While working in a Java app, I recently needed to assemble a comma-delimited list of values to pass to another web service without knowing how many elements there would be in advance. The best I could come up with off the top of my head was something like this:
public String appendWithDelimiter( String original, String addition, String delimiter ) {
if ( original.equals( "" ) ) {
return addition;
} else {
return original + delimiter + addition;
}
}
String parameterString = "";
if ( condition ) parameterString = appendWithDelimiter( parameterString, "elementName", "," );
if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, "anotherElementName", "," );
I realize this isn't particularly efficient, since there are strings being created all over the place, but I was going for clarity more than optimization.
In Ruby, I can do something like this instead, which feels much more elegant:
parameterArray = [];
parameterArray << "elementName" if condition;
parameterArray << "anotherElementName" if anotherCondition;
parameterString = parameterArray.join(",");
But since Java lacks a join command, I couldn't figure out anything equivalent.
So, what's the best way to do this in Java?
//Note: if you have access to Java5+,
//use StringBuilder in preference to StringBuffer.
//All that has to be replaced is the class name.
//StringBuffer will work in Java 1.4, though.
appendWithDelimiter( StringBuffer buffer, String addition,
String delimiter ) {
if ( buffer.length() == 0) {
buffer.append(addition);
} else {
buffer.append(delimiter);
buffer.append(addition);
}
}
StringBuffer parameterBuffer = new StringBuffer();
if ( condition ) {
appendWithDelimiter(parameterBuffer, "elementName", "," );
}
if ( anotherCondition ) {
appendWithDelimiter(parameterBuffer, "anotherElementName", "," );
}
//Finally, to return a string representation, call toString() when returning.
return parameterBuffer.toString();
So a couple of things you might do to get the feel that it seems like you're looking for:
1) Extend List class - and add the join method to it. The join method would simply do the work of concatenating and adding the delimiter (which could be a param to the join method)
2) It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class: so you could write that join method and add it as an extension method to List or even to Collection.
Solution 1 is probably the only realistic one, now, though since Java 7 isn't out yet :) But it should work just fine.
To use both of these, you'd just add all your items to the List or Collection as usual, and then call the new custom method to 'join' them.
So basically something like this:
public static String appendWithDelimiter(String original, String addition, String delimiter) {
if (original.equals("")) {
return addition;
} else {
StringBuilder sb = new StringBuilder(original.length() + addition.length() + delimiter.length());
sb.append(original);
sb.append(delimiter);
sb.append(addition);
return sb.toString();
}
}
Why don't you do in Java the same thing you are doing in ruby, that is creating the delimiter separated string only after you've added all the pieces to the array?
ArrayList<String> parms = new ArrayList<String>();
if (someCondition) parms.add("someString");
if (anotherCondition) parms.add("someOtherString");
// ...
String sep = ""; StringBuffer b = new StringBuffer();
for (String p: parms) {
b.append(sep);
b.append(p);
sep = "yourDelimiter";
}
You may want to move that for loop in a separate helper method, and also use StringBuilder instead of StringBuffer...
Edit: fixed the order of appends.
You can try something like this:
StringBuilder sb = new StringBuilder();
if (condition) { sb.append("elementName").append(","); }
if (anotherCondition) { sb.append("anotherElementName").append(","); }
String parameterString = sb.toString();
You can generalize it, but there's no join in Java, as you well say.
This might work better.
public static String join(Iterable<? extends CharSequence> s, String delimiter) {
Iterator<? extends CharSequence> iter = s.iterator();
if (!iter.hasNext()) return "";
StringBuilder buffer = new StringBuilder(iter.next());
while (iter.hasNext()) buffer.append(delimiter).append(iter.next());
return buffer.toString();
}
in Java 8 you can do this like:
list.stream().map(Object::toString)
.collect(Collectors.joining(delimiter));
if list has nulls you can use:
list.stream().map(String::valueOf)
.collect(Collectors.joining(delimiter))
it also supports prefix and suffix:
list.stream().map(String::valueOf)
.collect(Collectors.joining(delimiter, prefix, suffix));
With Java 5 variable args, so you don't have to stuff all your strings into a collection or array explicitly:
import junit.framework.Assert;
import org.junit.Test;
public class StringUtil
{
public static String join(String delim, String... strings)
{
StringBuilder builder = new StringBuilder();
if (strings != null)
{
for (String str : strings)
{
if (builder.length() > 0)
{
builder.append(delim).append(" ");
}
builder.append(str);
}
}
return builder.toString();
}
@Test
public void joinTest()
{
Assert.assertEquals("", StringUtil.join(",", null));
Assert.assertEquals("", StringUtil.join(",", ""));
Assert.assertEquals("", StringUtil.join(",", new String[0]));
Assert.assertEquals("test", StringUtil.join(",", "test"));
Assert.assertEquals("foo, bar", StringUtil.join(",", "foo", "bar"));
Assert.assertEquals("foo, bar, x", StringUtil.join(",", "foo", "bar", "x"));
}
}
Your approach is not too bad, but you should use a StringBuffer instead of using the + sign. The + has the big disadvantage that a new String instance is being created for each single operation. The longer your string gets, the bigger the overhead. So using a StringBuffer should be the fastest way:
public StringBuffer appendWithDelimiter( StringBuffer original, String addition, String delimiter ) {
if ( original == null ) {
StringBuffer buffer = new StringBuffer();
buffer.append(addition);
return buffer;
} else {
buffer.append(delimiter);
buffer.append(addition);
return original;
}
}
After you have finished creating your string simply call toString() on the returned StringBuffer.
The Google's Guava library has com.google.common.base.Joiner class which helps to solve such tasks.
Samples:
"My pets are: " + Joiner.on(", ").join(Arrays.asList("rabbit", "parrot", "dog"));
// returns "My pets are: rabbit, parrot, dog"
Joiner.on(" AND ").join(Arrays.asList("field1=1" , "field2=2", "field3=3"));
// returns "field1=1 AND field2=2 AND field3=3"
Joiner.on(",").skipNulls().join(Arrays.asList("London", "Moscow", null, "New York", null, "Paris"));
// returns "London,Moscow,New York,Paris"
Joiner.on(", ").useForNull("Team held a draw").join(Arrays.asList("FC Barcelona", "FC Bayern", null, null, "Chelsea FC", "AC Milan"));
// returns "FC Barcelona, FC Bayern, Team held a draw, Team held a draw, Chelsea FC, AC Milan"
Here is an article about Guava's string utilities.
