//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(); 

谷歌的Guava库有com.google.common.base.Joiner类，它可以帮助解决这样的任务。

样品:

"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"

这是一篇关于Guava的字符串实用程序的文章。

izb版本的速度略有提高:

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();
}

在Java 8中，你可以使用String.join():

List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"

还有一个流API示例的答案。

你可以尝试这样做:

StringBuilder sb = new StringBuilder();
if (condition) { sb.append("elementName").append(","); }
if (anotherCondition) { sb.append("anotherElementName").append(","); }
String parameterString = sb.toString();

为什么你不在Java中做你在ruby中做的同样的事情，那就是在你将所有的片段添加到数组后才创建分隔符分隔的字符串?

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";
}

你可能想要在一个单独的帮助方法中移动for循环，并且使用StringBuilder而不是StringBuffer…

编辑:修正了追加的顺序。