Java: join array of primitives with separator

问题:

Suppose, I have an array:

int[] arr = new int[] {1, 2, 3, 4, 5, 6, 7};

And I need to join its elements using separator, for example, " - ", so as the result I should get string like this:

"1 - 2 - 3 - 4 - 5 - 6 - 7"

How could I do this?

PS: yes, I know about this and this posts, but its solutions won't work with an array of primitives.

回答1:

Here what I came up with. There are several way to do this and they are depends on the tools you using.

---

Using StringUtils and ArrayUtils from Common Lang:

int[] 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:

int[] 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 achive the result using streams.

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

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

---

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

public 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();
}

Than you can do:

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

回答2:

In Java 8+ you could use an IntStream and a StringJoiner. Something like,

int[] arr = new int[] { 1, 2, 3, 4, 5, 6, 7 };
StringJoiner sj = new StringJoiner(" - ");
IntStream.of(arr).forEach(x -> sj.add(String.valueOf(x)));
System.out.println(sj.toString());

Output is (as requested)

1 - 2 - 3 - 4 - 5 - 6 - 7

回答3:

You can use Guava for joining elements. More examples and docs you can find there. https://github.com/google/guava/wiki/StringsExplained

Joiner.on("-")
      .join(texts);

To be more precise you should firstly wrap your array into a List with Arrays.asList() or Guava's primitive-friendly equivalents.

Joiner.on("-")
      .join(Arrays.asList(1, 2, 3, 4, 5, 6, 7));

Joiner.on("-")
      .join(Ints.asList(arr));

回答4:

I'm sure there's a way to do this in Kotlin/Scala or other JVM languages as well but you could always stick to keeping things simple for a small set of values like you have above:

int i, arrLen = arr.length;
 StringBuilder tmp = new StringBuilder();
 for (i=0; i<arrLen-1; i++)
    tmp.append(arr[i] +" - ");
 tmp.append(arr[arrLen-1]);

 System.out.println( tmp.toString() );

回答5:

I've been looking for a way to join primitives in a Stream without first instantiating strings for each one of them. I've come to this but it still requires boxing them.

LongStream.range(0, 500).boxed().collect(Collector.of(StringBuilder::new, (sb, v) -> {
    if (sb.length() != 0)
        sb.append(',');
    sb.append(v.longValue());
}, (a, b) -> a.length() == 0 ? b : b.length() != 0 ? a.append(',').append(b) : a, StringBuilder::toString));

回答6:

int[] arr = new int[] { 1, 2, 3, 4, 5, 6, 7 };

IntStream.of(arr).mapToObj(i -> String.valueOf(i)).collect(Collectors.joining(",")) ;