So basically something like this:
public static String appendWithDelimiter(String original, String addition, String delimiter) {
if (original.equals("")) {
return addition;
} else {
StringBuilder sb = new StringBuilder(original.length() + addition.length() + delimiter.length());
sb.append(original);
sb.append(delimiter);
sb.append(addition);
return sb.toString();
}
}
Don't know if this really is any better, but at least it's using StringBuilder, which may be slightly more efficient.
Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting.
// Answers real question
public String appendWithDelimiters(String delimiter, String original, String addition) {
StringBuilder sb = new StringBuilder(original);
if(sb.length()!=0) {
sb.append(delimiter).append(addition);
} else {
sb.append(addition);
}
return sb.toString();
}
// A more generic case.
// ... means a list of indeterminate length of Strings.
public String appendWithDelimitersGeneric(String delimiter, String... strings) {
StringBuilder sb = new StringBuilder();
for (String string : strings) {
if(sb.length()!=0) {
sb.append(delimiter).append(string);
} else {
sb.append(string);
}
}
return sb.toString();
}
public void testAppendWithDelimiters() {
String string = appendWithDelimitersGeneric(",", "string1", "string2", "string3");
}
Instead of using string concatenation, you should use StringBuilder if your code is not threaded, and StringBuffer if it is.
You should probably use a StringBuilder with the append method to construct your result, but otherwise this is as good of a solution as Java has to offer.
public static String join(String[] strings, char del)
{
StringBuffer sb = new StringBuffer();
int len = strings.length;
boolean appended = false;
for (int i = 0; i < len; i++)
{
if (appended)
{
sb.append(del);
}
sb.append(""+strings[i]);
appended = true;
}
return sb.toString();
}
If you're using Eclipse Collections, you can use makeString() or appendString().
makeString() returns a String representation, similar to toString().
It has three forms
makeString(start, separator, end)makeString(separator) defaults start and end to empty stringsmakeString() defaults the separator to ", " (comma and space)Code example:
MutableList<Integer> list = FastList.newListWith(1, 2, 3);
assertEquals("[1/2/3]", list.makeString("[", "/", "]"));
assertEquals("1/2/3", list.makeString("/"));
assertEquals("1, 2, 3", list.makeString());
assertEquals(list.toString(), list.makeString("[", ", ", "]"));
appendString() is similar to makeString(), but it appends to an Appendable (like StringBuilder) and is void. It has the same three forms, with an additional first argument, the Appendable.
MutableList<Integer> list = FastList.newListWith(1, 2, 3);
Appendable appendable = new StringBuilder();
list.appendString(appendable, "[", "/", "]");
assertEquals("[1/2/3]", appendable.toString());
If you can't convert your collection to an Eclipse Collections type, just adapt it with the relevant adapter.
List<Object> list = ...;
ListAdapter.adapt(list).makeString(",");
Note: I am a committer for Eclipse collections.
You should probably use a StringBuilder with the append method to construct your result, but otherwise this is as good of a solution as Java has to offer.
In Java 8 you can use String.join():
List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"
Also have a look at this answer for a Stream API example.
In the case of Android, the StringUtils class from commons isn't available, so for this I used
android.text.TextUtils.join(CharSequence delimiter, Iterable tokens)
http://developer.android.com/reference/android/text/TextUtils.html
public static String join(String[] strings, char del)
{
StringBuffer sb = new StringBuffer();
int len = strings.length;
boolean appended = false;
for (int i = 0; i < len; i++)
{
if (appended)
{
sb.append(del);
}
sb.append(""+strings[i]);
appended = true;
}
return sb.toString();
}
Don't know if this really is any better, but at least it's using StringBuilder, which may be slightly more efficient.
Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting.
// Answers real question
public String appendWithDelimiters(String delimiter, String original, String addition) {
StringBuilder sb = new StringBuilder(original);
if(sb.length()!=0) {
sb.append(delimiter).append(addition);
} else {
sb.append(addition);
}
return sb.toString();
}
// A more generic case.
// ... means a list of indeterminate length of Strings.
public String appendWithDelimitersGeneric(String delimiter, String... strings) {
StringBuilder sb = new StringBuilder();
for (String string : strings) {
if(sb.length()!=0) {
sb.append(delimiter).append(string);
} else {
sb.append(string);
}
}
return sb.toString();
}
public void testAppendWithDelimiters() {
String string = appendWithDelimitersGeneric(",", "string1", "string2", "string3");
}
If you're using Eclipse Collections, you can use makeString() or appendString().
makeString() returns a String representation, similar to toString().
It has three forms
makeString(start, separator, end)makeString(separator) defaults start and end to empty stringsmakeString() defaults the separator to ", " (comma and space)Code example:
MutableList<Integer> list = FastList.newListWith(1, 2, 3);
assertEquals("[1/2/3]", list.makeString("[", "/", "]"));
assertEquals("1/2/3", list.makeString("/"));
assertEquals("1, 2, 3", list.makeString());
assertEquals(list.toString(), list.makeString("[", ", ", "]"));
appendString() is similar to makeString(), but it appends to an Appendable (like StringBuilder) and is void. It has the same three forms, with an additional first argument, the Appendable.
MutableList<Integer> list = FastList.newListWith(1, 2, 3);
Appendable appendable = new StringBuilder();
list.appendString(appendable, "[", "/", "]");
assertEquals("[1/2/3]", appendable.toString());
If you can't convert your collection to an Eclipse Collections type, just adapt it with the relevant adapter.
List<Object> list = ...;
ListAdapter.adapt(list).makeString(",");
Note: I am a committer for Eclipse collections.
You can generalize it, but there's no join in Java, as you well say.
