Java 如何向数组添加新元素？

How to add new elements to an array?

提问by Pentium10

I have the following code:

我有以下代码：

String[] where;
where.append(ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1");
where.append(ContactsContract.Contacts.IN_VISIBLE_GROUP + "=1");

Those two appends are not compiling. How would that work correctly?

这两个附加没有编译。这将如何正确工作？

采纳答案by tangens

The size of an array can't be modified. If you want a bigger array you have to instantiate a new one.

无法修改数组的大小。如果你想要一个更大的数组，你必须实例化一个新的。

A better solution would be to use an ArrayListwhich can grow as you need it. The method ArrayList.toArray( T[] a )gives you back your array if you need it in this form.

更好的解决方案是使用ArrayList可以根据需要增长的 。ArrayList.toArray( T[] a )如果您需要这种形式的数组，该方法会返回给您。

List<String> where = new ArrayList<String>();
where.add( ContactsContract.Contacts.HAS_PHONE_NUMBER+"=1" );
where.add( ContactsContract.Contacts.IN_VISIBLE_GROUP+"=1" );

If you need to convert it to a simple array...

如果您需要将其转换为一个简单的数组...

String[] simpleArray = new String[ where.size() ];
where.toArray( simpleArray );

But most things you do with an array you can do with this ArrayList, too:

但是你用数组做的大多数事情你也可以用这个 ArrayList 做：

// iterate over the array
for( String oneItem : where ) {
    ...
}

// get specific items
where.get( 1 );

回答by polygenelubricants

Use a List<String>, such as an ArrayList<String>. It's dynamically growable, unlike arrays (see: Effective Java 2nd Edition, Item 25: Prefer lists to arrays).

使用List<String>，例如ArrayList<String>。它是动态增长的，与数组不同（参见：Effective Java 2nd Edition，Item 25: Prefer lists to arrays）。

import java.util.*;
//....

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
System.out.println(list); // prints "[1, 2, 3]"

If you insist on using arrays, you can use java.util.Arrays.copyOfto allocate a bigger array to accomodate the additional element. This is really not the best solution, though.

如果你坚持使用数组，你可以使用java.util.Arrays.copyOf分配一个更大的数组来容纳额外的元素。不过，这确实不是最好的解决方案。

static <T> T[] append(T[] arr, T element) {
    final int N = arr.length;
    arr = Arrays.copyOf(arr, N + 1);
    arr[N] = element;
    return arr;
}

String[] arr = { "1", "2", "3" };
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3]"
arr = append(arr, "4");
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3, 4]"

This is O(N)per append. ArrayList, on the other hand, has O(1)amortized cost per operation.

这是O(N)每append. ArrayList，另一方面，已O(1)摊销每次操作的成本。

See also

也可以看看

• Java Tutorials/Arrays
  • An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.
• Java Tutorials/The List interface

• Java 教程/数组
  • 数组是一个容器对象，它包含固定数量的单一类型的值。数组的长度是在创建数组时确定的。创建后，其长度是固定的。
• Java 教程/列表界面

回答by Simon

As tangens said, the size of an array is fixed. But you have to instantiate it first, else it will be only a null reference.

正如 tangens 所说，数组的大小是固定的。但是您必须先实例化它，否则它将只是一个空引用。

String[] where = new String[10];

This array can contain only 10 elements. So you can append a value only 10 times. In your code you're accessing a null reference. That's why it doesnt work. In order to have a dynamically growing collection, use the ArrayList.

此数组只能包含 10 个元素。因此，您只能附加一个值 10 次。在您的代码中，您正在访问一个空引用。这就是为什么它不起作用。为了拥有动态增长的集合，请使用 ArrayList。

回答by npinti

I'm not that experienced in Java but I have always been told that arrays are static structures that have a predefined size. You have to use an ArrayList or a Vector or any other dynamic structure.

我在 Java 方面经验不多，但我一直被告知数组是具有预定义大小的静态结构。您必须使用 ArrayList 或 Vector 或任何其他动态结构。

回答by Paligulus

You need to use a Collection List. You cannot re-dimension an array.

您需要使用集合列表。您不能重新调整数组的尺寸。

回答by Robert

There is no method append()on arrays. Instead as already suggested a List object can service the need for dynamically inserting elements eg.

append()数组没有方法。相反，正如已经建议的那样，List 对象可以满足动态插入元素的需要，例如。

List<String> where = new ArrayList<String>();
where.add(ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1");
where.add(ContactsContract.Contacts.IN_VISIBLE_GROUP + "=1");

Or if you are really keen to use an array:

或者，如果您真的很想使用数组：

String[] where = new String[]{
    ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1",
    ContactsContract.Contacts.IN_VISIBLE_GROUP + "=1"
};

but then this is a fixed size and no elements can be added.

但这是一个固定大小，不能添加任何元素。

回答by RMachnik

If you would like to store your data in simple array like this

如果您想将数据存储在这样的简单数组中

String[] where = new String[10];

and you want to add some elements to it like numbers please us StringBuilder which is much more efficient than concatenating string.

并且您想向其中添加一些元素，例如数字，请使用 StringBuilder，这比连接字符串更有效。

StringBuilder phoneNumber = new StringBuilder();
phoneNumber.append("1");
phoneNumber.append("2");
where[0] = phoneNumber.toString();

This is much better method to build your string and store it into your 'where' array.

这是构建字符串并将其存储到“where”数组中的更好方法。

回答by ratzip

you can create a arraylist, and use Collection.addAll()to convert the string array to your arraylist

您可以创建一个Collection.addAll()数组列表，并用于将字符串数组转换为您的数组列表

回答by Jiao

Size of array cannot be modified. If you have to use an array, you can use:

数组的大小不能修改。如果必须使用数组，可以使用：

System.arraycopy(src, srcpos, dest, destpos, length); 

回答by dforce

String[] source = new String[] { "a", "b", "c", "d" };
String[] destination = new String[source.length + 2];
destination[0] = "/bin/sh";
destination[1] = "-c";
System.arraycopy(source, 0, destination, 2, source.length);

for (String parts : destination) {
  System.out.println(parts);
}