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    <p>The following program will sometimes throw exceptions when using StringBuilder, but will never throw an exception when using StringBuffer.</p> <p>Program:</p> <pre><code>public class StringBuilderConcurrent { static final StringBuilder sb = new StringBuilder(); // shared memory public static void main(String[] args) throws Exception { int NUM_WRITERS = 300; ArrayList&lt;WriterThread&gt; threads = new ArrayList&lt;WriterThread&gt;(NUM_WRITERS); for (int i = 0; i &lt; NUM_WRITERS; i++) { WriterThread wt = new WriterThread("writerThread" + i); threads.add(wt); wt.start(); } for (int i = 0; i &lt; threads.size(); i++) { threads.get(i).join(); } System.out.println(sb); } public static class WriterThread extends Thread { public WriterThread(String name) { super(name); } public void run() { String nameNl = this.getName() + "\n"; for (int i = 1; i &lt; 20; i++) { sb.append(nameNl); } } }; } </code></pre> <p>Because StringBuilder (<code>sb</code>) is not thread safe, having multiple threads write data to <code>sb</code> may result in <code>sb</code> becoming corrupted (e.g. unexpected null characters, a word's letters interspersed with some other word's letters). It's also possible for <code>sb</code>'s internal state to become inconsistent enough that an exception might be thrown:</p> <pre><code>Exception in thread "writerThread0" java.lang.ArrayIndexOutOfBoundsException at java.lang.System.arraycopy(Native Method) at java.lang.String.getChars(String.java:854) at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:391) at java.lang.StringBuilder.append(StringBuilder.java:119) at test.StringBuilderConcurrent$WriterThread.run(StringBuilderConcurrent.java:35) </code></pre> <p>The following program is identical to the first except it uses StringBuffer instead of StringBuilder. It will never encounter an ArrayIndexOutOfBoundsException.</p> <pre><code>public class StringBufferConcurrent { static final StringBuffer sb = new StringBuffer(); // shared memory public static void main(String[] args) throws Exception { int NUM_WRITERS = 300; ArrayList&lt;WriterThread&gt; threads = new ArrayList&lt;WriterThread&gt;(NUM_WRITERS); for (int i = 0; i &lt; NUM_WRITERS; i++) { WriterThread wt = new WriterThread("writerThread" + i); threads.add(wt); wt.start(); } for (int i = 0; i &lt; threads.size(); i++) { threads.get(i).join(); } System.out.println(sb); } public static class WriterThread extends Thread { public WriterThread(String name) { super(name); } public void run() { String nameNl = this.getName() + "\n"; for (int i = 1; i &lt; 20; i++) { sb.append(nameNl); } } }; } </code></pre> <p>Whether these programs are representative of a "real world" problem is a fairly subjective question. I'll leave that judgement up to the audience. </p>
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    1. POImagine a real concurrent scenario where StringBuffer should be used than StringBuilder?
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