This might work better.
public static String join(Iterable<? extends CharSequence> s, String delimiter) {
Iterator<? extends CharSequence> iter = s.iterator();
if (!iter.hasNext()) return "";
StringBuilder buffer = new StringBuilder(iter.next());
while (iter.hasNext()) buffer.append(delimiter).append(iter.next());
return buffer.toString();
}
I personally quite often use the following simple solution for logging purposes:
List lst = Arrays.asList("ab", "bc", "cd");
String str = lst.toString().replaceAll("[\\[\\]]", "");
With Java 5 variable args, so you don't have to stuff all your strings into a collection or array explicitly:
import junit.framework.Assert;
import org.junit.Test;
public class StringUtil
{
public static String join(String delim, String... strings)
{
StringBuilder builder = new StringBuilder();
if (strings != null)
{
for (String str : strings)
{
if (builder.length() > 0)
{
builder.append(delim).append(" ");
}
builder.append(str);
}
}
return builder.toString();
}
@Test
public void joinTest()
{
Assert.assertEquals("", StringUtil.join(",", null));
Assert.assertEquals("", StringUtil.join(",", ""));
Assert.assertEquals("", StringUtil.join(",", new String[0]));
Assert.assertEquals("test", StringUtil.join(",", "test"));
Assert.assertEquals("foo, bar", StringUtil.join(",", "foo", "bar"));
Assert.assertEquals("foo, bar, x", StringUtil.join(",", "foo", "bar", "x"));
}
}
You're making this a little more complicated than it has to be. Let's start with the end of your example:
String parameterString = "";
if ( condition ) parameterString = appendWithDelimiter( parameterString, "elementName", "," );
if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, "anotherElementName", "," );
With the small change of using a StringBuilder instead of a String, this becomes:
StringBuilder parameterString = new StringBuilder();
if (condition) parameterString.append("elementName").append(",");
if (anotherCondition) parameterString.append("anotherElementName").append(",");
...
When you're done (I assume you have to check a few other conditions as well), just make sure you remove the tailing comma with a command like this:
if (parameterString.length() > 0)
parameterString.deleteCharAt(parameterString.length() - 1);
And finally, get the string you want with
parameterString.toString();
You could also replace the "," in the second call to append with a generic delimiter string that can be set to anything. If you have a list of things you know you need to append (non-conditionally), you could put this code inside a method that takes a list of strings.
So basically something like this:
public static String appendWithDelimiter(String original, String addition, String delimiter) {
if (original.equals("")) {
return addition;
} else {
StringBuilder sb = new StringBuilder(original.length() + addition.length() + delimiter.length());
sb.append(original);
sb.append(delimiter);
sb.append(addition);
return sb.toString();
}
}
Your approach is not too bad, but you should use a StringBuffer instead of using the + sign. The + has the big disadvantage that a new String instance is being created for each single operation. The longer your string gets, the bigger the overhead. So using a StringBuffer should be the fastest way:
public StringBuffer appendWithDelimiter( StringBuffer original, String addition, String delimiter ) {
if ( original == null ) {
StringBuffer buffer = new StringBuffer();
buffer.append(addition);
return buffer;
} else {
buffer.append(delimiter);
buffer.append(addition);
return original;
}
}
After you have finished creating your string simply call toString() on the returned StringBuffer.
Apache commons StringUtils class has a join method.
Use StringBuilder and class Separator
StringBuilder buf = new StringBuilder();
Separator sep = new Separator(", ");
for (String each : list) {
buf.append(sep).append(each);
}
Separator wraps a delimiter. The delimiter is returned by Separator's toString method, unless on the first call which returns the empty string!
Source code for class Separator
public class Separator {
private boolean skipFirst;
private final String value;
public Separator() {
this(", ");
}
public Separator(String value) {
this.value = value;
this.skipFirst = true;
}
public void reset() {
skipFirst = true;
}
public String toString() {
String sep = skipFirst ? "" : value;
skipFirst = false;
return sep;
}
}
Why not write your own join() method? It would take as parameters collection of Strings and a delimiter String. Within the method iterate over the collection and build up your result in a StringBuffer.
Your approach is not too bad, but you should use a StringBuffer instead of using the + sign. The + has the big disadvantage that a new String instance is being created for each single operation. The longer your string gets, the bigger the overhead. So using a StringBuffer should be the fastest way:
public StringBuffer appendWithDelimiter( StringBuffer original, String addition, String delimiter ) {
if ( original == null ) {
StringBuffer buffer = new StringBuffer();
buffer.append(addition);
return buffer;
} else {
buffer.append(delimiter);
buffer.append(addition);
return original;
}
}
After you have finished creating your string simply call toString() on the returned StringBuffer.
You should probably use a StringBuilder with the append method to construct your result, but otherwise this is as good of a solution as Java has to offer.
Why don't you do in Java the same thing you are doing in ruby, that is creating the delimiter separated string only after you've added all the pieces to the array?
ArrayList<String> parms = new ArrayList<String>();
if (someCondition) parms.add("someString");
if (anotherCondition) parms.add("someOtherString");
// ...
String sep = ""; StringBuffer b = new StringBuffer();
for (String p: parms) {
b.append(sep);
b.append(p);
sep = "yourDelimiter";
}
You may want to move that for loop in a separate helper method, and also use StringBuilder instead of StringBuffer...
Edit: fixed the order of appends.
You can generalize it, but there's no join in Java, as you well say.
This might work better.
public static String join(Iterable<? extends CharSequence> s, String delimiter) {
Iterator<? extends CharSequence> iter = s.iterator();
if (!iter.hasNext()) return "";
StringBuilder buffer = new StringBuilder(iter.next());
while (iter.hasNext()) buffer.append(delimiter).append(iter.next());
return buffer.toString();
}
You can try something like this:
StringBuilder sb = new StringBuilder();
if (condition) { sb.append("elementName").append(","); }
if (anotherCondition) { sb.append("anotherElementName").append(","); }
String parameterString = sb.toString();
I would use Google Collections. There is a nice Join facility.
http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/base/Join.html
But if I wanted to write it on my own,
package util;
import java.util.ArrayList;
import java.util.Iterable;
import java.util.Collections;
import java.util.Iterator;
public class Utils {
// accept a collection of objects, since all objects have toString()
public static String join(String delimiter, Iterable<? extends Object> objs) {
if (objs.isEmpty()) {
return "";
}
Iterator<? extends Object> iter = objs.iterator();
StringBuilder buffer = new StringBuilder();
buffer.append(iter.next());
while (iter.hasNext()) {
buffer.append(delimiter).append(iter.next());
}
return buffer.toString();
}
// for convenience
public static String join(String delimiter, Object... objs) {
ArrayList<Object> list = new ArrayList<Object>();
Collections.addAll(list, objs);
return join(delimiter, list);
}
}
I think it works better with an object collection, since now you don't have to convert your objects to strings before you join them.
