Java Object [] sorting

Question

I am trying to sort an array of created objects each time an object is added. I wrote a compareTo method, and it is printing out each line, but throws an exception when I try to sort it. I am initializing an Element[] of 99 elements(maxLen), then using topIndex as a counter to find the "real" length. A scanner is used to get user input to create the Element. EDIT-- I added the full code.

import java.util.Arrays;
import java.util.Scanner;



public class mainmenu {
static Element[] data = new Element[100];
static int maxLen= 99; 
static int topIndex= -1;
@SuppressWarnings("rawtypes")


public static void main(String[] args){


    menu();


}


public static void menu(){
    boolean leave = true;
    Scanner in = new Scanner(System.in);
    while(leave){
        //loop. unless value is 1-7, keeps recycling    
        System.out.println("Please select a menu option: ");
        System.out.println("'1'- enque ");
        System.out.println("'2'- deque");
        System.out.println("'3'- peek");
        System.out.println("'4'- display");
        System.out.println("'5' empty queue");
        System.out.println("'6'- check if the queue is empty");
        System.out.println("'7' to exit the program");
        String select = in.next();
        if ((select.equals("1") || select.equals("2")|| select.equals("3")|| select.equals("4")|| 
                select.equals("5")|| select.equals("6")|| select.equals("7")))
            leave = callMethods(select );
        else{
            System.out.println("Please enter a valid menu option.");
        }
    }
}

public static  boolean callMethods(String select )
{
    boolean leave= true;
    int sel = Integer.parseInt(select);

    switch(sel){
    case 1:

        enqueue();

        break;
    case 2:
        dequeue();
        break;
    case 3:
        peek();

        break;
    case 4:
        display();

        break;
    case 5:
        empty();

        break;
    case 6:
        if( isEmpty()){
            System.out.println("The structure is empty");
        }
        else{
            System.out.println("The structure is not empty.");
        }
        break;
    case 7:
        System.out.println("Bye!");
        leave = false;
    }
    return leave;
}

public static void enqueue(){
    boolean isInt = false;
    String st = null;
    int index = 0;
    Scanner in = new Scanner(System.in);

    while(!isInt){  //loop continues until input is an integer type
        System.out.println("Please enter a priority level for this element");
        st = in.next();
        if (isInteger(st) == true){// calls method to check if value is integer
            index = Integer.parseInt(st);//parses the string into a integer
            isInt = true;
        }
        else //if value isnt integer, try again
            System.out.println("Invalid Input.");
    }
    System.out.println("Please enter the string of information.");
    String info = in.next();

    Element e = new Element(info, index);

    if (topIndex == maxLen){
        System.out.println("Data Structure is full.");
    }
    else if (isEmpty()){
        data[0] = e;
        topIndex++;
    }
    else {
        topIndex++;
        data[topIndex]=e;
        System.out.println("Added "+ e.getPriority() + " at "+ e.getInfo());
        System.out.println(topIndex);

        Arrays.sort(data);

    }

}

private static boolean isInteger(String s) {
    //checks to see if string can be parsed as an integer.


    try{
        Integer.parseInt(s);
        return true;
    }
    catch( Exception e ){
        return false;
    }


}
public static void dequeue(){
    if(isEmpty()){
        System.out.println("The structure is empty.");
    }
    else{
        Element e = data[topIndex];
        System.out.println("Removing Element: " + e.getInfo()+ " Priority Level: " + e.getPriority());
        --topIndex;
    }
}

public static void peek(){
    if (isEmpty()){
        System.out.println("The structure is empty.");

    }
    else {
        Element e = (data[0]);
        System.out.println("Element: " +  e.getInfo()+ " Priority Level: " + e.getPriority());
    }
}

public static void display(){
    System.out.println("topIndex " + topIndex);
    if (topIndex==-1){
        System.out.println("The structure is empty.");

    }
    else {
        for(int i = 0; i <= topIndex; i++){
            System.out.println("Index: " +i);
            Element e = data[i];
            System.out.println("Element: " + e.getInfo()+ " Priority Level: " + e.getPriority());

        }
    }
}

public static void empty(){
    System.out.println("Erasing data.");
    topIndex=-1;
}

public static boolean isEmpty(){
    return (topIndex==-1);
}

}

And the Element class:

public class Element implements Comparable<Element>  {

private String info;
private int index;

public Element ( String st, int ind) {
    super();
    this.info = st;
    this.index= ind;

}


public String getInfo() {
    return info;
}
public void setInfo(String st) {
    this.info = st;
}

public int getPriority(){
    return index;
}
public void setPriority(int pr){
this.index = pr;
}


public int compareTo(Element e) {
    System.out.println(e.index+ ""+ e.info);
//  if (!(e instanceof Element))
//      throw new ClassCastException("An Element object expected.");
    int ePriority = e.getPriority();  
    System.out.println(ePriority);
    System.out.println(this.index);
    int balls= this.index - ePriority;    
    System.out.println(balls);
    return balls;
}

}

Answer 1

Arrays.sort requires all of the array elements to be non-null. You want to sort only the non-null part, so replace Array.sort(data) with Arrays.sort(data, 0, topIndex + 1).

Arrays.sort(Object[], int, int)

Do not modify compareTo to allow a null argument as others have suggested, because the contract of Comparable dictates that your implementation should throw NullPointerException.

Comparable.compareTo(T)

Answer 2

You have given Element[] array data to Array.sort() method which has size 100, but all of the element of data array are not initialized. Hence a call such as e.getPriority(); will result in NullPointerException as e is null. Initialize all of the element of data array first.

for(int i=0 ; i< data.length; i++)
   e = new Element(info, index); // replace with relevant info and index of your contest

Use Arrays.sort(Object[] array, int fromIndexInclusive, int toIndexExclusive) to sort parts of an array if needed:

Arrays.sort(data, 0, topIndex+1);

Answer 3

From your error description I would guess the Element object is getting null.

As per your comment

It will throw the Element object expected exception I wrote in. If I comment out the throw declaration in the compareTo method, I get a null pointer exception.

if (!(e instanceof Element))
    throw new ClassCastException("An Element object expected.");

The above statement will throw the exception only the object e is NULL

Answer 4

When doing a comparison, you're deciding if one object outranks another by some arbitrary condition.

I notice that you don't check if the object you're comparing against is null - you should do that.

public int compareTo(Element e) {
    if(e == null) {
        return this.index;
    } else {
        // rest of your logic goes here
    }
}