How do I remove objects from an array in Java?

Question

Given an array of n Objects, let's say it is an array of strings, and it has the following values:

foo[0] = "a";
foo[1] = "cc";
foo[2] = "a";
foo[3] = "dd";

What do I have to do to delete/remove all the strings/objects equal to "a" in the array?

Answer 1

My little contribution to this problem.

public class DeleteElementFromArray {
public static String foo[] = {"a","cc","a","dd"};
public static String search = "a";


public static void main(String[] args) {
    long stop = 0;
    long time = 0;
    long start = 0;
    System.out.println("Searched value in Array is: "+search);
    System.out.println("foo length before is: "+foo.length);
    for(int i=0;i<foo.length;i++){ System.out.println("foo["+i+"] = "+foo[i]);}
    System.out.println("==============================================================");
    start = System.nanoTime();
    foo = removeElementfromArray(search, foo);
    stop = System.nanoTime();
    time = stop - start;
    System.out.println("Equal search took in nano seconds = "+time);
    System.out.println("==========================================================");
    for(int i=0;i<foo.length;i++){ System.out.println("foo["+i+"] = "+foo[i]);}
}
public static String[] removeElementfromArray( String toSearchfor, String arr[] ){
     int i = 0;
     int t = 0;
     String tmp1[] = new String[arr.length];     
         for(;i<arr.length;i++){
              if(arr[i] == toSearchfor){     
              i++;
              }
             tmp1[t] = arr[i];
             t++;
     }   
     String tmp2[] = new String[arr.length-t];   
     System.arraycopy(tmp1, 0, tmp2, 0, tmp2.length);
     arr = tmp2; tmp1 = null; tmp2 = null;
    return arr;
}

}

Answer 2

class sd 
{
 public static void main(String[ ] args)
 {
     System.out.println("Search and Delete");

    int key;
    System.out.println("Enter the length of array:");
    Scanner in=new Scanner(System.in);
    int n=in.nextInt();
    int numbers[]=new int[n];

      int i = 0;
      boolean found = false;  
      System.out.println("Enter the elements in Array :");
      for ( i = 0; i < numbers.length; i++)
      {
          numbers[i]=in.nextInt();
      }
      System.out.println("The elements in Array are:");
      for ( i = 0; i < numbers.length; i++)
      {
          System.out.println(numbers[i]);
      }
      System.out.println("Enter the element to be searched:");
      key=in.nextInt();
      for ( i = 0; i < numbers.length; i++)
      {
             if (numbers[ i ]  == key)
            {
                     found = true;      
                     break;
             }
       }
      if (found)   
      {
            System.out.println("Found " + key + " at index " + i + ".");
            numbers[i]=0;//haven't deleted the element in array
            System.out.println("After Deletion:");
        for ( i = 0; i < numbers.length; i++)
          {
              if (numbers[ i ]!=0)
            {   //it skips displaying element in array
                        System.out.println(numbers[i]);
            }
          }
      }
      else
      {
            System.out.println(key + "is not in this array.");
      }
  }
}//Sorry.. if there are mistakes.

Answer 3

You can use external library:

org.apache.commons.lang.ArrayUtils.remove(java.lang.Object[] array, int index)

It is in project Apache Commons Lang http://commons.apache.org/lang/

Answer 4

Make a List out of the array with Arrays.asList(), and call remove() on all the appropriate elements. Then call toArray() on the 'List' to make back into an array again.

Not terribly performant, but if you encapsulate it properly, you can always do something quicker later on.

Answer 5

If you need to remove multiple elements from array without converting it to List nor creating additional array, you may do it in O(n) not dependent on count of items to remove.

Here, a is initial array, int... r are distinct ordered indices (positions) of elements to remove:

public int removeItems(Object[] a, int... r) {
    int shift = 0;                             
    for (int i = 0; i < a.length; i++) {       
        if (shift < r.length && i == r[shift])  // i-th item needs to be removed
            shift++;                            // increment `shift`
        else 
            a[i - shift] = a[i];                // move i-th item `shift` positions left
    }
    for (int i = a.length - shift; i < a.length; i++)
        a[i] = null;                            // replace remaining items by nulls

    return a.length - shift;                    // return new "length"
}  

Small testing:

String[] a = {"0", "1", "2", "3", "4"};
removeItems(a, 0, 3, 4);                     // remove 0-th, 3-rd and 4-th items
System.out.println(Arrays.asList(a));        // [1, 2, null, null, null]

In your task, you can first scan array to collect positions of "a", then call removeItems().

Answer 6

You can always do:

int i, j;
for (i = j = 0; j < foo.length; ++j)
  if (!"a".equals(foo[j])) foo[i++] = foo[j];
foo = Arrays.copyOf(foo, i);