Shorten array length once element is remove in Java_问答_开发者

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; 开发者_如何学JAVAi++) {
               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

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.

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"

All that setElements[i] = ""; does is change the value of an element in the array. It does not actually remove anything from the array. If you were using a collection class and called remove(i) on it, then you would actually be removing that element from the collection. But here, you're just changing its value. However, arrays in Java have a fixed size and cannot be resized, so there is no way to remove elements from them. The solution, therefore, is to create a new array with a length one shorter than the old one, and then copy all of the values that you want to keep into the new one. So,

Create new array with a length of setElements.length - 1.
Copy all of the elements in setElements into the new array, except for the one which you're looking to remove. Be careful of the fact that the indices into the two arrays will be off by one rather than equal once you've reached the index for the element that you wish to remove.
Set setElements to the new array if you want to keep using the same variable for your array.

void remove(String newValue) {
    if(setElements.length == 0) return;
    String [] array = new String[setElements.length-1];
    int j = 0;
    for ( int i = 0; i < setElements.length; i++) {
       if ( setElements[i] != newValue ) {
            array[j++] = setElements[i];
       }
    }
    setElements = array;
}