问题
What is the clearest way to comma-delimit a list in Java?
I know several ways of doing it, but I\'m wondering what the best way is (where \"best\" means clearest and/or shortest, not the most efficient.
I have a list and I want to loop over it, printing each value. I want to print a comma between each item, but not after the last one (nor before the first one).
List --> Item ( , Item ) *
List --> ( Item , ) * Item
Sample solution 1:
boolean isFirst = true;
for (Item i : list) {
if (isFirst) {
System.out.print(i); // no comma
isFirst = false;
} else {
System.out.print(\", \"+i); // comma
}
}
Sample solution 2 - create a sublist:
if (list.size()>0) {
System.out.print(list.get(0)); // no comma
List theRest = list.subList(1, list.size());
for (Item i : theRest) {
System.out.print(\", \"+i); // comma
}
}
Sample solution 3:
Iterator<Item> i = list.iterator();
if (i.hasNext()) {
System.out.print(i.next());
while (i.hasNext())
System.out.print(\", \"+i.next());
}
These treat the first item specially; one could instead treat the last one specially.
Incidentally, here is how List toString is implemented (it\'s inherited from AbstractCollection), in Java 1.6:
public String toString() {
Iterator<E> i = iterator();
if (! i.hasNext())
return \"[]\";
StringBuilder sb = new StringBuilder();
sb.append(\'[\');
for (;;) {
E e = i.next();
sb.append(e == this ? \"(this Collection)\" : e);
if (! i.hasNext())
return sb.append(\']\').toString();
sb.append(\", \");
}
}
It exits the loop early to avoid the comma after the last item. BTW: this is the first time I recall seeing \"(this Collection)\"; here\'s code to provoke it:
List l = new LinkedList();
l.add(l);
System.out.println(l);
I welcome any solution, even if they use unexpected libraries (regexp?); and also solutions in languages other than Java (e.g. I think Python/Ruby have an intersperse function - how is that implemented?).
Clarification: by libraries, I mean the standard Java libraries. For other libraries, I consider them with other languages, and interested to know how they\'re implemented.
EDIT toolkit mentioned a similar question: Last iteration of enhanced for loop in java
And another: Does the last element in a loop deserve a separate treatment?
回答1:
Java 8 and later
Using StringJoiner class :
StringJoiner joiner = new StringJoiner(",");
for (Item item : list) {
joiner.add(item.toString());
}
return joiner.toString();
Using Stream, and Collectors:
return list.stream().
map(Object::toString).
collect(Collectors.joining(",")).toString();
Java 7 and earlier
See also #285523
String delim = "";
for (Item i : list) {
sb.append(delim).append(i);
delim = ",";
}
回答2:
org.apache.commons.lang3.StringUtils.join(list,",");
回答3:
Java 8 provides several new ways to do this:
- The join method on
String. - A joining collector for streams from the
Collectorsclass. - The StringJoiner class.
Example:
// Util method for strings and other char sequences
List<String> strs = Arrays.asList("1", "2", "3");
String listStr1 = String.join(",", strs);
// For any type using streams and collectors
List<Object> objs = Arrays.asList(1, 2, 3);
String listStr2 = objs.stream()
.map(Object::toString)
.collect(joining(",", "[", "]"));
// Using the new StringJoiner class
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.setEmptyValue("<empty>");
for (Integer i : objs) {
joiner.add(i.toString());
}
String listStr3 = joiner.toString();
The approach using streams assumes import static java.util.stream.Collectors.joining;.
回答4:
Joiner.on(",").join(myList)
Joiner.
回答5:
If you use the Spring Framework you can do it with StringUtils:
public static String arrayToDelimitedString(Object[] arr)
public static String arrayToDelimitedString(Object[] arr, String delim)
public static String collectionToCommaDelimitedString(Collection coll)
public static String collectionToCommaDelimitedString(Collection coll, String delim)
回答6:
Based on Java's List toString implementation:
Iterator i = list.iterator();
for (;;) {
sb.append(i.next());
if (! i.hasNext()) break;
ab.append(", ");
}
It uses a grammar like this:
List --> (Item , )* Item
By being last-based instead of first-based, it can check for skip-comma with the same test to check for end-of-list. I think this one is very elegant, but I'm not sure about clarity.
