1. Arrays of Objects
Fall CS2302: Programming Principles

2. Wrappers: Java’s Wrapper classes for the primitive types
Contents
Wrappers: Java’s Wrapper classes for the primitive types
Object class
Arrays of Object
Fall CS2302: Programming Principles

3. Java has a wrapper class for each of the eight primitive data types:
Primitives & Wrappers
Java has a wrapper class for each of the eight primitive data types:
Primitive Type
Wrapper Class
boolean
Boolean
float
Float
byte
Byte
int
Integer
char
Character
long
Long
double
Double
short
Short
Fall CS2302: Programming Principles

4. Primitives vs. Wrappers
int x = 25; Integer y = new Integer(33);
Fall CS2302: Programming Principles

5. Use of the Wrapper Classes
Java’s primitive data types (boolean, int, etc.) are not classes.
Wrapper classes are used in situations where objects are required, such as for elements of a Collection:
ArrayList<Integer> a = new ArrayList<Integer>();
a.add(new Integer(2));
Fall CS2302: Programming Principles

6. Value => Object: Wrapper Object Creation
Wrapper.valueOf() takes a value (or string) and returns an object of that class:
Integer i1 = Integer.valueOf(42);
Integer i2 = Integer.valueOf(“42”);
Boolean b1 = Boolean.valueOf(true);
Boolean b2 = Boolean.valueOf(“true”);
Fall CS2302: Programming Principles

7. Object => Value
Each wrapper class Type has a method typeValue to obtain the object’s value:
Integer i1 = Integer.valueOf(42);
System.out.println(i1.intValue());
Boolean b1 = Boolean.valueOf(“false”);
System.out.println(b1.booleanValue());
=> 42
false
Fall CS2302: Programming Principles

8. The Object Class and Its Methods
java.lang.Object class
The toString() method returns a string representation of the object. The default implementation returns a string consisting of a class name of which the object is an instance, the at sign and a number representing this object.
Circle c1 = new Circle();
System.out.println(c1.toString());
Fall CS2302: Programming Principles

9. The elements of an array can be object references
Array of Object
The elements of an array can be object references
Declaration:
ArrayList<Object> arrayRefVar;
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10. Declaring and Creating in one step:
Basic Operations
Creation:
arrayRefVar = new ArrayList<Object>();
Declaring and Creating in one step:
ArrayList<Object> myList = new ArrayList<Object>();
Fall CS2302: Programming Principles

11. Class definition
public class PartialArrayOfObject { /*Where to store the data.*/ private ArrayList<Object> data; /* Constructor: Initialize the array to the given size. This will be the maximum number that can be held.*/ public PartialArrayOfObject(int size) { data = new ArrayList<Object>(); numStored = 0; } /*Insert the string val into the array so that it ends up with the given*/ public void insertAt(Object val, int index) { if(index < 0 || index > numStored) { System.out.println(“Insert index out of bounds”); } else if(numStored >= data.length) {// no more room System.out.println("Partial array is full"); }else { for(int j = numStored; j > index; j--) { data[j] = data[j-1]; data.add(index,val); // put the new value in place
Fall CS2302: Programming Principles

12. Create an integer array
public class Test {
public static void main(String[] args) {
/*create an object array with size 5*/
PartialArrayOfObject intArr = new PartialArrayOfObject();
/*Fill the array and display each element*/
for(int i=0; i<5;i++){
intArr.add(new Integer(i));
System.out.println(“Element " + i + “ is: ” + intArr.get(i)); }
/*Calculate total*/
int total = 0;
total += (Integer)intArr.get(i);
System.out.println("Total is " + total);
Fall CS2302: Programming Principles