Exam objectives. Java Certification - Week 2. Operators and Assignments. Chris Harris. Overview. Exam objectives (continued)

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1 Exam objectives Java Certification - Week 2 Operators and Assignments Chris Harris Determine the result of applying any operator,including assignment operators,instance of,and casts to operands of any type,class,scope,or accessibility,or any combination of these. Determine the result of applying the boolean equals (Object)method to objects of any combination of the classes java.lang.string,java.lang.boolean,and java.lang.object Exam objectives (continued) In an expression involving the operators &,,&&,,and variables of known values,state which operands are evaluated and the value of the expression Determine the effect upon objects and primitive values of passing variables into methods and performing assignments or other modifying operations in that method Overview Conversions Rules for Operators Assignment Operators Arithmetic Operators Boolean Expressions and Operators Shift Operators Parameter passing

2 Conversions - Casting Java is a strongly typed language. Type checking at compile time. Runtime type checking also needed. Casting is used to explicitly perform a type conversion in cases where an operator would have incompatible operands. Example: int myint = (int)3.14; Casts can be applied to primitive and reference types. Conversions - Casting (continued) Primitive types Narrowing Conversion loss of information Widening Conversion no loss of information Reference types Converting up and down the inheritance hierarchy. Downcasting Upcasting Conversions can be explicit or implicit. Conversions - Numeric Promotions Unary Numeric Promotion If the single operand is of a narrower type than int, it is converted implicitly to int. Will occur in the following contexts: Unary arithmetic operators + - Bitwise complement ~ Array creation ( dimension must be int ) Array indexing ( index must be int ) Shift operators >> >> >>> Conversions - Numeric Promotions (continued) Binary Numeric Promotion Let T be the broadest numeric type of the two operands. If T is broader than int, both operands are converted to T, otherwise they are both converted to int. Will occur in the following contexts: Arithmetic operators *,%,/,+ and - Relational operators <,<=,> and >= Numerical equality operators == and!= Bitwise operators &, ^ and

3 Rules for Operators Precedence and Associativity Rules Determine the order of evaluation Precedence and Associativity rules can be overridden using parentheses. Put them in if you are not sure. Rules for Operators - Operator Precedence In decreasing order of precendence. Postfix Operators [ ],., (parameters), exp++, exp-- Prefix Unary Operators ++exp, --exp, +exp, -exp Object creation and cast new, (type) Multiplication and Division *. / Addition and Substraction +, = Shift <<, >>, >>> Relational Operators <, <=, >, >=, instanceof Equality Operators ==,!= bitwise / boolean AND & bitwise / boolean XOR ^ bitwise / boolean OR logical AND && logical OR Conditional Operator?: Assigment =, +=, -=, *=, /=, %=, <<=, >>=, &=, ^=, = Rules for Operators (continued) Associativity Rules Determines the order of evaluation when two operators have the same precedence. Unary postfix operators and all binary operators, except for assignment operators, associate from left to right. All unary operators ( except for postfix ), all assignment operators and the ternary conditional operator ( including object creation and cast ) associate right to left. Assignment Operators - = Simple assignment operator. Both primitive and reference types <variable> = <expression> The types must be compatible Can perform multiple assignment. An assignment always returns the result of the assignment. Examples: int j, k; k = j = 12; // ( k = (j =12 ) )

4 Assignment Operators - = (continued) Numeric Type conversion Widening conversion If the source and destination are of different types and a widening conversion is permitted, it is performed implicitly. Narrowing conversion Implicit narrowing conversions can occur If the source is an int const whose value can be determined to be in the range of the destination type at compile time. Other narrowing conversions will produce a compile error. - Need a explicit cast. Assignment Operators - = (continued) Integer to floating-point values can result in loss of precision. - Don't confuse magnitude and precision. Narrowing conversions between char, byte or short, a cast is always required. Arithmetic Operators The arithmetic operators are listed below in order of precedence, the top row having the highest precedence. Unary + - Binary * / % + - Associativity Unary - right to left Binary - left to right Operands are evaluated left to right before operators are applied. Arithmetic Expressions - Numeric Promotion Unary operators byte, short and char are first promoted to int type, the result is of type int. So an explicit cast is often needed. Example: byte b = 4; b = (byte)-b;

5 Arithmetic Expressions - Numeric Promotion (continued) Binary operators Automatic type promotion of operands. The result is of the promoted type, which is always at least type int. An implicit widening conversion may be applied if the result is being assigned. Variable increment and decrement operators. Two flavours, prefix and postfix. Variable increment ( ++ ) Prefix - ++i Adds one to i, then uses the new value of i. Postfix - i++ Uses value of i, then adds one to i. Variable increment and decrement operators. Variable decrement ( -- ) Prefix = --i Subtracts one from i, then uses the new value of i. Postfix - i-- Uses value of i, then subtracts one from i. Can be applied to byte, char, short and int In the case of byte, char and short, binary numeric promotion and an implicit narrowing conversion are performed Example: byte b; int i = ++b; Boolean Expressions Have boolean datatype and can only evaluate to true or false. Boolean expressions can be formed using the following operators: Relational operators Equality operators Logical operators Conditional operators Assignment operator