Apache commons StringUtils class has a join method.
So a couple of things you might do to get the feel that it seems like you're looking for:
1) Extend List class - and add the join method to it. The join method would simply do the work of concatenating and adding the delimiter (which could be a param to the join method)
2) It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class: so you could write that join method and add it as an extension method to List or even to Collection.
Solution 1 is probably the only realistic one, now, though since Java 7 isn't out yet :) But it should work just fine.
To use both of these, you'd just add all your items to the List or Collection as usual, and then call the new custom method to 'join' them.
You should probably use a StringBuilder with the append method to construct your result, but otherwise this is as good of a solution as Java has to offer.
Instead of using string concatenation, you should use StringBuilder if your code is not threaded, and StringBuffer if it is.
Java 8 Native Type
List<Integer> example;
example.add(1);
example.add(2);
example.add(3);
...
example.stream().collect(Collectors.joining(","));
Java 8 Custom Object:
List<Person> person;
...
person.stream().map(Person::getAge).collect(Collectors.joining(","));
public static String join(String[] strings, char del)
{
StringBuffer sb = new StringBuffer();
int len = strings.length;
boolean appended = false;
for (int i = 0; i < len; i++)
{
if (appended)
{
sb.append(del);
}
sb.append(""+strings[i]);
appended = true;
}
return sb.toString();
}
Instead of using string concatenation, you should use StringBuilder if your code is not threaded, and StringBuffer if it is.
Why don't you do in Java the same thing you are doing in ruby, that is creating the delimiter separated string only after you've added all the pieces to the array?
ArrayList<String> parms = new ArrayList<String>();
if (someCondition) parms.add("someString");
if (anotherCondition) parms.add("someOtherString");
// ...
String sep = ""; StringBuffer b = new StringBuffer();
for (String p: parms) {
b.append(sep);
b.append(p);
sep = "yourDelimiter";
}
You may want to move that for loop in a separate helper method, and also use StringBuilder instead of StringBuffer...
Edit: fixed the order of appends.
You can try something like this:
StringBuilder sb = new StringBuilder();
if (condition) { sb.append("elementName").append(","); }
if (anotherCondition) { sb.append("anotherElementName").append(","); }
String parameterString = sb.toString();
Why not write your own join() method? It would take as parameters collection of Strings and a delimiter String. Within the method iterate over the collection and build up your result in a StringBuffer.
Instead of using string concatenation, you should use StringBuilder if your code is not threaded, and StringBuffer if it is.
I personally quite often use the following simple solution for logging purposes:
List lst = Arrays.asList("ab", "bc", "cd");
String str = lst.toString().replaceAll("[\\[\\]]", "");
Use an approach based on java.lang.StringBuilder! ("A mutable sequence of characters. ")
Like you mentioned, all those string concatenations are creating Strings all over. StringBuilder won't do that.
Why StringBuilder instead of StringBuffer? From the StringBuilder javadoc:
Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
You're making this a little more complicated than it has to be. Let's start with the end of your example:
String parameterString = "";
if ( condition ) parameterString = appendWithDelimiter( parameterString, "elementName", "," );
if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, "anotherElementName", "," );
With the small change of using a StringBuilder instead of a String, this becomes:
StringBuilder parameterString = new StringBuilder();
if (condition) parameterString.append("elementName").append(",");
if (anotherCondition) parameterString.append("anotherElementName").append(",");
...
When you're done (I assume you have to check a few other conditions as well), just make sure you remove the tailing comma with a command like this:
if (parameterString.length() > 0)
parameterString.deleteCharAt(parameterString.length() - 1);
And finally, get the string you want with
parameterString.toString();
You could also replace the "," in the second call to append with a generic delimiter string that can be set to anything. If you have a list of things you know you need to append (non-conditionally), you could put this code inside a method that takes a list of strings.
Slight improvement [speed] of version from izb:
public static String join(String[] strings, char del)
{
StringBuilder sb = new StringBuilder();
int len = strings.length;
if(len > 1)
{
len -= 1;
}else
{
return strings[0];
}
for (int i = 0; i < len; i++)
{
sb.append(strings[i]).append(del);
}
sb.append(strings[i]);
return sb.toString();
}
Why don't you do in Java the same thing you are doing in ruby, that is creating the delimiter separated string only after you've added all the pieces to the array?
ArrayList<String> parms = new ArrayList<String>();
if (someCondition) parms.add("someString");
if (anotherCondition) parms.add("someOtherString");
// ...
String sep = ""; StringBuffer b = new StringBuffer();
for (String p: parms) {
b.append(sep);
b.append(p);
sep = "yourDelimiter";
}
You may want to move that for loop in a separate helper method, and also use StringBuilder instead of StringBuffer...
Edit: fixed the order of appends.
You could write a little join-style utility method that works on java.util.Lists
public static String join(List<String> list, String delim) {
StringBuilder sb = new StringBuilder();
String loopDelim = "";
for(String s : list) {
sb.append(loopDelim);
sb.append(s);
loopDelim = delim;
}
return sb.toString();
}
Then use it like so:
List<String> list = new ArrayList<String>();
if( condition ) list.add("elementName");
if( anotherCondition ) list.add("anotherElementName");
join(list, ",");
//Note: if you have access to Java5+,
//use StringBuilder in preference to StringBuffer.
//All that has to be replaced is the class name.
