Conversion & Casting

Converting between types comes in two flavors with opposite risk profiles.

Widening (safe, automatic) — smaller type into bigger type, no data loss:

int apples = 12;
double avg = apples;        // int → double, always fine: 12.0
long big = apples;          // int → long, fine

Narrowing (lossy, you must ask for it) — the compiler refuses silent damage; an explicit cast takes responsibility:

double price = 19.99;
int rounded = (int) price;  // 19 — the fraction is TRUNCATED, not rounded
long huge = 9_876_543_210L;
int small = (int) huge;     // compiles; may silently wrap around

(int) truncates toward zero — for real rounding use Math.round.

String ↔ number is the other daily conversion, and it can fail:

int n = Integer.parseInt("42");      // 42
double d = Double.parseDouble("3.5");// 3.5
String s = String.valueOf(42);       // "42"

Integer.parseInt("4x2");             // NumberFormatException at runtime

parseInt throws when the text is not a number — you will meet the try/catch that handles this politely in Module 12; for now, only parse text you trust.

One more trap: 1 + 2 + "A" is "3A" but "A" + 1 + 2 is "A12" — + works left to right, and once a String appears, everything after it is glued as text.

Next: arithmetic and its surprises.