Java constructor and modify the object properties at runtime
- by lupin
Note: This is an assignment
Hi,
I have the following class/constructor.
import java.io.*;
class Set {
public int numberOfElements = 0;
public String[] setElements = new String[5];
public int maxNumberOfElements = 5;
// constructor for our Set class
public Set(int numberOfE, int setE, int maxNumberOfE) {
int numberOfElements = numberOfE;
String[] setElements = new String[setE];
int maxNumberOfElements = maxNumberOfE;
}
// Helper method to shorten/remove element of array since we're using basic array instead of ArrayList or HashSet from collection interface :(
static String[] removeAt(int k, String[] arr) {
final int L = arr.length;
String[] ret = new String[L - 1];
System.arraycopy(arr, 0, ret, 0, k);
System.arraycopy(arr, k + 1, ret, k, L - k - 1);
return ret;
}
int findElement(String element) {
int retval = 0;
for ( int i = 0; i < setElements.length; i++) {
if ( setElements[i] != null && setElements[i].equals(element) ) {
return retval = i;
}
retval = -1;
}
return retval;
}
void add(String newValue) {
int elem = findElement(newValue);
if( numberOfElements < maxNumberOfElements && elem == -1 ) {
setElements[numberOfElements] = newValue;
numberOfElements++;
}
}
int getLength() {
if ( setElements != null ) {
return setElements.length;
}
else {
return 0;
}
}
String[] emptySet() {
setElements = new String[0];
return setElements;
}
Boolean isFull() {
Boolean True = new Boolean(true);
Boolean False = new Boolean(false);
if ( setElements.length == maxNumberOfElements ){
return True;
} else { return False; }
}
Boolean isEmpty() {
Boolean True = new Boolean(true);
Boolean False = new Boolean(false);
if ( setElements.length == 0 ) {
return True;
} else { return False; }
}
void remove(String newValue) {
for ( int i = 0; i < setElements.length; i++) {
if ( setElements[i].equals(newValue) ) {
setElements = removeAt(i,setElements);
}
}
}
int isAMember(String element) {
int retval = -1;
for ( int i = 0; i < setElements.length; i++ ) {
if (setElements[i] != null && setElements[i].equals(element)) {
return retval = i;
}
}
return retval;
}
void printSet() {
for ( int i = 0; i < setElements.length; i++) {
System.out.println("Member elements on index: "+ i +" " + setElements[i]);
}
}
String[] getMember() {
String[] tempArray = new String[setElements.length];
for ( int i = 0; i < setElements.length; i++) {
if(setElements[i] != null) {
tempArray[i] = setElements[i];
}
}
return tempArray;
}
Set union(Set x, Set y) {
String[] newXtemparray = new String[x.getLength()];
String[] newYtemparray = new String[y.getLength()];
Set temp = new Set(1,20,20);
newXtemparray = x.getMember();
newYtemparray = x.getMember();
for(int i = 0; i < newXtemparray.length; i++) {
temp.add(newYtemparray[i]);
}
for(int j = 0; j < newYtemparray.length; j++) {
temp.add(newYtemparray[j]);
}
return temp;
}
}
// This is the SetDemo class that will make use of our Set class
class SetDemo {
public static void main(String[] args) {
//get input from keyboard
BufferedReader keyboard;
InputStreamReader reader;
String temp = "";
reader = new InputStreamReader(System.in);
keyboard = new BufferedReader(reader);
try
{
System.out.println("Enter string element to be added" );
temp = keyboard.readLine( );
System.out.println("You entered " + temp );
}
catch (IOException IOerr)
{
System.out.println("There was an error during input");
}
/*
**************************************************************************
* Test cases for our new created Set class.
*
**************************************************************************
*/
Set setA = new Set(1,10,10);
setA.add(temp);
setA.add("b");
setA.add("b");
setA.add("hello");
setA.add("world");
setA.add("six");
setA.add("seven");
setA.add("b");
int size = setA.getLength();
System.out.println("Set size is: " + size );
Boolean isempty = setA.isEmpty();
System.out.println("Set is empty? " + isempty );
int ismember = setA.isAMember("sixb");
System.out.println("Element six is member of setA? " + ismember );
Boolean output = setA.isFull();
System.out.println("Set is full? " + output );
setA.printSet();
int index = setA.findElement("world");
System.out.println("Element b located on index: " + index );
setA.remove("b");
setA.emptySet();
int resize = setA.getLength();
System.out.println("Set size is: " + resize );
setA.printSet();
Set setB = new Set(0,10,10);
Set SetA = setA.union(setB,setA);
SetA.printSet();
}
}
I have two question here, why I when I change the class property declaration to:
class Set {
public int numberOfElements;
public String[] setElements;
public int maxNumberOfElements;
// constructor for our Set class
public Set(int numberOfE, int setE, int maxNumberOfE) {
int numberOfElements = numberOfE;
String[] setElements = new String[setE];
int maxNumberOfElements = maxNumberOfE;
}
I got this error:
\
javaprojects>java SetDemo
Enter string element to be added
a
You entered a
Exception in thread "main" java.lang.NullPointerException
at Set.findElement(Set.java:30)
at Set.add(Set.java:43)
at SetDemo.main(Set.java:169)
Second, on the union method, why the result of SetA.printSet still printing null, isn't it getting back the return value from union method?
Thanks in advance for any explaination.
lupin
