Type Conversions

• When writing an assignment statement, one rule we have to follow is that the data type of the richt-hand value (rvalue) has to match the data type of the left-hand value (lvalue).

• Sometimes this rule can cause problems, for instance when we get input. Let's suppose we want to have the user enter a numerical value, then use it to perform mathematical calculations.

• When we get input, the data is of type String, but mathematical calculations require numerical primitives like int or double. What we need to do is convert the data from String to int or String to double.

• In Java, the primitive data types each have a corresponding class that begins with an upper-case letter. For instance, the primitive type int has a corresponding class called Integer and the primitive type double has a corresponding class called Double. These classes can be used to convert String data to primitives. They are called type-wrapper classes.

• Here's a short code example:

  Converting Strings to Primitives

  	int num1;
  	double num2;
  	String s1 = "17";
  	String s2 = "17.5";
  	num1 = Integer.parseInt(s1);
  	num2 = Double.parseDouble(s2);

• Look at the assignment statment that gives num1 a value. This statement converts the object s1 to an int and stores it in the integer primitive num1.

• Notice this form of dot-notation uses class.method(arg), where the class is Integer, the method name is parseInt, and the argument is s1.

• Next, look at the assignment statment that gives num2 a value. This statement converts the object s2 to a double and stores it in the double primitive num2.

• Notice this also uses the same form of dot-notation as the parseInt() method call. In this statement the class is Double, the method name is parseDouble, and the argument is s2.

• Now the values are in primitive objects they can be manipulated in mathematical formulas.

• In later versions of Java, arithmetic operations can be done with objects of the type-wrapper classes (like Double).