Type Casting in Java

What is type casting

Type casting means converting a value from one data type to another — for example, an int to a double. Java has two kinds: widening (small to big, automatic) and narrowing (big to small, manual). Knowing the difference avoids data loss and errors.

Widening (automatic)

int i = 10;
double d = i;      // int -> double, automatic
System.out.println(d);

Converting a smaller type to a bigger one (int to double) is widening. It happens automatically because no data can be lost — a bigger type easily holds a smaller value.

Narrowing (manual)

double d = 9.7;
int i = (int) d;   // double -> int, manual cast
System.out.println(i);

Converting a bigger type to a smaller one (double to int) is narrowing. You must write the target type in brackets, (int), because data can be lost.

Data loss in narrowing

System.out.println((int) 9.7);    // 9  (decimal dropped)
System.out.println((int) 9.2);    // 9
System.out.println(Math.round(9.7)); // 10 (rounded)

Casting a double to int truncates — it drops the decimal, it does not round. So (int) 9.7 is 9. To round properly, use Math.round() instead.

String and number conversion

// String to number: parse, not cast
int n = Integer.parseInt("42");
// number to String:
String s = String.valueOf(42);
System.out.println(n + " " + s);

You cannot cast between a String and a number. To go from text to a number, parse it with Integer.parseInt. To go from a number to text, use String.valueOf.

Object casting

Object obj = "Hello";
String str = (String) obj;   // downcast back to String
System.out.println(str.length());

Objects can also be cast, usually when a reference is stored as a general type (like Object) and you cast it back to the specific type. Cast only to a type the object really is, or you get a ClassCastException.

Common mistakes

• Expecting narrowing to round. It truncates — use Math.round() to round.
• Casting a String to a number. You must parse, not cast.
• Forgetting the cast for narrowing. int i = 9.7; errors — write (int) 9.7.
• Wrong object cast. Casting to a type the object is not throws ClassCastException.

Summary

• Type casting converts a value between data types.
• Widening (small → big) is automatic and safe.
• Narrowing (big → small) needs a manual cast and may lose data.
• Casting a double to int truncates; use Math.round() to round.
• Convert strings and numbers by parsing, not casting.

Next, work with number objects in wrapper classes, or revise Java data types.