Data types, declarations, and expressions in Java

Variables A variable is a named memory location capable of storing data As we have already seen, object variables refer to objects, which are created by instantiating classes with the new operator We can also store data in simple variables, which represent data only, without any associated methods

Data declaration syntax The syntax for the declaration of a variable is: Data type identifier; –“data type” may be the name of a class, as we have seen, or may be one of the simple types, which we’ll see in a moment –“identifier” is a legal Java identifier; the rules for simple variable identifiers are the same as those for object identifiers

Variable declaration: examples For example: int age; // int means integer double cashAmount; // double is a real # We can also declare multiple variables of the same type using a single instruction; for example: int x, y, z; //or int x, y, z; The second way is preferable, because it’s easier to document the purpose of each variable this way.

Numeric data types in Java: integers Data type nameMinimum valueMaximum value byte short-32,76832,767 int-2,147,483,6482,147,483,647 long-9,223,372,036,854,775,8089,223,372,036,854,775,807

Numeric data types in Java: floating-point numbers Data type nameMinimum valueMaximum value float x x double x x

Numeric data types: some notes Most programmers use int for whole numbers and double for real numbers Numeric data types in Java are primitive (non- object) types; this means that a numeric variable is somewhat different from an object: –You don’t use the new operator to initialize a numeric variable – just assign it a value –Memory for a numeric variable is allocated at declaration –Numeric variables actually store values; object names store addresses

Scientific notation and real numbers Both float and double have wide ranges to the values they can represent In order to save space, particularly large or small values are often displayed by default using a variation of scientific notation For example, the value would appear as 2.58 x in conventional notation – as output from a Java program, the number would appear as 2.58e-5 The ‘e’ is for exponent, and can be upper or lowercase

Assignment statements We can store a value in a variable using an assignment statement Assignment statement syntax: variableName = expression; –variableName must be the name of a declared variable –expression must evaluate to an appropriate value for storage within the type of variable specified

Arithmetic expressions An expression is a set of symbols that represents a value An arithmetic expression represents a numeric value Simple expressions are single values; examples: e3 Previously-declared and initialized variables or constants can also be simple expressions

Arithmetic operators in Java Compound expressions are formed by combining simple expressions using arithmetic operators OperationSymbol Addition+ Subtraction- Multiplication* Division/ Modulus%

Arithmetic operations in Java As in algebra, multiplication and division (and modulus, which we’ll look at momentarily) take precedence over addition and subtraction We can form larger expressions by adding more operators and more operands –Parentheses are used to group expressions, using the same rule as in algebra: evaluate the innermost parenthesized expression first, and work your way out through the levels of nesting –The one complication with this is we have only parentheses to group with; you can’t use curly or square brackets, as they have other specific meanings in Java

Examples int x = 4, y = 9, z; z = x + y * 2;// result is 22 z = (x + y) * 2;// result is 26 y = y – 1;// result is 8

Integer division When one real number is divided by another, the result is a real number; for example: double x = 5.2, y = 2.0, z; z = x / y;// result is 2.6 When dividing integers, we get an integer result For example: int x = 4, y = 9, z; z = x / 2;// result is 2 z = y / x;// result is 2, again z = x / y;// result is 0

Integer division There are two ways to divide integers –using the / operator, produces the quotient of the two operands –using the % operator, produces the remainder when the operands are divided. This is called modular division, or modulus (often abbreviated mod). For example: int x = 4, y = 9, z; z = x % 2;// result is 0 z = y % x;// result is 1 z = x % y;// result is 4

Mixed-type expressions A mixed-type expression is one that involves operands of different data types –Like other expressions, such an expression will evaluate to a single result –The data type of that value will be the type of the operand with the highest precision –What this means, for all practical purposes, is that, if an expression that involves both real numbers and whole numbers, the result will be a real number. The numeric promotion that takes place in a mixed-type expression is also known as implicit type casting

Explicit type casting We can perform a deliberate type conversion of an operand or expression through the explicit cast mechanism Explicit casts mean the operand or expression is evaluated as a value of the specified type rather than the type of the actual result The syntax for an explicit cast is: (data type) operand-or- (data type) (expression)

Explicit type casts - examples int x = 2, y = 5; double z; z = (double) y / z;// z = 2.5 z = (double) (y / z);// z = 2.0