//StringBuffer will work in Java 1.4, though.
appendWithDelimiter( StringBuffer buffer, String addition,
String delimiter ) {
if ( buffer.length() == 0) {
buffer.append(addition);
} else {
buffer.append(delimiter);
buffer.append(addition);
}
}
StringBuffer parameterBuffer = new StringBuffer();
if ( condition ) {
appendWithDelimiter(parameterBuffer, "elementName", "," );
}
if ( anotherCondition ) {
appendWithDelimiter(parameterBuffer, "anotherElementName", "," );
}
//Finally, to return a string representation, call toString() when returning.
return parameterBuffer.toString();
So basically something like this:
public static String appendWithDelimiter(String original, String addition, String delimiter) {
if (original.equals("")) {
return addition;
} else {
StringBuilder sb = new StringBuilder(original.length() + addition.length() + delimiter.length());
sb.append(original);
sb.append(delimiter);
sb.append(addition);
return sb.toString();
}
}
If you are using Spring MVC then you can try following steps.
import org.springframework.util.StringUtils;
List<String> groupIds = new List<String>;
groupIds.add("a");
groupIds.add("b");
groupIds.add("c");
String csv = StringUtils.arrayToCommaDelimitedString(groupIds.toArray());
It will result to a,b,c
Apache commons StringUtils class has a join method.
Your approach is not too bad, but you should use a StringBuffer instead of using the + sign. The + has the big disadvantage that a new String instance is being created for each single operation. The longer your string gets, the bigger the overhead. So using a StringBuffer should be the fastest way:
public StringBuffer appendWithDelimiter( StringBuffer original, String addition, String delimiter ) {
if ( original == null ) {
StringBuffer buffer = new StringBuffer();
buffer.append(addition);
return buffer;
} else {
buffer.append(delimiter);
buffer.append(addition);
return original;
}
}
After you have finished creating your string simply call toString() on the returned StringBuffer.
using Dollar is simple as typing:
String joined = $(aCollection).join(",");
NB: it works also for Array and other data types
Internally it uses a very neat trick:
@Override
public String join(String separator) {
Separator sep = new Separator(separator);
StringBuilder sb = new StringBuilder();
for (T item : iterable) {
sb.append(sep).append(item);
}
return sb.toString();
}
the class Separator return the empty String only the first time that it is invoked, then it returns the separator:
class Separator {
private final String separator;
private boolean wasCalled;
public Separator(String separator) {
this.separator = separator;
this.wasCalled = false;
}
@Override
public String toString() {
if (!wasCalled) {
wasCalled = true;
return "";
} else {
return separator;
}
}
}
I would use Google Collections. There is a nice Join facility.
http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/base/Join.html
But if I wanted to write it on my own,
package util;
import java.util.ArrayList;
import java.util.Iterable;
import java.util.Collections;
import java.util.Iterator;
public class Utils {
// accept a collection of objects, since all objects have toString()
public static String join(String delimiter, Iterable<? extends Object> objs) {
if (objs.isEmpty()) {
return "";
}
Iterator<? extends Object> iter = objs.iterator();
StringBuilder buffer = new StringBuilder();
buffer.append(iter.next());
while (iter.hasNext()) {
buffer.append(delimiter).append(iter.next());
}
return buffer.toString();
}
// for convenience
public static String join(String delimiter, Object... objs) {
ArrayList<Object> list = new ArrayList<Object>();
Collections.addAll(list, objs);
return join(delimiter, list);
}
}
I think it works better with an object collection, since now you don't have to convert your objects to strings before you join them.
In the case of Android, the StringUtils class from commons isn't available, so for this I used
android.text.TextUtils.join(CharSequence delimiter, Iterable tokens)
http://developer.android.com/reference/android/text/TextUtils.html
//Note: if you have access to Java5+,
//use StringBuilder in preference to StringBuffer.
//All that has to be replaced is the class name.
//StringBuffer will work in Java 1.4, though.
appendWithDelimiter( StringBuffer buffer, String addition,
String delimiter ) {
if ( buffer.length() == 0) {
buffer.append(addition);
} else {
buffer.append(delimiter);
buffer.append(addition);
}
}
StringBuffer parameterBuffer = new StringBuffer();
if ( condition ) {
appendWithDelimiter(parameterBuffer, "elementName", "," );
}
if ( anotherCondition ) {
appendWithDelimiter(parameterBuffer, "anotherElementName", "," );
}
//Finally, to return a string representation, call toString() when returning.
return parameterBuffer.toString();
With Java 5 variable args, so you don't have to stuff all your strings into a collection or array explicitly:
import junit.framework.Assert;
import org.junit.Test;
public class StringUtil
{
public static String join(String delim, String... strings)
{
StringBuilder builder = new StringBuilder();
if (strings != null)
{
for (String str : strings)
{
if (builder.length() > 0)
{
builder.append(delim).append(" ");
}
builder.append(str);
}
}
return builder.toString();
}
@Test
public void joinTest()
{
Assert.assertEquals("", StringUtil.join(",", null));
Assert.assertEquals("", StringUtil.join(",", ""));
Assert.assertEquals("", StringUtil.join(",", new String[0]));
Assert.assertEquals("test", StringUtil.join(",", "test"));
Assert.assertEquals("foo, bar", StringUtil.join(",", "foo", "bar"));
Assert.assertEquals("foo, bar, x", StringUtil.join(",", "foo", "bar", "x"));
}
}
I would use Google Collections. There is a nice Join facility.
http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/base/Join.html
But if I wanted to write it on my own,
package util;
import java.util.ArrayList;
import java.util.Iterable;
import java.util.Collections;
import java.util.Iterator;
public class Utils {
// accept a collection of objects, since all objects have toString()
public static String join(String delimiter, Iterable<? extends Object> objs) {
if (objs.isEmpty()) {
return "";
}
Iterator<? extends Object> iter = objs.iterator();
StringBuilder buffer = new StringBuilder();
buffer.append(iter.next());
while (iter.hasNext()) {
buffer.append(delimiter).append(iter.next());
}
return buffer.toString();
}
// for convenience
public static String join(String delimiter, Object... objs) {
ArrayList<Object> list = new ArrayList<Object>();
Collections.addAll(list, objs);
return join(delimiter, list);
}
}
I think it works better with an object collection, since now you don't have to convert your objects to strings before you join them.
