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Arrays of Objects

Fall 2012 CS2302: Programming Principles

Contents

Wrappers: Java’s Wrapper classes for the primitive types

Object class

Arrays of Object

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Primitives & Wrappers

Java has a wrapper class for each of the eight primitive data types:

Primitive Type

Wrapper Class

Primitive Type

Wrapper Class

boolean Boolean float Float

byte Byte int Integer

char Character long Long

double Double short Short

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Primitives vs. Wrappers

int x = 25;Integer y = new Integer(33);

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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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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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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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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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Array of Object

The elements of an array can be object references

Declaration:– Object [ ] arrayRefVar;

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Basic Operations

Creation:– arrayRefVar = new Object[arraySize];

Declaring and Creating in one step:– Object[ ] myList = new Object[10];

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Class definition

public class PartialArrayOfObject { /*Where to store the data.*/ private Object[] data; /*How many are actually stored.*/ private int numStored; /* Constructor: Initialize the array to the given size. This will be the maximum number that can be held.*/ public PartialArrayOfObject(int size) { data = new Object[size]; numStored = 0; } /*Get the element at index i.*/ public Object get(int i) { if(0 <= i && i < numStored) {

return data[i]; } else { System.out.println(“Index is out of range.”); } } /*Add an element to the end of the array.*/ public void add(Object val) { ………… }}

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Class definition

/*Add an element to the end of the array.*/ public void add(Object val) {

if(numStored < data.length) {data[numStored] = val;numStored++;

} else {// no more roomSystem.out.println("Partial array is full");

}}/*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 roomSystem.out.println("Partial array is full");

}else {for(int j = numStored; j > index; j--) {

data[j] = data[j-1];}data[index] = val; // put the new value in placenumStored++; // one more element stored}

}}

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Create an integer array

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public class Test {

public static void main(String[] args) { /*create an object array with size 5*/ PartialArrayOfObject intArr = new PartialArrayOfObject(5); /*Fill the array and display each element*/ for(int i=0; i<5;i++){

intArr.add(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); }}