1Z0-771: Java Fundamentals and Data Types

Domain 1 tests whether you can read a short snippet and decide whether it compiles and what it prints: primitive widths, literal syntax, promotion and casting, var, arrays, and String equality.

Primitives and literal rules

Eight primitives: byte (8 bits), short (16), int (32), long (64), float (32), double (64), char (16, unsigned), boolean.
Integer literals are int unless suffixed L; floating-point literals are double unless suffixed f. Prefixes: 0x hex, 0b binary, leading 0 octal.
Underscores may only separate digits: 1_000_000 compiles; _100, 100_, 1_.0 and 1.0_f do not.

Promotion and casting

Binary numeric promotion widens to double, else float, else long, else int. byte, short and char always promote to int in arithmetic.
Widening is implicit; narrowing needs a cast. Compound assignment (+=) and ++/-- apply an implicit cast, which is why b += 3 compiles when b = b + 3 does not.

var, arrays, String

var is legal only for local variables with an initializer; not for fields, parameters, or null.
Arrays are covariant: Object[] o = new String[1]; compiles, but storing an Integer throws ArrayStoreException at runtime. length is a field, not a method.
String is immutable and pooled; == compares references, so use equals for content.

Worked example

byte b = 5;
b = b + 3;      // compile error: int cannot be assigned to byte
b += 3;         // compiles; implicit cast to byte
long n = 1_000 * 1_000_000_000L; // int * long -> long

The first assignment fails because b + 3 is an int. The compound form hides the cast, so no error.

Traps

switch accepts int-compatible types, String and enums, never long, float or boolean.
char c = 'a' + 1; is legal (constant expression in range); char c = 'a' + 1L; is not.
var is resolved at compile time, so var x = 1; x = "s"; fails.