You can use Java's StringBuilder type for this. There's also StringBuffer, but it contains extra thread safety logic that is often unnecessary.
stringCollection.stream().collect(Collectors.joining(", "));
You could write a little join-style utility method that works on java.util.Lists
public static String join(List<String> list, String delim) {
StringBuilder sb = new StringBuilder();
String loopDelim = "";
for(String s : list) {
sb.append(loopDelim);
sb.append(s);
loopDelim = delim;
}
return sb.toString();
}
Then use it like so:
List<String> list = new ArrayList<String>();
if( condition ) list.add("elementName");
if( anotherCondition ) list.add("anotherElementName");
join(list, ",");
You're making this a little more complicated than it has to be. Let's start with the end of your example:
String parameterString = "";
if ( condition ) parameterString = appendWithDelimiter( parameterString, "elementName", "," );
if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, "anotherElementName", "," );
With the small change of using a StringBuilder instead of a String, this becomes:
StringBuilder parameterString = new StringBuilder();
if (condition) parameterString.append("elementName").append(",");
if (anotherCondition) parameterString.append("anotherElementName").append(",");
...
When you're done (I assume you have to check a few other conditions as well), just make sure you remove the tailing comma with a command like this:
if (parameterString.length() > 0)
parameterString.deleteCharAt(parameterString.length() - 1);
And finally, get the string you want with
parameterString.toString();
You could also replace the "," in the second call to append with a generic delimiter string that can be set to anything. If you have a list of things you know you need to append (non-conditionally), you could put this code inside a method that takes a list of strings.
For those who are in a Spring context their StringUtils class is useful as well:
There are many useful shortcuts like:
and many others.
This can be helpful if you are not already using Java 8 and you are already in a Spring context.
I prefer it against the Apache Commons (although very good as well) for the Collection support which is easier like this:
// Encoding Set<String> to String delimited
String asString = org.springframework.util.StringUtils.collectionToDelimitedString(codes, ";");
// Decoding String delimited to Set
Set<String> collection = org.springframework.util.StringUtils.commaDelimitedListToSet(asString);
And a minimal one (if you don't want to include Apache Commons or Gauva into project dependencies just for the sake of joining strings)
/**
*
* @param delim : String that should be kept in between the parts
* @param parts : parts that needs to be joined
* @return a String that's formed by joining the parts
*/
private static final String join(String delim, String... parts) {
StringBuilder builder = new StringBuilder();
for (int i = 0; i < parts.length - 1; i++) {
builder.append(parts[i]).append(delim);
}
if(parts.length > 0){
builder.append(parts[parts.length - 1]);
}
return builder.toString();
}
I would use Google Collections. There is a nice Join facility.
http://google-collections.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/base/Join.html
But if I wanted to write it on my own,
package util;
import java.util.ArrayList;
import java.util.Iterable;
import java.util.Collections;
import java.util.Iterator;
public class Utils {
// accept a collection of objects, since all objects have toString()
public static String join(String delimiter, Iterable<? extends Object> objs) {
if (objs.isEmpty()) {
return "";
}
Iterator<? extends Object> iter = objs.iterator();
StringBuilder buffer = new StringBuilder();
buffer.append(iter.next());
while (iter.hasNext()) {
buffer.append(delimiter).append(iter.next());
}
return buffer.toString();
}
// for convenience
public static String join(String delimiter, Object... objs) {
ArrayList<Object> list = new ArrayList<Object>();
Collections.addAll(list, objs);
return join(delimiter, list);
}
}
I think it works better with an object collection, since now you don't have to convert your objects to strings before you join them.
For those who are in a Spring context their StringUtils class is useful as well:
There are many useful shortcuts like:
and many others.
This can be helpful if you are not already using Java 8 and you are already in a Spring context.
I prefer it against the Apache Commons (although very good as well) for the Collection support which is easier like this:
// Encoding Set<String> to String delimited
String asString = org.springframework.util.StringUtils.collectionToDelimitedString(codes, ";");
// Decoding String delimited to Set
Set<String> collection = org.springframework.util.StringUtils.commaDelimitedListToSet(asString);
You can use Java's StringBuilder type for this. There's also StringBuffer, but it contains extra thread safety logic that is often unnecessary.
Why not write your own join() method? It would take as parameters collection of Strings and a delimiter String. Within the method iterate over the collection and build up your result in a StringBuffer.
using Dollar is simple as typing:
String joined = $(aCollection).join(",");
NB: it works also for Array and other data types
Internally it uses a very neat trick:
@Override
public String join(String separator) {
Separator sep = new Separator(separator);
StringBuilder sb = new StringBuilder();
for (T item : iterable) {
sb.append(sep).append(item);
}
return sb.toString();
}
the class Separator return the empty String only the first time that it is invoked, then it returns the separator:
class Separator {
private final String separator;
private boolean wasCalled;
public Separator(String separator) {
this.separator = separator;
this.wasCalled = false;
}
@Override
public String toString() {
if (!wasCalled) {
wasCalled = true;
return "";
} else {
return separator;
}
}
}
Use an approach based on java.lang.StringBuilder! ("A mutable sequence of characters. ")
Like you mentioned, all those string concatenations are creating Strings all over. StringBuilder won't do that.
Why StringBuilder instead of StringBuffer? From the StringBuilder javadoc:
Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
And a minimal one (if you don't want to include Apache Commons or Gauva into project dependencies just for the sake of joining strings)
/**
*
* @param delim : String that should be kept in between the parts
* @param parts : parts that needs to be joined
* @return a String that's formed by joining the parts
*/
private static final String join(String delim, String... parts) {
StringBuilder builder = new StringBuilder();
for (int i = 0; i < parts.length - 1; i++) {
builder.append(parts[i]).append(delim);
}
if(parts.length > 0){
builder.append(parts[parts.length - 1]);
}
return builder.toString();
}
You're making this a little more complicated than it has to be. Let's start with the end of your example:
String parameterString = "";
if ( condition ) parameterString = appendWithDelimiter( parameterString, "elementName", "," );
if ( anotherCondition ) parameterString = appendWithDelimiter( parameterString, "anotherElementName", "," );
With the small change of using a StringBuilder instead of a String, this becomes:
StringBuilder parameterString = new StringBuilder();
if (condition) parameterString.append("elementName").append(",");
if (anotherCondition) parameterString.append("anotherElementName").append(",");
...
