Java How to Convert String to Double - Simple Example

In Java, convert a string to a double using Double.parseDouble(yourString), which returns the double value represented by the string.

Examples

Input"3.14"

Output3.14

Input"0"

Output0.0

Input"-123.456"

Output-123.456

How to Think About It

To convert a string to a double, think of reading the number as text and turning it into a decimal number type. Java provides a built-in method that reads the string and returns its double value if the string is a valid number.

Algorithm

1

Take the input string that represents a number.

2

Use the built-in method to parse the string into a double.

3

Return the double value.

Code

java

public class StringToDouble {
    public static void main(String[] args) {
        String numberStr = "3.14";
        double number = Double.parseDouble(numberStr);
        System.out.println(number);
    }
}

Output

3.14

Dry Run

Let's trace converting the string "3.14" to a double.

1

Input string

numberStr = "3.14"

2

Parse string

Double.parseDouble("3.14") returns 3.14

3

Print result

System.out.println(3.14) outputs 3.14

Step	Action	Value
1	Input string	"3.14"
2	Parse to double	3.14
3	Print output	3.14

Why This Works

Step 1: Parsing the string

The method Double.parseDouble() reads the string and converts it to a double number.

Step 2: Valid string format

The string must represent a valid decimal number, otherwise it throws a NumberFormatException.

Step 3: Using the double value

Once converted, you can use the double value in calculations or print it.

Alternative Approaches

Using Double.valueOf()

java

public class StringToDoubleValueOf {
    public static void main(String[] args) {
        String numberStr = "3.14";
        double number = Double.valueOf(numberStr);
        System.out.println(number);
    }
}

Returns a Double object which auto-unboxes to double; useful if you need an object.

Using new Scanner

java

import java.util.Scanner;
public class StringToDoubleScanner {
    public static void main(String[] args) {
        String numberStr = "3.14";
        Scanner scanner = new Scanner(numberStr);
        double number = scanner.nextDouble();
        System.out.println(number);
        scanner.close();
    }
}

Uses Scanner to parse double from string; more flexible for complex input but heavier.

Complexity: O(n) time, O(1) space

Time Complexity

Parsing the string scans each character once, so it takes linear time relative to string length.

Space Complexity

No extra space is needed besides a few variables, so space is constant.

Which Approach is Fastest?

Double.parseDouble() is fastest and simplest; Double.valueOf() adds object creation overhead; Scanner is slower and more complex.

Approach	Time	Space	Best For
Double.parseDouble()	O(n)	O(1)	Simple, fast conversion
Double.valueOf()	O(n)	O(1)	When Double object needed
Scanner.nextDouble()	O(n)	O(1)	Parsing complex input streams