Shorten array length once element is remove in Java

Question

Note: Following is my homework/assignment, feel free not to answer if you will.

I want to delete/remove an element from an String array(Set) basic, I'm not allowed to use Collections..etc.

Now I have this:

void remove(String newValue) {

            for ( int i = 0; i < setElements.length; i++) {
               if ( setElements[i] == newValue ) {
                    setElements[i] = "";

               }
            }

       }   

I does what I want as it remove the element from an array but it doesn't shorten the length. The following is the output, basically it remove the element indexed #1.

D:\javaprojects>java SetsDemo
Enter string element to be added
A
You entered A
Set size is: 5
Member elements on index: 0 A
Member elements on index: 1 b
Member elements on index: 2 hello
Member elements on index: 3 world
Member elements on index: 4 six
Set size is: 5
Member elements on index: 0 A
Member elements on index: 1
Member elements on index: 2 hello
Member elements on index: 3 world
Member elements on index: 4 six

Answer 1

You can't change the length of an array object once it's created. Here's an excerpt from JLS 10.2. Array Variables:

Once an array object is created, its length never changes. To make an array variable refer to an array of different length, a reference to a different array must be assigned to the variable.

This means that for this problem, you'd have to allocate a new array that's one-element shorter than the original array, and copy over the remaining elements.

If you need to remove element at index k, and the original array has L elements, then you need to copy over elements (upper bounds are exclusive):

• From [0,k) to [0,k) (k elements)
• From [k+1,L) to [k,L-1) (L-k-1 elements).
• For a total of L-1 elements copied

---

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;
}
static void print(String[] arr) {
    System.out.println(Arrays.toString(arr));       
}   
public static void main(String[] args) {
    String[] arr = { "a", "b", "c", "d", "e" };
    print(arr); // prints "[a, b, c, d, e]"

    arr = removeAt(0, arr);
    print(arr); // prints "[b, c, d, e]"

    arr = removeAt(3, arr);
    print(arr); // prints "[b, c, d]"

    arr = removeAt(1, arr);
    print(arr); // prints "[b, d]"

    arr = removeAt(0, arr);
    arr = removeAt(0, arr);
    print(arr); // prints "[]"
}

This uses System.arraycopy; you can always write your own if this isn't allowed.

static void arraycopy(String[] src, int from, String[] dst, int to, int L) {
    for (int i = 0; i < L; i++) {
        dst[to + i] = src[from + i];
    }
}

This is a simplistic implementation that doesn't handle src == dst, but it's sufficient in this case.

See also

• In java to remove an element in an array can you set it to null?
  • Answer: NO!

---

Note on == for String comparison

Most of the time, using == to compare String objects is a mistake. You should use equals instead.

String ha1 = new String("ha");
String ha2 = new String("ha");
System.out.println(ha1 == ha2); // prints "false"
System.out.println(ha1.equals(ha2)); // prints "true"

See also

• Java String.equals versus ==
• Difference Between Equals and ==
• why equals() method when we have == operator?