Cannot assign string to null within a string array

Question

Good day,

I have to create a dynamic array class which will represent a string array with dynamic size. I have created the add method and now I have some difficulties with the removeElement method.

Here is my code:

public class DynamicArray {

public String[] dynamicString = new String[0];

public void addElement(String input) {
    int loc = findFreeSpace();

    if (loc >= 0) {
        dynamicString[loc] = input;
    }
    else {
        dynamicString = Arrays.copyOf(dynamicString, dynamicString.length + 1);
        dynamicString[dynamicString.length - 1] = input;
    }

}

public void removeElement(String input){
    for(String eachElement : dynamicString){
        if(eachElement.equalsIgnoreCase(input)) ; eachElement = null;
    }
}

private int findFreeSpace() {
    for (int i = 0; i < dynamicString.length; i++) {
        if (dynamicString[i] == null) {
            return i;
        }
    }
    return -1;
}

public static void main(String[] args){
    DynamicArray array = new DynamicArray();

    array.addElement("a");
    array.addElement("b");
    array.addElement("c");
    array.addElement("d");

    array.removeElement("a");
    System.out.println(array.dynamicString[0]);
}

In the main method 4 String elements are added to the array object. I am using the removeElement method afterwards to remove a particular string element from that object's dynamicString. However, this method fails to work and the system.out.println prints "a" (instead of null) in the console.

Is there any particular reason you're doing this instead of using an ArrayList? — matt
– matt, Commented Apr 13, 2014 at 10:18
You made a typo. Remove the ; before eachElement = null; in if(eachElement.equalsIgnoreCase(input)) ; eachElement = null; (note that this by itself doesn't solve your problem, look at @JBNizet's answer) — Erwin Bolwidt
– Erwin Bolwidt, Commented Apr 13, 2014 at 10:18
@Human I guess that the idea is to learn about arrays. I'm a bit partial against lessons that force the student to use a method that should never be used in real life though. You learn to use an array but now you have to unlearn that an array can be used as a list. Hopefully the next lesson... — Maarten Bodewes
– Maarten Bodewes, Commented Apr 13, 2014 at 10:26

JB Nizet · Accepted Answer · 2014-04-13 10:19:23Z

2

eachElement is a copy of the reference in the array, and you're assigning a new value (null) to this copy. So the original reference is left unmodified. You need a traditional loop, which modifies dynamicString[i]:

for (int i = 0; i < dynamicString.length; i++){
    if (dynamicString[i].equalsIgnoreCase(input)) {
        dynamicString[i] = null;
    }
}

answered Apr 13, 2014 at 10:19

JB Nizet

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Comments

score 1 · Accepted Answer · 2014-04-13 10:25:03Z

1

You should remove the ";" after the if.

Also, as said in the comments you are modifying only the local variable. Use a for loop instead of a foreach loop to iterate through the index of your array and then once you've got the index of the element you want to change you can update your array

edited Apr 13, 2014 at 10:25

answered Apr 13, 2014 at 10:19

user2137101

1 Comment

Erwin Bolwidt Over a year ago

That statement is correct but it doesn't solve the problem as he's still not modifying the array - he's only modifying a local variable eachElement.

Tassos Bassoukos · Accepted Answer · 2014-04-13 10:19:26Z

0

Your code is equivalent to this:

for(Iterator<String> it = dynamicString.iterator; it.hasNext(); ){
    String eachElement = it.next();
    if(eachElement.equalsIgnoreCase(input)) ;
    eachElement = null;
}

Can you see why it won't work?

answered Apr 13, 2014 at 10:19

Tassos Bassoukos

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Comments

Puru-- · Accepted Answer · 2014-04-13 10:25:40Z

0

Multiple problems

1.';' after if means eachElement = null gets executed for every iteration

2.eachElement = null will not have no affect on array it is a seperate reference, you should use old style for loop in this case and access array element using index.

public void removeElement(String input){
    for(String eachElement : dynamicString){
        if(eachElement.equalsIgnoreCase(input)) ; eachElement = null;
    }
}

Other best option is to use ArrayList.

answered Apr 13, 2014 at 10:25

Puru--

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1 Comment

Bravo Over a year ago

thanks mate, I know ArrayList is better but this is a mini-assignment in which I have to create a dynamicArray

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