6 Boolean Expressions - Relational Operators Also known as the comparison operators. a < b - a less than b? a <= b - a less than or equal to b? a > b - a greater than b? a >= b - a great than or equal to b? Binary operators Operands are numeric. Binary numeric promotion is applied. a<=b<=c must written as a<=b && b<=c Boolean Expressions - Equality Operators Primitive Data Value Equality If a and b are primitive datatypes a == b - a and b are equal? I.e. have the same primitive value. a!= b - a and b are not equal? Binary numeric promotion is applied to non-boolean operands. Care should be taken when comparing floating point numbers as certain values can only be stored as approximations. ( /3.0 == 1.0/3.0 ) returns false Boolean Expressions - Equality Operators (continued) Object Reference Equality References the same object If r and s are object references r == s - r and s are equal? r!= s - r and s are not equal? The operators must be type compatible. Must be possible to cast one into the other. Can check a reference for null. Protect against NullPointerExceptions. Boolean Expressions - Equality Operators (continued) Object Value Equality The equals(... ) method is defined in java.lang.object. The default implementation is equivalent to == Can be overridden by subclasses String, Date, File etc.

7 Boolean Expressions - Boolean Logical Operators NOT -!x Returns the complement of the truth-value of x. AND - x & y true if both operands are true, false otherwise. OR - x y true if either or both are true, false otherwise. XOR - x ^ y true if and only if one operand is true, false otherwise. Operators always evaluate both operands. Boolean Expressions - Boolean Logical Extended Assignment Operators b &= a b ^= a b = a b = ( b & (a)) b = (b ^ (a)) b = (b (a)) Boolean Expressions - Conditional Operators &&, Logically equivalent to & and Their evaluation is short-circuited. Left to right evaluation. If the result of the boolean expression can be evaluated from the left hand operand, the right one is not evaluated. Integer Bitwise Operators Binary bitwise operators Manipulate the individual bit values in the operands. Numeric promotion is applied. result int or long Bitwise NOT ~A Invert the bit value. Bitwise AND A & B 1 if both bits are 1, 0 otherwise. Bitwise OR A B 1 if either or both bits are 1, 0 otherwise Bitwise XOR A ^ B 1 if and only if one of the bits is 1, otherwise 0.

8 Integer Bitwise Extend Assignment Operators Shift Operators T is the integer type of b. b &= a b ^= a b = a b = (T) (b & (a)) b = (T) (b ^ (a)) b = (T) (b (a)) An implicit narrowing conversion is performed when destination type is either byte, short or char. Shift a << n Shifts all bits in a left n times, filling with 0 from the right. Shift right with sign bit a >> n Shifts all bits in a right n times but filling with the sign bit from the left. Shift right with zero fill a >>> n Shifts all bits in a right n times but filling with 0 from the left Unary numeric promotion is applied to each operand. The returned value has the promoted type of the left operand. Left operand is not changed. Shift Operators (continued) The number of bits shifted is always in the range modulus 32 for an int value, and in the range modulus 64 for a long value. Shifting a positive value can result in a negative value. Shift Extended Assignment Operators T is the integral type of x. x <<= a x >>= a x >>>= a x = (T) ( x << (a)) x = (T) (x >> (a)) x = (T) (x >>> (a)) An implicit narrowing conversion may be performed.

9 The Conditional Operator The Other Operators <condition>?<expression 1>:<expression 2> If condition is true then expression 1 is evaluated, otherwise expression 2 is evaluated. The new operator. Object creation. The [ ] operator. Declaring and constructing arrays. Array access. The instanceof operator. Used to test an objects type. Parameter Passing Formal parameters Parameters defined in the method definition. Local to method. Actual parameters Parameters passed to the method when it is called. Actual and Formal parameters must be compatible in two respects: Number of actual param must equal the number of formal. Actual and formal param must be type compatible. Parameter Passing (continued) In Java all parameters are passed by value. Primitive datatype The data value of the actual parameter is passed. Class type The reference value is passed not the object. Array type The reference value is passed not the array data. Arrays are object. Type conversion of parameters are similar to those for numeric assignment conversions, except no implicit narrowing conversions are made for int constants.

10 Parameter Passing - final Parameters Formal parameters can be declared as final. These are known as blank final variables. Value cannot be changed during the lifetime of the variable. The compile can perform optimizations on the variables.