COSC: 12.6 Primitive and reference types

12.7.3: ArrayList definition, initialization, and use. # Question Your answer 1 In a single statement, define and initialize a reference variable for an ArrayList named frameScores that stores items of type Integer. CheckShow answer 2 Assign the Integer element at index 8 of ArrayList frameScores to a variable currFrame. currFrame = frameScores.get(8); The get() method returns the object at the specified index within the ArrayList. CheckShow answer 3 Assign the value 10 to element at index 2 of ArrayList frameScores. CheckShow answer 4 Expand the size of ArrayList frameScores by appending an element with an Integer value of 9. frameScores.add(new Integer(9)); The add() method expands the size of the ArrayList by 1. The new element is added at the end of the list.

1. ArrayList<Integer> frameScores = new ArrayList<Integer>(); Creates an empty ArrayList for Integer objects. 2. currFrame = frameScores.get(8); The get() method returns the object at the specified index within the ArrayList. 3. frameScores.set(2, new Integer(10)); Creates a new Integer object and sets the ArrayList's element at index 2 to be equal to that Integer object. 4. frameScores.add(new Integer(9)); The add() method expands the size of the ArrayList by 1. The new element is added at the end of the list.

12.6.6: String representations. # Question Your answer 1 Write a statement that assigns the String representation of the value held by the Double trackRadius to a String variable called radiusText. radiusText =; CheckShow answer 2 Write a statement that converts the text representation of an integer within String numberText, storing the value in an int variable numLanes. numLanes=

1. trackRadius.toString() The toString() method returns a String representing the value held by the primitive wrapper object. 2. Integer.parseInt(numberText) Integer.parseInt() returns an int representing the value encoded in a String argument.

12.6.4: Comparing primitive wrapper objects. Given the following primitive wrapper objects, determine whether the provided comparisons evaluate to true or false: Integer score1 = new Integer(95); Integer score2 = new Integer(91); Integer score3 = 95; int maxScore = score1; 1 score1 < score2 True False 2 score1 <= score3 True False 3 score2 < maxScore True False 4 score1 == score3 True False 5 98 < score3 True False 6 score1.equals(score3) True False 7 score2.compareTo(score1) > 0

1. False < operator compares value of objects: 95 > 91 2. True <= operator compares value of objects: 95 = 95 3. True < operator compares value of objects: 91 < 95 4. False == operator compares object references. score1 and score3 refer to two different Integer objects. 5. False Compares integer literal to value of object: 98 > 95 6. True equals() methods compares values of objects: 95 = 95 7. False compareTo() returns a negative value because score2 < score1.

12.6.5: Converting to primitive types. # Question Your answer 1 Write a statement that assigns the int representation of the value held by the Integer objects totalPins to a primitive int variable pinScore. CheckShow answer 2 Write a statement that assigns the double representation of the value held by the Integer objects numElements to a double variable named calcSize.

1. pinScore = totalPins.intValue(); intValue returns value of numElements as a primitive int value. 2. calcSize = numElements.doubleValue(); doubleValue returns value of numElements as a primitive double value.

charValue() and booleanValue

Character and Boolean classes support the charValue() and booleanValue() methods, respectively, which perform similar functions.

12.6.2: Time calculation using Integer class.

Integer timeMins = 0; Integer timeHrs = 0; timeMins = 400; timeHrs = timeMins/60; -timeMins creates an integer object When the result of an expression is assigned to an Integer reference variable, memory for a new Integer object with the computed value is allocated, and the reference (or address) of this new object is assigned to the reference variable A new memory allocation occurs every time a new value is assigned to an Integer variable, and the previous memory location to which the variable referred, remains unmodified.

Table 12.6.1: Commonly used primitive wrapper classes

Reference type Associated primitive type Character char Integer int Double double Boolean boolean Long long

equals() and compareTo()

Reference variables of primitive wrapper classes can also be compared using the equals() and compareTo() methods. These method descriptions are presented for the Integer class, but apply equally well to the other primitive wrapper classe

12.6.3: Defining primitive wrapper objects. # Question Your answer 1 Define a variable called gameScore that refers to a primitive wrapper object with an int value of 81. Use the object initialization style. 2 Define a variable called trackCircumference that refers to a primitive wrapper object with a double value of 29.5. Do not use the object initialization style. 3 Define a variable called logoChar that refers to a primitive wrapper object with a char value of 'U'. Do not use the object initializ

Solutions: 1. Integer gameScore = new Integer(81); Using the "new" operator makes it clear that Integer is a class. 2. Double trackCircumference = 29.5; Primitive wrapper objects can be initialized in the same manner as variables of primitive types. 3. Character logoChar = 'U'; Primitive wrapper objects can be initialized in the same manner as variables of primitive types

primitive wrapper classes

are built-in reference types that augment the primitive types -allow the program to create objects that store a single primitive type value, such as an integer or floating-point value. -provide methods for converting between primitive types (e.g., int to double), between number systems (e.g., decimal to binary), and between a primitive type and a String representation.