回答7:
String delimiter = ",";
StringBuilder sb = new StringBuilder();
for (Item i : list) {
sb.append(delimiter).append(i);
}
sb.toString().replaceFirst(delimiter, "");
回答8:
There is a pretty way to achieve this using Java 8:
List<String> list = Arrays.asList(array);
String joinedString = String.join(",", list);
回答9:
for(int i=0, length=list.size(); i<length; i++)
result+=(i==0?"":", ") + list.get(i);
回答10:
One option for the foreach loop is:
StringBuilder sb = new StringBuilder();
for(String s:list){
if (sb.length()>0) sb.append(",");
sb.append(s);
}
回答11:
StringBuffer result = new StringBuffer();
for(Iterator it=list.iterator; it.hasNext(); ) {
if (result.length()>0)
result.append(", ");
result.append(it.next());
}
Update: As Dave Webb mentioned in the comments this may not produce correct results if the first items in the list are empty strings.
回答12:
I usually do this :
StringBuffer sb = new StringBuffer();
Iterator it = myList.iterator();
if (it.hasNext()) { sb.append(it.next().toString()); }
while (it.hasNext()) { sb.append(",").append(it.next().toString()); }
Though I think I'll to a this check from now on as per the Java implementation ;)
回答13:
If you can use Groovy (which runs on the JVM):
def list = ['a', 'b', 'c', 'd']
println list.join(',')
回答14:
(Copy paste of my own answer from here.) Many of the solutions described here are a bit over the top, IMHO, especially those that rely on external libraries. There is a nice clean, clear idiom for achieving a comma separated list that I have always used. It relies on the conditional (?) operator:
Edit: Original solution correct, but non-optimal according to comments. Trying a second time:
int[] array = {1, 2, 3};
StringBuilder builder = new StringBuilder();
for (int i = 0 ; i < array.length; i++)
builder.append(i == 0 ? "" : ",").append(array[i]);
There you go, in 4 lines of code including the declaration of the array and the StringBuilder.
2nd Edit: If you are dealing with an Iterator:
List<Integer> list = Arrays.asList(1, 2, 3);
StringBuilder builder = new StringBuilder();
for (Iterator it = list.iterator(); it.hasNext();)
builder.append(it.next()).append(it.hasNext() ? "," : "");
回答15:
I usually use something similar to version 3. It works well c/c++/bash/... :P
回答16:
I didn't compile it... but should work (or be close to working).
public static <T> String toString(final List<T> list,
final String delim)
{
final StringBuilder builder;
builder = new StringBuilder();
for(final T item : list)
{
builder.append(item);
builder.append(delim);
}
// kill the last delim
builder.setLength(builder.length() - delim.length());
return (builder.toString());
}
回答17:
This is very short, very clear, but gives my sensibilities the creeping horrors. It's also a bit awkward to adapt to different delimiters, especially if a String (not char).
for (Item i : list)
sb.append(',').append(i);
if (sb.charAt(0)==',') sb.deleteCharAt(0);
Inspired by: Last iteration of enhanced for loop in java
回答18:
StringBuffer sb = new StringBuffer();
for (int i = 0; i < myList.size(); i++)
{
if (i > 0)
{
sb.append(", ");
}
sb.append(myList.get(i));
}
回答19:
I somewhat like this approach, which I found on a blog some time ago. Unfortunately I don't remember the blog's name/URL.