When you're done (I assume you have to check a few other conditions as well), just make sure you remove the tailing comma with a command like this:
if (parameterString.length() > 0)
parameterString.deleteCharAt(parameterString.length() - 1);
And finally, get the string you want with
parameterString.toString();
You could also replace the "," in the second call to append with a generic delimiter string that can be set to anything. If you have a list of things you know you need to append (non-conditionally), you could put this code inside a method that takes a list of strings.
Use StringBuilder and class Separator
StringBuilder buf = new StringBuilder();
Separator sep = new Separator(", ");
for (String each : list) {
buf.append(sep).append(each);
}
Separator wraps a delimiter. The delimiter is returned by Separator's toString method, unless on the first call which returns the empty string!
Source code for class Separator
public class Separator {
private boolean skipFirst;
private final String value;
public Separator() {
this(", ");
}
public Separator(String value) {
this.value = value;
this.skipFirst = true;
}
public void reset() {
skipFirst = true;
}
public String toString() {
String sep = skipFirst ? "" : value;
skipFirst = false;
return sep;
}
}
In Java 8 you can use String.join():
List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"
Also have a look at this answer for a Stream API example.
You can generalize it, but there's no join in Java, as you well say.
This might work better.
public static String join(Iterable<? extends CharSequence> s, String delimiter) {
Iterator<? extends CharSequence> iter = s.iterator();
if (!iter.hasNext()) return "";
StringBuilder buffer = new StringBuilder(iter.next());
while (iter.hasNext()) buffer.append(delimiter).append(iter.next());
return buffer.toString();
}
You could write a little join-style utility method that works on java.util.Lists
public static String join(List<String> list, String delim) {
StringBuilder sb = new StringBuilder();
String loopDelim = "";
for(String s : list) {
sb.append(loopDelim);
sb.append(s);
loopDelim = delim;
}
return sb.toString();
}
Then use it like so:
List<String> list = new ArrayList<String>();
if( condition ) list.add("elementName");
if( anotherCondition ) list.add("anotherElementName");
join(list, ",");
Don't know if this really is any better, but at least it's using StringBuilder, which may be slightly more efficient.
Down below is a more generic approach if you can build up the list of parameters BEFORE doing any parameter delimiting.
// Answers real question
public String appendWithDelimiters(String delimiter, String original, String addition) {
StringBuilder sb = new StringBuilder(original);
if(sb.length()!=0) {
sb.append(delimiter).append(addition);
} else {
sb.append(addition);
}
return sb.toString();
}
// A more generic case.
// ... means a list of indeterminate length of Strings.
public String appendWithDelimitersGeneric(String delimiter, String... strings) {
StringBuilder sb = new StringBuilder();
for (String string : strings) {
if(sb.length()!=0) {
sb.append(delimiter).append(string);
} else {
sb.append(string);
}
}
return sb.toString();
}
public void testAppendWithDelimiters() {
String string = appendWithDelimitersGeneric(",", "string1", "string2", "string3");
}
The Google's Guava library has com.google.common.base.Joiner class which helps to solve such tasks.
Samples:
"My pets are: " + Joiner.on(", ").join(Arrays.asList("rabbit", "parrot", "dog"));
// returns "My pets are: rabbit, parrot, dog"
Joiner.on(" AND ").join(Arrays.asList("field1=1" , "field2=2", "field3=3"));
// returns "field1=1 AND field2=2 AND field3=3"
Joiner.on(",").skipNulls().join(Arrays.asList("London", "Moscow", null, "New York", null, "Paris"));
// returns "London,Moscow,New York,Paris"
Joiner.on(", ").useForNull("Team held a draw").join(Arrays.asList("FC Barcelona", "FC Bayern", null, null, "Chelsea FC", "AC Milan"));
// returns "FC Barcelona, FC Bayern, Team held a draw, Team held a draw, Chelsea FC, AC Milan"
Here is an article about Guava's string utilities.
Fix answer Rob Dickerson.
It's easier to use:
public static String join(String delimiter, String... values)
{
StringBuilder stringBuilder = new StringBuilder();
for (String value : values)
{
stringBuilder.append(value);
stringBuilder.append(delimiter);
}
String result = stringBuilder.toString();
return result.isEmpty() ? result : result.substring(0, result.length() - 1);
}
Use an approach based on java.lang.StringBuilder! ("A mutable sequence of characters. ")
Like you mentioned, all those string concatenations are creating Strings all over. StringBuilder won't do that.
Why StringBuilder instead of StringBuffer? From the StringBuilder javadoc:
Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
Apache commons StringUtils class has a join method.
You can use Java's StringBuilder type for this. There's also StringBuffer, but it contains extra thread safety logic that is often unnecessary.
If you want to apply comma in a List of object's properties. This is the way i found most useful.
here getName() is a string property of a class i have been trying to add "," to.
String message = listName.stream().map(list -> list.getName()).collect(Collectors.joining(", "));
Why not write your own join() method? It would take as parameters collection of Strings and a delimiter String. Within the method iterate over the collection and build up your result in a StringBuffer.
public static String join(String[] strings, char del)
{
StringBuffer sb = new StringBuffer();
int len = strings.length;
boolean appended = false;
for (int i = 0; i < len; i++)
{
if (appended)
{
sb.append(del);
}
sb.append(""+strings[i]);
appended = true;
}
return sb.toString();
}
You can use Java's StringBuilder type for this. There's also StringBuffer, but it contains extra thread safety logic that is often unnecessary.
With Java 5 variable args, so you don't have to stuff all your strings into a collection or array explicitly:
import junit.framework.Assert;
import org.junit.Test;
public class StringUtil
{
public static String join(String delim, String... strings)
{
StringBuilder builder = new StringBuilder();
if (strings != null)
{
for (String str : strings)
{
if (builder.length() > 0)
{
builder.append(delim).append(" ");
}
builder.append(str);
}
}
return builder.toString();
}
@Test
public void joinTest()
{
Assert.assertEquals("", StringUtil.join(",", null));
Assert.assertEquals("", StringUtil.join(",", ""));
Assert.assertEquals("", StringUtil.join(",", new String[0]));
Assert.assertEquals("test", StringUtil.join(",", "test"));
Assert.assertEquals("foo, bar", StringUtil.join(",", "foo", "bar"));
Assert.assertEquals("foo, bar, x", StringUtil.join(",", "foo", "bar", "x"));
}
}
stringCollection.stream().collect(Collectors.joining(", "));
Use an approach based on java.lang.StringBuilder! ("A mutable sequence of characters. ")
Like you mentioned, all those string concatenations are creating Strings all over. StringBuilder won't do that.
