Lecture 20: Wrapper
Classes and More Loops
Building Java Programs: A Back to Basics Approach
by Stuart Reges and Marty Stepp
Copyright (c) Pearson 2013.
All rights reserved.
Wrapper Classes
• A wrapper class takes an existing value of primitive type and
“wraps” or “boxes” it in an object, and provides a new set of
methods for that type.
• It can be used in Java container classes that requires the item
to be objects. (Arraylist)
2
Wrapper Classes
The wrapper class allows
1. The construction of an object from a single value(wrapping or
boxing the primitive in a wrapper object.
2. The retrieval of a primitive value(unwrapping or unboxing
from a wrapper object.)
3
Wrapper Classes
Primitive Type Wrapper Type
int
Integer
double
Double
char
Character
boolean
Boolean
Java allows wrapper object for each of its primitive types. The
two that will be on the AP Exam are Integer and Double classes.
4
Integer Class
The Integer class wraps a value of type int in an object. This class has only
one instance variable whose type is int. Here are its methods:
Integer(int value): Constructs an Integer object from an int(boxing).
int compareTo(Integer other): Returns 0 if the value of this Integer
is equal to the value of the other, a negative integer if it is less than the value
of other and a positive integer if it is greater than the value of other.
5
Integer Class
int intValue(): Returns the value of this Integer as an int(unboxing).
boolean equals(Object obj): Returns true if and only if this Integer
has the same int value as obj.
– This method overrides the equals method from class Object.
– This method throws a ClassCastException if obj is not an Integer.
String toString(): Returns a String representing the value of this
Integer.
6
Double Class
The Double class wraps a value of type double in an object. This class has
only one instance variable whose type is double. Here are its methods:
Double(double value): Constructs an Double object from an
double(boxing).
int compareTo(Double other): Returns 0 if the value of this Double is
equal to the value of the other, a negative integer if it is less than the value of
other and a positive integer if it is greater than the value of other.
7
Double Class
double doubleValue(): Returns the value of this Double as a
double(unboxing).
boolean equals(Object obj): Returns true if and only if this Double
has the same double value as obj.
– This method overrides the equals method from class Object.
– This method throws a ClassCastException if obj is not a Double.
String toString(): Returns a String representing the value of this
Double.
8
Examples
Integer intObj=new Integer(6);//boxes 6 in Integer object
int j=intObj.intValue(); //unboxes 6 from Integer object
System.out.println(“Integer value is ”+intObj);
//calls toString() for intObj
//output is Integer value is 6
9
Examples
Object a=new Integer(5);
int j=4;
Integer b=new Integer(5);
int k=b.intValue();
if(a==b) //false
if(a.equals(b)) //true, polymorphism, which equals version?
if(a.intValue()==b.intValue()) //error, Object has no intValue
if(k==b.intValue()) //ok,comparing primitives
if(k.equals(j)) //error, j and k not objects
if(b.compareTo(a)<0) //error, need to cast
if(b.compareTo((Integer)a)<0)//ok
10
Examples
Double dObj=new Double(2.5);//boxes 2.5 in Double object
double d=dObj.doubleValue(); //unboxes 2.5 from Double object
Object object=new Double(7.3);
Object intObj=new Integer(4);
if(dObj.compareTo((Double)object>0) //remember to cast
if(dObj.compareTo(intObj)>0) //class cast exception
//can’t compare Integer
//with Double
11
Auto-Boxing and Unboxing
• Auto-boxing is the automatic wrapping of primitive types in
their wrapper classes.
• To retrieve the value of an Integer(or Double), the intValue()
(or doubleValue()) method must be invoked(unwrapped). Autounboxing is the automatic conversion of a wrapper class to its
corresponding primitive type. This means you don’t need to
explicitly call the intValue() or doubleValue().
12
Auto-Boxing and Unboxing
• Auto-boxing and unboxing reduce code clutter but are still
performed behind the scenes and can decrease run time
efficiency.
