Arrays of Objects
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Fall 2012 CS2302: Programming Principles
Contents
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
Wrappers: Java’s Wrapper classes
for the primitive types

Object class

Arrays of Object
Fall 2012 CS2302: Programming Principles
Primitives & Wrappers

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Java has a wrapper class for each of the
eight primitive data types:
Primitive
Type
Wrapper
Class
Primitive
Type
Wrapper
Class
boolean
byte
char
Boolean
Byte
Character
float
int
long
Float
Integer
Long
double
Double
short
Short
Fall 2012 CS2302: Programming Principles
Primitives vs. Wrappers
int x = 25;
Integer y = new Integer(33);
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Fall 2012 CS2302: Programming Principles
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));
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Fall 2012 CS2302: Programming Principles
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”);
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Fall 2012 CS2302: Programming Principles
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
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Fall 2012 CS2302: Programming Principles
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());
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Fall 2012 CS2302: Programming Principles
Array of Object

The elements of an array can be object
references

Declaration:
–
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ArrayList<Object> arrayRefVar;
Fall 2012 CS2302: Programming Principles
Basic Operations

Creation:
–

Declaring and Creating in one step:
–
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arrayRefVar = new ArrayList<Object>();
ArrayList<Object> myList = new
ArrayList<Object>();
Fall 2012 CS2302: Programming Principles
Class definition
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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 2012 CS2302: Programming Principles
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;
for(int i=0; i<5;i++){
total += (Integer)intArr.get(i);
}
System.out.println("Total is " + total);
}
}
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Fall 2012 CS2302: Programming Principles