Chuyển đổi范围和

Changing a value's type

• Casting 类型转换 converts a value from one type to another.
• Write the target type in parentheses: (double) 5 gives 5.0; (int) 3.9 gives 3.
• Casting to int truncates 截断 — it drops the decimal part, it does not round.
• (int) 3.9 is 3, and (int) -2.7 is -2 (toward zero).

Widening happens automatically

• Going from a smaller type to a larger one (int → double) is widening 拓宽.
• Widening is safe, so Java does it automatically: double d = 5; stores 5.0.
• No information is lost going from int to double.
• You rarely need to write a widening cast, but it's allowed.

Casting truncates toward zero; int arithmetic overflows 溢出 past Integer.MAX_VALUE

Narrowing needs an explicit cast

• Going from larger to smaller (double → int) is narrowing 收窄 — it can lose data.
• Java requires an explicit cast: int n = (int) 3.9; (without the cast, it won't compile).
• The cast is you promising "I know I'm dropping the decimals."
• Narrowing truncates toward zero.

Range and overflow

• Each type has a fixed range 取值范围: int holds roughly ±2.1 billion.
• Exceeding the range causes overflow — the value wraps around to a wrong number.
• No error is raised; the result is just silently wrong.
• For huge whole numbers, a bigger type (like long) is needed.