Here's what I came up with. There are several ways to do this and they depend on the tools you are using.

Using StringUtils and ArrayUtils from Commons Lang:

Use with AIint[] arr = new int[] {1, 2, 3, 4, 5, 6, 7};
String result = StringUtils.join(ArrayUtils.toObject(arr), " - ");

You can't just use StringUtils.join(arr, " - "); because StringUtils doesn't have that overloaded version of method. Though, it has method StringUtils.join(int[], char).

Works at any Java version, from 1.2.

Using Java 8 streams:

Something like this:

Use with AIint[] arr = new int[] {1, 2, 3, 4, 5, 6, 7};
String result = Arrays.stream(arr)
        .mapToObj(String::valueOf)
        .collect(Collectors.joining(" - "));

In fact, there are lot of variations to achieve it using streams.

Java 8's method String.join() works only with strings, so to use it you still have to convert int[] to String[].

Use with AIString[] sarr = Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new);
String result = String.join(" - ", sarr);

If you're stuck using Java 7 or earlier with no libraries, you could write your own utility method:

Use with AIpublic static String myJoin(int[] arr, String separator) {
    if (null == arr || 0 == arr.length) return "";

    StringBuilder sb = new StringBuilder(256);
    sb.append(arr[0]);

    //if (arr.length == 1) return sb.toString();

    for (int i = 1; i < arr.length; i++) sb.append(separator).append(arr[i]);

    return sb.toString();
}

Then you can do:

Use with AIint[] arr = new int[] {1, 2, 3, 4, 5, 6, 7};
String result = myJoin(arr, " - ");

Just multiply and add the digits:

int result = 1000 * arr[3] + 100 * arr[2] + 10 * arr[1] + arr[0];

Or, if you need it to work for any length of array (up to the number of digits in Integer.MAX_VALUE):

int result = 0;
for (int i = arr.length - 1; i >= 0; --i) {
  result = 10*result + arr[i];
}

You can't add them directly, you have to make a new array and then copy each of the arrays into the new one. System.arraycopy is a method you can use to perform this copy.

int[] array1and2 = new int[array1.length + array2.length];
System.arraycopy(array1, 0, array1and2, 0, array1.length);
System.arraycopy(array2, 0, array1and2, array1.length, array2.length);

This will work regardless of the size of array1 and array2.

If you wanna do it using reduce convert Integers into Strings and then use Accumulator function

Optional<String> result = numbers.stream().map(i->i.toString()).reduce((i,j)->i+", "+j);

Or you can simply use Collectors.joining

String str = numbers.stream().map(i->i.toString()).collect(Collectors.joining(", "));