.45

NOTE:
When we look under the hood of:
- Double.parseDouble("123.45")
- Double.valueOf("123.45")
- new Double("123.45")
Then we can see that all of those methods call:
sun.misc.FloatingDecimal.parseDouble()

Reference

1. Type conversion - wiki
2. Double.parseDouble() - docs
3. Double.valueOf() - docs
4. new Double() - docs
5. DecimalFormat parse() - docs