Assignment conversion Another kind of implicit conversion can take place when an expression of one type is assigned to a variable of another type For example, an integer can be assigned to a real-number type variable; in this case, an implicit promotion of the integer value occurs

No demotions in assignment conversions In Java we are not allowed to “demote” a higher- precision type value by assigning it to a lower- precision type variable Instead, we must do an explicit type cast. Some examples: int x = 10; double y = x;// this is allowed; y = 10.0 x = y;// error: can’t demote value to int y = y / 3;// y now contains x = (int)y;// allowed; x = 3

Compound arithmetic/assignment operators Previous examples in the notes have included the following statements: y = y + 1; y = y / 3; In each case, the current value of the variable is used to evaluate the expression, and the resulting value is assigned to the variable (erasing the previously-stored value) This type of operation is extremely common; so much so, that Java (like C++ and C before it) provides a set of shorthand operators to perform this type of operation. The table on the next slide illustrates the use and meaning of these operators

Compound arithmetic/assignment operators OperatorUseMeaning +=X += 1;X = X + 1; -=X -= 1;X = X – 1; *=X *= 5;X = X * 5; /=X /= 2;X = X / 2; %=X %= 10;X = X % 10;

Named constants A variable is a named memory location that can hold a value of a specific data type; as we have seen, the value stored at this location can change throughout the execution of a program If we want to maintain a value in a named location, we use the Java keyword final in the declaration and immediately assign the desired value; with this mechanism, we declare a named constant. Some examples: final int LUCKY = 7; final double PI = ; final double LIGHTSPEED = 3.0e10.0 ;

Named constants The name of the constant is used in expressions but cannot be assigned a new value. For example, to calculate the value of variable circleArea using the variable radius and the value , we could write: circleArea = 2 * PI * radius * radius; The use of named constants is considered good programming practice, because it: –eliminates (or at least minimizes) the use of “magic” numbers in a program; it is easier to read code that contains meaningful names –allows a programmer to make global changes in calculations easily

Using named constants: example Suppose, for example, that you are writing a program that involves adding sales tax and subtracting discounts from users’ totals If the tax rate is 5% and the discount rate is 10%, the calculation could look like this: total = total – (total *.1) + ((total *.1) * (1 +.05)); By itself, this isn’t too bad; but suppose there are several places in the program that use these values?

Example continued If, for example, the discount changes to 12%, the programmer who has to maintain the code would have to change the value.1 to.12 everywhere in the program – at least, everywhere that it actually refers to the discount. –The value.1 could very well mean something else in a different expression. –If we use named constants instead, the value has to change in just one place, and there is no ambiguity about what the number means in context; with named constants, the revised code might read: total = total – (total * discount) + ((total * discount) * (1 + taxrate));

Calculations using Java’s Math class The standard Java class Math contains class methods and constants that are useful in performing calculations that go beyond simple arithmetic operations The constants defined in the Math class are Math.PI and Math.E, which are defined values for  and e (the base for natural logs), respectively

Math class methods Math.abs(a): returns the absolute value of its argument (a), which can be of type int, long, float, or double Math.sin(a): returns the sine of its argument, a double value representing an angle in radians; similar trigonometric functions include Math.cos(a) for cosine, Math.tan(a) for tangent, Math.acos(a), Math.asin(a) and Math.atan(a), which provide arccosine, arcsine, and arctangent, respectively

Math class methods Math.toDegrees(a): converts a, a double value representing an angle in radians, to the corresponding value in degrees Math.toRadians(a): converts a, a double value representing an angle in degrees to the corresponding value in radians

Math class methods Math.sqrt(a): returns the square root of a, a value of type double Math.cbrt(a): returns the cube root of a, a value of type double Math.pow(a, b): returns the value of a b Math.log(a): returns the natural log of a, a double value Math.log10(a): returns the log base 10 of a, a double value

Example // computing the roots of a quadratic equation: doublea,// coefficient of x squared b,// coefficient of x c,// 3 rd term in equation x1,// first root x2;// second root // read in values for a, b, and c – not shown here … x1 = (-b + Math.sqrt(Math.pow(b, 2) – (4 * a * c))) / (2 * a); x2 = (-b - Math.sqrt(Math.pow(b, 2) – (4 * a * c))) / (2 * a);