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level: Chapter 2 - Data in Java

Questions and Answers List

level questions: Chapter 2 - Data in Java

QuestionAnswer
What does a variable consist of?Data type - int, double, String Identifier (name) - •Combination of letters, digits and special characters (without spaces etc. and operators like +, -,*, etc.) •Must start with a letter •Identifiers are case sensitive •Java keywords are not allowed (e.g. void, int) •Convention for compound names: myName
Does variable have initial values?Initially, variables have no well-defined value (not even 0!), but can have arbitrary (possibly also invalid) contents.
What are the primitive data types?Photo
Floating point numbers (float, double)Literals: • 0.23, -47.11 • Scientific notation: -1.23E-45 (= -1,23x10-45) • In principle double; float can be forced by a suffix f or F Operations: • additive: +, - (binary and unary) • multiplicative: *, / • comparison: == (attention – often imprecise!), !=, >, <, >=, <=
Logical values (boolean)Literals: • true or false Operations (boolean algebra): • Negation: ! (unary) • Conjunction (∧, AND): && • Disjunction (∨, OR): || • Antivalence (≢, XOR): ^ • Comparison: ==, !=
Characters (char)For storing individual characters of a text: •Upper and lower case letters, digits, special characters •Value range: 216 possible characters (Unicode) Literal: between ' ' Comparisons (lexicographic): ==, !=, >, <, >=, <= • +, - with whole numbers (»code arithmetic«): 'A' + 1 ➝ (Unicode of A) + 1 ➝ Unicode of B No possibility of input via a Scanner variable
Character Strings (String)For storing texts. Actually not a primitive data type, but a class Input via a Scanner variable in: in.next()
ExpressionsExpressions are combinations of operands and corresponding operators. operators: +, -, *, /, % operands: x, y, z, 1, 2, 3
Precedence and associativity of operatorsa
Type compatibility & Type conversions (1)Operands of an expression must be (data) type compatible to each other – each operation (+, *, ...) ultimately requires operands of the same data type. Two types are compatible if they are • equal, or • one can be converted to the other by implicit type conversion (performed autonomously by the compiler). Implicit (automatic) data type conversion (type promotion) • byte → short → int → long → float → double • char → int • All primitive types → String These conversions never cause data loss!
Type compatibility & Type conversions (2)If no implicit type conversion is possible, an explicit conversion must take place. Data loss is possible in this case. e.g int area = (int) (r*r*3.14159);
Conversion of strings to primitive typesint valueI = Integer.parseInt("123"); double valueD = Double.parseDouble("123.4");
Assignment as an expressionAssignments are actually expressions expression: result = a + b * c assignment: =
Shortcut assignment operatorsCompund assignment operators following the pattern •= (for any binary operator •): a += b; // is equivalent to a = a + b; a -= b; // is equivalent to a = a - b; a *= b; // is equivalent to a = a * b; a /= b; // is equivalent to a = a / b;
Shortcut assignment operatorsd
Evaluation order (1)Subexpressions are evaluated (with respect to the run-time) strictly from left to right. Logical expression are »lazyly« evaluated (conditional evaluation, lazy evaluation) – only as long as the final result is not yet determined: • x && y yields always false, if x == false • x || y yields always true, if x == true Therefore, y is not evaluated in these cases
Evaluation order (2)The order is especially important if side effects occur in the expression! Example 1: int i = 2; int j = (i=3) * i; // j = 9
RiddleWHICH ALGEBRAIC LAW FOR THE OPERATORS && AND || IS VIOLATED BECAUSE OF THIS?