• When possible, use arrays of integers rather than arraylist of
Integers.
13
Examples
Integer a=new Integer(5);
int x=a; //auto unboxing
Integer b=7; //auto-boxing
int y=a+x; //auto-unboxing
NOTE: Autoboxing and unboxing will not be test on the AP Exam.
However, it is ok to use these features of Java in writing your
answers to the free response section.
14
For-Each Loop
For-Each Loop
The for-each or enhanced for loop is used to iterate over an array or arraylist.
The general form of the loop is
for(SomeType element: collection)
{
...
}
16
For-Each Loop
int[] arr={1,4,3,5};
for(int i=0;i<arr.length;i++)
System.out.println(arr[i]);
//for each
for(int element : arr)
System.out.println(element);
• The for-each loop cannot be used for replacing or removing elements as
you traverse.
• The loop hides the index variable that is used with arrays.
17
For-Each Loop
What is the output of the following code?
int[] arr={1,4,3,5};
for(int element : arr)
element=7;
System.out.println(Arrays.toString(arr));
Output:
[1,4,3,5]
The for-each loop cannot be used for replacing or removing elements as you
traverse! In this case, element is a temporary variable and does not refer to
any element in the array.
18
For-Each Loop
For each is the same for both an array and an arraylist.
String[] array=new String[2];
array[0]=“hello”; array[1]=“goodbye”;
ArrayList<String> list=new ArrayList<String>();
list.add(“hi”); list.add(“hello”);
for(String s: array)
System.out.println(s);
for(String s: list)
System.out.println(s);
19
For Each
The for-each construction is the same for objects.
Point[] pts={new Point(),new Point(1,2),new Point(-1,4)};
for(Point element : pts)
System.out.println(element+” “); //toString()
20
For Each
The for-each construction can be used with 2D arrays. Use
nested for each loops.
int[][] matrix={{1,5,6}{-3,-1,7},{0,1,-1}}
int sum=0;
for(int[] row: matrix){
for(int element: row)
sum+=element;}
for(int i=0;i<matrix.length;i++)
for(int j=0;j<matrix[0].length;j++)
sum+=matrix[i][j];
21
Lab 3
Write the Test class with the instance variables and methods listed below. If
necessary, modify the Question class written previously.
public class Test{
private ArrayList<Question> questions;
private int[] points;
public Test(ArrayList<Question> questions) {…}
public int totalPoints(){…}
public void printTest() {…}
public void printAnswers() {…}
public void setPoints(int mcPoints, int shortPoints) {…}
public boolean addChoice(int questionNumber, String
choice) {…}
public void scrambleTest() {…}
}
22
Lab 3
A Test consists of an arraylist of questions and an array of point value for each
question.
public Test(ArrayList<Question> questions)
The Test class has one constructor to initialize the arraylist questions. This constructor
must call setPoints(int mcPoints, int shortPoints) to initialize the points array. Each
multiple choice is worth 5 points and each short answer 10 points.
public int totalPoints()
This method returns the total number of points for the test.
public void printTest()
This method prints the entire test with proper numbering of the questions.
23
Lab 3
public void printAnswers()
This method prints a numbered list of answers to each question on the test. It should
also prints the point value for each question. For example,
1. (5 points) B: George Washington.
2. (10 points) Teddy Roosevelt.
etc..
public void setPoints(int mcPoints, int shortPoints)
This method initializes the points array. Each multiple choice question is worth
mcPoints and each short answer question is worth shortPoints.
public boolean addChoice(int questionNumber, String choice)
This method adds a choice to the question given by questionNumber. Returns true if
added properly and false otherwise.
24
Lab 3
public void scrambleTest();
This method scrambles the test questions. You must create a new temporary arraylist
of questions. The method must pick a random question from the instance variable
arraylist questions, remove it and add it to the temporary arraylist. You must use
Math.random(). In addition, the method updates the arraylist questions and the
points array.
25