Double

class to calculate the amount of time necessary to drive or fly a certain distance.

integer

data type is a built in class in Java that augments the int primitive type

Table 12.6.3: equals() and compareTo() methods for primitive wrapper types. Given: Integer num1 = 10; Integer num2 = 8; Integer num3 = 10; int regularInt = 20;

equals(otherInt) : true if both Integers contain the same value. otherInteger may be an Integer object, int variable, or integer literal. num1.equals(num2) // Evaluates to false num1.equals(10) // Evaluates to true !(num2.equals(regularInt)) // Evaluates to true because 8 != 20 compareTo(otherInt): return 0 if the two Integer values are equal, returns a negative number if the Integer value is less than otherInteger's value, and returns a positive number if the Integer value is greater than otherInteger's value. otherInteger may be an Integer object, int variable, or integer literal. num1.compareTo(num2) // Returns value greater than 0, because 10 > 8 num2.compareTo(8) // Returns 0 because 8 == 8 num1.compareTo(regularInt) // Returns value less than 0, because 10 < 20

Figure 12.6.2: A program to convert a decimal number to binary.

import java.util.Scanner; public class DecimalToBinary { public static void main (String [] args) { Scanner scnr = new Scanner(System.in); int decimalInput = 0; String binaryOutput = ""; System.out.print("Enter a decimal number: "); decimalInput = scnr.nextInt(); binaryOutput = Integer.toBinaryString(decimalInput); System.out.println("The binary representation of " + decimalInput + " is " + binaryOutput); return; } } Enter a decimal number: 10 The binary representation of 10 is 1010 ... Enter a decimal number: 256 The binary representation of 256 is 100000000

Figure 12.6.1: Program using the Double class to calculate flight and driving times.

import java.util.Scanner; public class FlyDrive { public static void main (String [] args) { Scanner scnr = new Scanner(System.in); (**) Double distMiles = 0.0; (**) Double hoursFly = 0.0; (**) Double hoursDrive = 0.0; System.out.print("Enter a distance in miles: "); distMiles = scnr.nextDouble(); hoursFly = distMiles / 500.0; hoursDrive = distMiles / 60.0; System.out.println(distMiles + " miles would take:"); System.out.println(hoursFly + " hours to fly"); System.out.println(hoursDrive + " hours to drive"); return; } } (**) variable for a primitive wrapper class may be initialized when defined similarly to variables of primitive types, as in Double distMiles = 0.0;. Alternatively, the new operator can be used, as in Double distMiles = new Double(0.0);, which is equivalent to Double distMiles = 0.0;.

12.6.1: Time calculation using primitives.

int timeMins = 0; int timeHrs = 0; timeMins = 400; timeHrs = timeMins/60;

Table 12.6.4: Converting primitive wrapper objects to primitive types Given: Integer num1 = 14; Double num2 = 6.7643; Double num3 = 5.6e12;

intValue(): Returns the value of the primitive wrapper object as a primitive int value, type casting if necessary. num2.intValue() // Returns 6 doubleValue(): Returns the value of the primitive wrapper object as a primitive double value, type casting if necessary. num1.doubleValue() // Returns 14.0 longValue(): Returns the value of the primitive wrapper object as a primitive long value, type casting if necessary. num3.longValue() // Returns 5600000000000

immutable

meaning a programmer cannot change the object via methods or variable assignments after object creation. -A primitive wrapper object (as well as a String object) is immutable -Ex: jerseyNumber = 24; does not change the value in jerseyNumber's object. Instead, the assignment allocates a new Integer object with value 24, returns a reference to that new object, and assigns the reference to variable jerseyNumber. jerseyNumber refers to the new Integer object, and the original Integer object is unchanged. -When using a literal for initialization, the programmer must ensure that the literal's value falls within the appropriate numeric range

Table 12.6.2: Comparing primitive wrapper class objects using relational operators.

objectVar == objectVar (also applies to !=) DO NOT USE. Compares references to objects, not the value of the objects. objectVar == primitiveVar (also applies to !=) OK. Compares value of object to value of primitive variable. objectVar == 100 (also applies to !=) OK. Compares value of object to literal constant. objectVar < objectVar (also applies to <=, >, and >=) OK. Compares values of objects. objectVar < primitiveVar (also applies to <=, >, and >=) OK. Compares values of object to value of primitive. objectVar < 100 (also applies to <=, >, and >=) OK. Compares values of object to literal constant.

Table 12.6.5: Conversions: Strings and numeral systems Given: Integer num1 = 10; Double num2 = 3.14; String str1 = "32"; int regularInt = 20;

toString(): Returns a String containing the decimal representation of the value contained by the primitive wrapper object. num1.toString() // Returns "10" num2.toString() // Returns "3.14" Integer.toString(someInteger): Returns a String containing the decimal representation of the value of someInteger. someInteger may be be an Integer object, a int variable, or an integer literal. This static method is also available for the other primitive wrapper classes (e.g., Double.toString(someDouble)). Integer.toString(num1) // Returns "10" Integer.toString(regularInt) // Returns "20" Integer.toString(3) // Returns "3" Integer.parseInt(someString): Parses someString and returns an int representing the value encoded by someString. This static method is also available for the other primitive wrapper classes (e.g., Double.parseDouble(someString)), returning the corresponding primitive type. Integer.parseInt(str1) // Returns int value 32 Integer.parseInt("2001") // Returns int value 2001 Integer.valueOf(someString): Parses someString and returns a new Integer object with the value encoded by someString. This static method is also available for the other primitive wrapper classes (e.g., Double.valueOf(someString)), returning a new object of the corresponding type. Integer.valueOf(str1) // Returns Integer object with value 32 Integer.valueOf("2001") // Returns Integer object with value 2001 Integer.toBinaryString(someInteger): Returns a String containing the binary representation of someInteger. someInteger may be be an Integer object, a int variable, or an integer literal. This static method is also available for the Long classes (e.g., Long.toBinaryString(someLong)). Integer.toBinaryString(num1) // Returns "1010" Integer.toBinaryString(regularInt) // Returns "10100" Integer.toBinaryString(7) // Return "111"

reference type

variable can refer to an instance of a class, also known as an object. Ex: integer maxPlayer = 10; defines an integer reference variable named maxPlayers that refers to an instance of the integer class, also known as an Integer object. The integer object stores the integer value 10.

primitive type

variable directly stores the data for that variable type, such as int, double, or char. Ex: int numStudents = 20; defines an in that directly stores the data 20