Which statement is correct to convert a string str to a double?

In Java, to convert a string representation of a number into a double primitive type, the method Double.parseDouble(String s) is used. This method belongs to the Double class and specifically handles the conversion from a String to a double. The correct answer demonstrates this proper usage by directly invoking the method on the Double class, passing the string as the argument.

The other options either reference incorrect types or methods that do not exist in the Java language. The option that suggests using Integer.parseDouble(str) is incorrect because Integer class deals with integers, not doubles, and there's no double.parseDouble(str) since double is a primitive type and does not have methods. The first option incorrectly tries to call a method that does not exist on a string, leading to confusion. Thus, the correct method usage emphasizes understanding of class methods in Java and their correct applications for type conversion.