Why StringBuilder instead of StringBuffer? From the StringBuilder javadoc:
Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.
So a couple of things you might do to get the feel that it seems like you're looking for:
1) Extend List class - and add the join method to it. The join method would simply do the work of concatenating and adding the delimiter (which could be a param to the join method)
2) It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class: so you could write that join method and add it as an extension method to List or even to Collection.
Solution 1 is probably the only realistic one, now, though since Java 7 isn't out yet :) But it should work just fine.
To use both of these, you'd just add all your items to the List or Collection as usual, and then call the new custom method to 'join' them.
Use StringBuilder and class Separator
StringBuilder buf = new StringBuilder();
Separator sep = new Separator(", ");
for (String each : list) {
buf.append(sep).append(each);
}
Separator wraps a delimiter. The delimiter is returned by Separator's toString method, unless on the first call which returns the empty string!
Source code for class Separator
public class Separator {
private boolean skipFirst;
private final String value;
public Separator() {
this(", ");
}
public Separator(String value) {
this.value = value;
this.skipFirst = true;
}
public void reset() {
skipFirst = true;
}
public String toString() {
String sep = skipFirst ? "" : value;
skipFirst = false;
return sep;
}
}
So a couple of things you might do to get the feel that it seems like you're looking for:
1) Extend List class - and add the join method to it. The join method would simply do the work of concatenating and adding the delimiter (which could be a param to the join method)
2) It looks like Java 7 is going to be adding extension methods to java - which allows you just to attach a specific method on to a class: so you could write that join method and add it as an extension method to List or even to Collection.
Solution 1 is probably the only realistic one, now, though since Java 7 isn't out yet :) But it should work just fine.
To use both of these, you'd just add all your items to the List or Collection as usual, and then call the new custom method to 'join' them.
Fix answer Rob Dickerson.
It's easier to use:
public static String join(String delimiter, String... values)
{
StringBuilder stringBuilder = new StringBuilder();
for (String value : values)
{
stringBuilder.append(value);
stringBuilder.append(delimiter);
}
String result = stringBuilder.toString();
return result.isEmpty() ? result : result.substring(0, result.length() - 1);
}
Java 8 Native Type
List<Integer> example;
example.add(1);
example.add(2);
example.add(3);
...
example.stream().collect(Collectors.joining(","));
Java 8 Custom Object:
List<Person> person;
...
person.stream().map(Person::getAge).collect(Collectors.joining(","));
If you want to apply comma in a List of object's properties. This is the way i found most useful.
here getName() is a string property of a class i have been trying to add "," to.
String message = listName.stream().map(list -> list.getName()).collect(Collectors.joining(", "));
in Java 8 you can do this like:
list.stream().map(Object::toString)
.collect(Collectors.joining(delimiter));
if list has nulls you can use:
list.stream().map(String::valueOf)
.collect(Collectors.joining(delimiter))
it also supports prefix and suffix:
list.stream().map(String::valueOf)
.collect(Collectors.joining(delimiter, prefix, suffix));
//Note: if you have access to Java5+,
//use StringBuilder in preference to StringBuffer.
//All that has to be replaced is the class name.
//StringBuffer will work in Java 1.4, though.
appendWithDelimiter( StringBuffer buffer, String addition,
String delimiter ) {
if ( buffer.length() == 0) {
buffer.append(addition);
} else {
buffer.append(delimiter);
buffer.append(addition);
}
}
StringBuffer parameterBuffer = new StringBuffer();
if ( condition ) {
appendWithDelimiter(parameterBuffer, "elementName", "," );
}
if ( anotherCondition ) {
appendWithDelimiter(parameterBuffer, "anotherElementName", "," );
}
//Finally, to return a string representation, call toString() when returning.
return parameterBuffer.toString();
Slight improvement [speed] of version from izb:
public static String join(String[] strings, char del)
{
StringBuilder sb = new StringBuilder();
int len = strings.length;
if(len > 1)
{
len -= 1;
}else
{
return strings[0];
}
for (int i = 0; i < len; i++)
{
sb.append(strings[i]).append(del);
}
sb.append(strings[i]);
return sb.toString();
}
You could write a little join-style utility method that works on java.util.Lists
public static String join(List<String> list, String delim) {
StringBuilder sb = new StringBuilder();
String loopDelim = "";
for(String s : list) {
sb.append(loopDelim);
sb.append(s);
loopDelim = delim;
}
return sb.toString();
}
Then use it like so:
List<String> list = new ArrayList<String>();
if( condition ) list.add("elementName");
if( anotherCondition ) list.add("anotherElementName");
join(list, ",");
You can try something like this:
StringBuilder sb = new StringBuilder();
if (condition) { sb.append("elementName").append(","); }
if (anotherCondition) { sb.append("anotherElementName").append(","); }
String parameterString = sb.toString();
Use StringBuilder and class Separator
StringBuilder buf = new StringBuilder();
Separator sep = new Separator(", ");
for (String each : list) {
buf.append(sep).append(each);
}
Separator wraps a delimiter. The delimiter is returned by Separator's toString method, unless on the first call which returns the empty string!
Source code for class Separator
public class Separator {
private boolean skipFirst;
private final String value;
public Separator() {
this(", ");
}
public Separator(String value) {
this.value = value;
this.skipFirst = true;
}
public void reset() {
skipFirst = true;
}
public String toString() {
String sep = skipFirst ? "" : value;
skipFirst = false;
return sep;
}
}
If you are using Spring MVC then you can try following steps.
import org.springframework.util.StringUtils;
List<String> groupIds = new List<String>;
groupIds.add("a");
groupIds.add("b");
groupIds.add("c");
String csv = StringUtils.arrayToCommaDelimitedString(groupIds.toArray());
It will result to a,b,c
Source: Stackoverflow.com