在Java中删除元素后缩短数组长度 [英] Shorten array length once element is remove in Java

问题描述

注意：
以下是我的作业/作业，如果你愿意，请随时不回答。

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

我想删除/删除元素字符串数组（设置）基本，我不允许使用Collections..etc。

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

现在我有：

void remove(String newValue) {

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

               }
            }

       }   

我做我想要的，因为它从数组中删除元素，但它没有缩短长度。以下是输出，基本上它删除了索引＃1的元素。

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

推荐答案

你不能改变长度一个数组对象一旦创建。以下是 JLS 10.2的摘录。数组变量：

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

创建数组对象后，其长度永远不会改变。要使数组变量引用不同长度的数组，必须为变量分配对不同数组的引用。

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.

这意味着对于这个问题，你必须分配一个比原始数组短的一个元素的新数组，并复制剩余的元素。

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.

如果你需要删除索引 k 的元素，原始数组有 L 元素，然后你需要复制元素（上层）边界是独家的）：

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):

• 从 [0，k）到 [0，k）（ k 元素）


• 来自 [k + 1，L）至 [k，L-1）（ Lk-1 元素）。


• 总计 L-1 复制的元素

• 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 "[]"
}

这使用 System.arraycopy ;如果不允许，你总是可以写自己的。

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];
    }
}

这是一个简单的实现，无法处理 src == dst ，但在这种情况下就够了。

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

• 在java中删除数组中的元素可以将它设置为null吗？
  
  
  
  
  • 答案：NO！
  
  

  大多数时间，使用 == 比较字符串对象是错误的。您应该使用等于。

  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"
  

  
  
  

  参见

  
  
  
  
  
  • Java String.equals vs. ==
  
  
  • 等于和==之间的差异
  
  
  • 为什么我们有==运算符时的equals（）方法？
  
  

  See also

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

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