How to add spaces only between catenated values using java

Using Guava Libraries:

String[] names = {"bill", "bob", "steve" };
String fullString = Joiner.on(" ").join(names);

In Java 8:

String fullString = String.join(" ", names)

This is the way I'm doing it

public static boolean isEmpty(String string) {
    return string == null || string.trim().length() == 0;
}

public static String concatenate(final String separator, final String... parameters) {

    StringBuilder result = new StringBuilder("");

    for (String param : parameters) {
        if (!isEmpty(param)) {
            if (result.length() > 0) {
                result.append(separator);
            }
            result.append(param);
        }
    }

    return result.toString();
}

I would suggest using a StringBuilder, in case you're iterating through an array of String. Something like this:

StringBuilder buffer = new StringBuilder();

//Assuming you have a string array;

int length = stringArray.length;

for(int index = 0; index < length; index++){
    if(index > 0)
        buffer.append(" ");
    buffer.append(stringArray[index]);
}

return buffer.toString();

It sounds like you're iterating over the strings to concatenate, something like:

StringBuilder sb = new StringBuilder();
for (String str : strings)
    sb.append(str).append(" ");  
result = sb.toString();

And you end up with a space at the end. If so, you can use the String.trim method to get rid of that extra space:

result = result.trim();

trim removes all leading and trailing whitespace.

You can use a StringBuilder to build your string like this:

StringBuilder sb = new StringBuilder();
sb.append("bill").append(" ");
//and so on..

If your values are in a list or an array you can use a for loop to build your string. Something like:

foreach(String s: strings){
    sb.append(s).append(" ");
}
sb.toString.trim();

Because using "+" in loops isn't the best idea, you can try also with:

    String foo [] = {"asd", "bbb", "ccc"};
    StringBuilder sb = new StringBuilder();
    for (String string : foo) {
        sb.append(string).append(" ");
    }
    String result = sb.toString().trim();

Ah yes ... one of my favourites ... you are looking for a Separator. Here's one. It comes with some sample static utility methods to demonstrate its use. There are many more usages.

public class Separator {
  private final String sepString;
  private final String firstString;
  boolean first = true;

  // Use for url params ("?","&").
  public Separator(final String first, final String sep) {
    this.sepString = sep;
    this.firstString = first;
  }

  // Use for commas etc.
  public Separator(final String sep) {
    this("", sep);
  }

  public String sep() {
    // Return empty string first and then the separator on every subsequent invocation.
    if (first) {
      first = false;
      return firstString;
    }
    return sepString;
  }

  public void reset() {
    first = true;
  }

  @Override
  public String toString() {
    return first ? "(" + sepString + ")" : sepString;
  }

  // Utilities.
  public StringBuilder addAll(final StringBuilder buf, final Iterable<?> values) {
    if (null == values) {
      return buf;
    }
    for (final Object v : values) {
      buf.append(sep()).append(v);
    }
    return buf;
  }

  public StringBuilder addAll(final StringBuilder buf, final Iterator<?> values) {
    if (null == values) {
      return buf;
    }
    while ( values.hasNext() ) {
      buf.append(sep()).append(values.next());
    }
    return buf;
  }

  public StringBuilder addAll(final StringBuilder buf, final Object... values) {
    if (null == values) {
      return buf;
    }
    for (final Object v : values) {
      buf.append(sep()).append(v);
    }
    return buf;
  }

  public StringBuilder addAll(final StringBuilder buf, final int... values) {
    if (null == values) {
      return buf;
    }
    for (final int v : values) {
      buf.append(sep()).append(v);
    }
    return buf;
  }

  public static String separate ( String separator, int [] ints ) {
    return new Separator(separator).addAll(new StringBuilder(), ints).toString();
  }

  public static String separate ( String separator, long [] longs ) {
    return new Separator(separator).addAll(new StringBuilder(), longs).toString();
  }

  public static String separate ( String separator, Object [] objs ) {
    return new Separator(separator).addAll(new StringBuilder(), objs).toString();
  }

  public static String separate ( String separator, Set them ) {
    return new Separator(separator).addAll(new StringBuilder(), them).toString();
  }

  public static String separate ( String separator, Iterator<?> i ) {
    return new Separator(separator).addAll(new StringBuilder(), i).toString();
  }

}

use java.util.StringJoiner

import java.util.StringJoiner;  
StringJoiner sj = new StringJoiner(" ");
sj.add("aaa");
sj.add("bbb");
sj.add("ccc");
String result = sj.toString();
System.out.println(result);

output: aaa bbb ccc

no need to write any logic.