Type Conversion

Type conversion is the process of converting a value of one data type into another data type. There are many situations where data is available in one type and the program needs to treat it as another type. Common instances are situations where a mathematical operation needs a float value and one of the operands needed is an integer, or when processing user input that comes in as String data and it needs to be interpreted as some sort of number.

Converting Between Numeric Types

In certain situations, where the data types are compatible, Java can do type conversion automatically. This happens when a smaller data type, like int, is assigned to a variable of a longer data type, like long.

// Automatic type conversion
int x = 15;
long y = 0L;
y = x; // y is now 15L

This process is also known as widening and follows the following pattern:

byte -> short -> int -> long -> float -> double

Any data type can be “widened” to a data type to the right on this list.

Converting to a smaller data type, i.e. going to the left on this list, requires explicit conversion or type casting.

// Explicit type casting
long x = 15L;
int y = 0;
y = (int)x; // y is now 15

Converting Strings to Numeric Types

String values are not directly compatible with numeric values. However, Java provides Number subclasses for the primitive numeric types; Byte, Integer, Double, Float, Long and Short. Each of these provides a .parseXXXX() method that takes a string and provides the equivalent data type.

// Convert a string into a number using .parseXXXX()
String s = "15";
// Convert a string into an integer.
int x = Integer.parseInt(s); // x is now 15
// Convert a string into a float.
float y = Float.parseFloat(s); // y is now 15f

Converting Numeric Types Into Strings

Converting numbers to strings can occur in a few different ways.

When a number is concatenated with a string, it will be converted automatically:

// Automatically convert an integer to a string
int i = 15;
String s = "" + i; // s now has the value "15"

The String.valueOf() method can be used:

// Convert an integer to a string using the .valueOf() method.
int i = 15;
String s = String.valueOf(i); // s now has the value "15"

Also, each of the Number subclasses has a .toString() method that converts its associated primitive numeric type into a String:

// Convert an integer to a string using the .toString() method.
int i = 15;
String s = Integer.toString(i); // s now has the value "15"