Best alternative to String lightweight implementation in Java

NOTE: The examples used in this answer may not be relevant to modern runtime JVM libraries. In particular, the substringexample is no longer a problem in OpenJDK/Oracle 7+.

I know this goes against what people often tell you, but sometimes explicitly creating new Stringinstances can be an important way to reduce memory.

Since strings are immutable, there are several ways to take advantage of that fact and share an array of backing characters to save memory. However, sometimes this can actually increase memory by preventing garbage collection of those unused parts of the array.

For example, let's say you're parsing a log file for message IDs to extract warning IDs. Your code looks like this:

//Format:
//ID: [WARNING|ERROR|DEBUG] Message...
String testLine = "5AB729: WARNING Some really really really long message";

Matcher matcher = Pattern.compile("([A-Z0-9]*): WARNING.*").matcher(testLine);
if ( matcher.matches() ) {
    String id = matcher.group(1);
        //...do something with id...
}

But look at the actual stored data:

    //...
    String id = matcher.group(1);
    Field valueField = String.class.getDeclaredField("value");
    valueField.setAccessible(true);

    char[] data = ((char[])valueField.get(id));
    System.out.println("Actual data stored for string \"" + id + "\": " + Arrays.toString(data) );

That's the whole test line, because the matcher just wraps a new String instance around the same character data. String id = matcher.group(1);Use when you replace the comparison result String id = new String(matcher.group(1));.