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copied!<p>If you run with <code>-XX:-UseTLAB</code></p> <pre><code>public static void main(String... args) throws Exception { StringBuilder text = new StringBuilder(); text.append(new char[1024]); long free1 = free(); String str = text.toString(); long free2 = free(); String [] array = { str.substring(0, 100), str.substring(101, 200) }; long free3 = free(); if (free3 == free2) System.err.println("You must use -XX:-UseTLAB"); System.out.println("To create String with 1024 chars "+(free1-free2)+" bytes\nand to create an array with two sub-string was "+(free2-free3)); } private static long free() { return Runtime.getRuntime().freeMemory(); } </code></pre> <p>prints</p> <pre><code>To create String with 1024 chars 2096 bytes and to create an array with two sub-string was 88 </code></pre> <p>You can see its consuming more memory that you might expect if they shared the same back end store.</p> <p>If you look at the code in the <strong>String</strong> class.</p> <pre><code>public String substring(int start, int end) { // checks. return ((beginIndex == 0) && (endIndex == count)) ? this : new String(offset + beginIndex, endIndex - beginIndex, value); } String(int offset, int count, char value[]) { this.value = value; this.offset = offset; this.count = count; } </code></pre> <p>You can see that substring for <strong>String</strong> doesn't take a copy of the underlying value array.</p> <hr> <p>Another thing to consider is the <a href="http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html" rel="nofollow"><code>-XX:+UseCompressedStrings</code></a> which is on by default on newer versions of the JVM. This encourages the JVM to use byte[] instead of char[] where possible.</p> <p>The size of the headers for the String and array object varies for 32-bit JVMs, 64-bit JVM with 32-bit references and 64-bit JVMs with 64-bit references.</p>

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