How to add an element at the end of an array?

问题

I want to know how to add or append a new element to the end of an array. Is any simple way to add the element at the end? I know how to use a StringBuffer but I don't know how to use it to add an element in an array. I prefer it without an ArrayList or list. I wonder if the StringBuffer will work on integers.

回答1:

You can not add an element to an array, since arrays, in Java, are fixed-length. However, you could build a new array from the existing one using Arrays.copyOf(array, size) :

public static void main(String[] args) {
    int[] array = new int[] {1, 2, 3};
    System.out.println(Arrays.toString(array));
    array = Arrays.copyOf(array, array.length + 1); //create new array from old array and allocate one more element
    array[array.length - 1] = 4;
    System.out.println(Arrays.toString(array));
}

I would still recommend to drop working with an array and use a List.

回答2:

Arrays in Java have a fixed length that cannot be changed. So Java provides classes that allow you to maintain lists of variable length.

Generally, there is the List<T> interface, which represents a list of instances of the class T. The easiest and most widely used implementation is the ArrayList. Here is an example:

List<String> words = new ArrayList<String>();
words.add("Hello");
words.add("World");
words.add("!");

List.add() simply appends an element to the list and you can get the size of a list using List.size().

回答3:

The OP says, for unknown reasons, "I prefer it without an arraylist or list."

If the type you are referring to is a primitive (you mention integers, but you don't say if you mean int or Integer), then you can use one of the NIO Buffer classes like java.nio.IntBuffer. These act a lot like StringBuffer does - they act as buffers for a list of the primitive type (buffers exist for all the primitives but not for Objects), and you can wrap a buffer around an array and/or extract an array from a buffer.

Note that the javadocs say, "The capacity of a buffer is never negative and never changes." It's still just a wrapper around an array, but one that's nicer to work with. The only way to effectively expand a buffer is to allocate() a larger one and use put() to dump the old buffer into the new one.

If it's not a primitive, you should probably just use List, or come up with a compelling reason why you can't or won't, and maybe somebody will help you work around it.

回答4:

As many others pointed out if you are trying to add a new element at the end of list then something like, array[array.length-1]=x; should do. But this will replace the existing element.

For something like continuous addition to the array. You can keep track of the index and go on adding elements till you reach end and have the function that does the addition return you the next index, which in turn will tell you how many more elements can fit in the array.

Of course in both the cases the size of array will be predefined. Vector can be your other option since you do not want arraylist, which will allow you all the same features and functions and additionally will take care of incrementing the size.

Coming to the part where you want StringBuffer to array. I believe what you are looking for is the getChars(int srcBegin, int srcEnd,char[] dst,int dstBegin) method. Look into it that might solve your doubts. Again I would like to point out that after managing to get an array out of it, you can still only replace the last existing element(character in this case).

回答5:

To clarify the terminology right: arrays are fixed length structures (and the length of an existing cannot be altered) the expression add at the end is meaningless (by itself).

What you can do is create a new array one element larger and fill in the new element in the last slot:

public static int[] append(int[] array, int value) {
     int[] result = Arrays.copyOf(array, array.length + 1);
     result[result.length - 1] = value;
     return result;
}

This quickly gets inefficient, as each time append is called a new array is created and the old array contents is copied over.

One way to drastically reduce the overhead is to create a larger array and keep track of up to which index it is actually filled. Adding an element becomes as simple a filling the next index and incrementing the index. If the array fills up completely, a new array is created with more free space.

And guess what ArrayList does: exactly that. So when a dynamically sized array is needed, ArrayList is a good choice. Don't reinvent the wheel.

来源：https://stackoverflow.com/questions/28549489/how-to-add-an-element-at-the-end-of-an-array

标签

java

arrays

append

stringbuffer