You can create a utility/helper class that looks like this:
private class Delimiter
{
private final String delimiter;
private boolean first = true;
public Delimiter(String delimiter)
{
this.delimiter = delimiter;
}
@Override
public String toString()
{
if (first) {
first = false;
return "";
}
return delimiter;
}
}
Using the helper class is simple as this:
StringBuilder sb = new StringBuilder();
Delimiter delimiter = new Delimiter(", ");
for (String item : list) {
sb.append(delimiter);
sb.append(item);
}
回答20:
Because your delimiter is ", " you could use any of the following:
public class StringDelim {
public static void removeBrackets(String string) {
System.out.println(string.substring(1, string.length() - 1));
}
public static void main(String... args) {
// Array.toString() example
String [] arr = {"Hi" , "My", "name", "is", "br3nt"};
String string = Arrays.toString(arr);
removeBrackets(string);
// List#toString() example
List<String> list = new ArrayList<String>();
list.add("Hi");
list.add("My");
list.add("name");
list.add("is");
list.add("br3nt");
string = list.toString();
removeBrackets(string);
// Map#values().toString() example
Map<String, String> map = new LinkedHashMap<String, String>();
map.put("1", "Hi");
map.put("2", "My");
map.put("3", "name");
map.put("4", "is");
map.put("5", "br3nt");
System.out.println(map.values().toString());
removeBrackets(string);
// Enum#toString() example
EnumSet<Days> set = EnumSet.allOf(Days.class);
string = set.toString();
removeBrackets(string);
}
public enum Days {
MON("Monday"),
TUE("Tuesday"),
WED("Wednesday"),
THU("Thursday"),
FRI("Friday"),
SAT("Saturday"),
SUN("Sunday");
private final String day;
Days(String day) {this.day = day;}
public String toString() {return this.day;}
}
}
If your delimiter is ANYTHING else then this isn't going to work for you.
回答21:
I like this solution:
String delim = " - ", string = "";
for (String item : myCollection)
string += delim + item;
string = string.substring(delim.length());
I assume it can make use of StringBuilder too.
回答22:
In Python its easy
",".join( yourlist )
In C# there is a static method on the String class
String.Join(",", yourlistofstrings)
Sorry, not sure about Java but thought I'd pipe up as you asked about other languages. I'm sure there would be something similar in Java.
回答23:
You can also unconditionally add the delimiter string, and after the loop remove the extra delimiter at the end. Then an "if list is empty then return this string" at the beginning will allow you to avoid the check at the end (as you cannot remove characters from an empty list)
So the question really is:
"Given a loop and an if, what do you think is the clearest way to have these together?"
回答24:
public static String join (List<String> list, String separator) {
String listToString = "";
if (list == null || list.isEmpty()) {
return listToString;
}
for (String element : list) {
listToString += element + separator;
}
listToString = listToString.substring(0, separator.length());
return listToString;
}
回答25:
if (array.length>0) // edited in response Joachim's comment
sb.append(array[i]);
for (int i=1; i<array.length; i++)
sb.append(",").append(array[i]);
Based on Clearest way to comma-delimit a list (Java)?
Using this idea: Does the last element in a loop deserve a separate treatment?
回答26:
public String toString(List<Item> items)
{
StringBuilder sb = new StringBuilder("[");
for (Item item : items)
{
sb.append(item).append(", ");
}
if (sb.length() >= 2)
{
//looks cleaner in C# sb.Length -= 2;
sb.setLength(sb.length() - 2);
}
sb.append("]");
return sb.toString();
}
回答27:
None of the answers uses recursion so far...
public class Main {
public static String toString(List<String> list, char d) {
int n = list.size();
if(n==0) return "";
return n > 1 ? Main.toString(list.subList(0, n - 1), d) + d
+ list.get(n - 1) : list.get(0);
}
public static void main(String[] args) {
List<String> list = Arrays.asList(new String[]{"1","2","3"});
System.out.println(Main.toString(list, ','));
}
}
回答28:
In my opinion, this is the simplest to read and understand:
StringBuilder sb = new StringBuilder();
for(String string : strings) {
sb.append(string).append(',');
}
sb.setLength(sb.length() - 1);
String result = sb.toString();
回答29:
private String betweenComma(ArrayList<String> strings) {
String united = "";
for (String string : strings) {
united = united + "," + string;
}
return united.replaceFirst(",", "");
}
回答30:
Method
String join(List<Object> collection, String delimiter){
StringBuilder stringBuilder = new StringBuilder();
int size = collection.size();
for (Object value : collection) {
size --;
if(size > 0){
stringBuilder.append(value).append(delimiter);
}
}
return stringBuilder.toString();
}
Usage
Given the array of [1,2,3]
join(myArray, ",") // 1,2,3
来源：https://stackoverflow.com/questions/668952/the-simplest-way-to-comma-delimit-a-list