Module 1 · Java from zero Lesson 2 of 15
Types, variables and casting
Java is statically typed: every variable has a type fixed at compile time. That is why Java programs fail at compile time instead of at 3am, and why interviews start with types.
The concept
Java is statically typed: you declare the type and it does not change. int, long, double, boolean, char and String cover fresher work — note that String starts with a capital, because it is a class and the others are primitives. Declaring the type is what stops a whole class of bugs before they compile.
Narrowing needs an explicit cast and loses information: (int) 9.8 gives 9, and integer division is why 7 / 2 is 3. Widening is automatic and safe. If you need exact money arithmetic, neither is good enough — use BigDecimal, which is what a service-company interviewer is checking when they ask about double.
int age = 21; // whole number
double marks = 88.5; // decimal
boolean placed = false;
char grade = 'A';
String name = "Asha"; // a class, not a primitive
int rounded = (int) marks; // narrowing CAST drops the decimals
String text = Integer.toString(age);
int back = Integer.parseInt(text); // text -> numberCommon mistakes
The mistakes are comparing strings with == instead of .equals() — == compares references, which happen to match inside the string pool and fail on text from a file — and reading a number as a String from Scanner and then doing arithmetic on it. Integer.parseInt() is the bridge.
Worked example
int a = 7; int b = 2; a / b is 3, not 3.5 — integer division truncates. Write (double) a / b and it becomes 3.5. Knowing this one line separates candidates who have run Java from candidates who have read about it.
Your practice task
Write a small calculator that takes the four operations and prints results as decimals, then check the division case carefully.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
Java is statically typed: you declare the type and it does not change.
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