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    <p>Performance testing is king to determine what's fastest. </p> <p>A little code to benchmark three different ways to write this...</p> <pre><code>import java.io.IOException; import java.util.Properties; import java.util.Vector; public class BenchmarkStringStuff { public static void main(String[] args) { try { System.getProperties().store(System.out, ""); } catch (IOException ioe ) { System.out.println("Failed to write properties to STDOUT"); return; } BenchmarkStringStuff bss = new BenchmarkStringStuff(); bss.benchmark(10); bss.benchmark(1000); bss.benchmark(100000); bss.benchmark(1000000); } public void benchmark(int numTests) { Vector&lt;Test&gt; tests = new Vector&lt;Test&gt;(); tests.add(new OriginalCode()); tests.add(new TwoSearches()); tests.add(new SearchOnceAndSubstring()); Vector&lt;String&gt; testStrings = new Vector&lt;String&gt;(); // we have a very poor sample, here. You should test with a better // representation of your expected values testStrings.add("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[time]aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"); Performance overallRuntime = new Performance("overall combined runtime"); overallRuntime.setCount(numTests * testStrings.size() * tests.size()); overallRuntime.start(); for( Test test : tests ) { Performance codePerformance = new Performance(test.getClass().getName()); codePerformance.setCount(numTests * testStrings.size()); codePerformance.start(); for(String testString : testStrings) { //Performance stringPerformance = new Performance(testString); //stringPerformance.setCount(numTests); //stringPerformance.start(); for(int i=0; i&lt;numTests; i++) { test.test(testString); } //stringPerformance.end(); //System.out.println(stringPerformance.toString()); } codePerformance.end(); System.out.println(codePerformance.toString()); } overallRuntime.end(); System.out.println(overallRuntime.toString()); System.out.println(); } private static String getTime() { return "a date"; // static value to avoid any caching behavior - we // want to test the algorithm, not the external stuff. // you should use your real getTime method to see real numbers // for your application } private static String getName(){ return "a name"; } private interface Test { public String test(String in); } public class OriginalCode implements Test { public String test(String in) { in=in.replace("[time]",getTime()); //just some methods returning strings in=in.replace("[name]",getName()); return in; } } public class TwoSearches implements Test { public String test(String in) { if (in.contains("[time]")) { in=in.replace("[time]",getTime()); } return in; } } public class SearchOnceAndSubstring implements Test { public String test(String in) { String REPLACEME = "[time]"; int idx = in.indexOf(REPLACEME); if( idx == 0 ) { in=getTime() + in.substring(REPLACEME.length()); } else if( idx &gt; 0 ) { in = in.substring(0,idx) + getTime(); if( idx+REPLACEME.length() &lt; in.length()) { in += in.substring(idx+REPLACEME.length()); } } return in; } } private class Performance { long start = 0; long end = 0; String name = null; public int count = 0; public Performance(String name) { this.name = name; } public void start(){ start = System.currentTimeMillis(); } public void end() { end = System.currentTimeMillis(); } public void setCount(int count){ this.count = count; } /** be sure to call start &amp; stop first **/ public long total(){ return end - start; } public String toString() { return count + "cycles start:"+start+" end:"+end+" total:"+total() + " &lt;---- " + name; } } } </code></pre> <p>And the results of running this code (performance-unrelatedstuff redacted)...</p> <pre><code># #Thu Mar 14 18:45:37 EDT 2013 java.runtime.name=Java(TM) SE Runtime Environment java.vm.version=20.5-b03 java.vm.vendor=Sun Microsystems Inc. java.vendor.url=http\://java.sun.com/ java.vm.name=Java HotSpot(TM) Client VM sun.java.launcher=SUN_STANDARD sun.os.patch.level=Service Pack 1 java.vm.specification.name=Java Virtual Machine Specification java.runtime.version=1.6.0_30-b12 os.arch=x86 java.vm.specification.vendor=Sun Microsystems Inc. user.variant= os.name=Windows 7 java.specification.name=Java Platform API Specification java.class.version=50.0 sun.management.compiler=HotSpot Client Compiler os.version=6.1 java.specification.version=1.6 java.class.path=REDACTED java.vm.specification.version=1.0 sun.java.command=BenchmarkStringStuff sun.arch.data.model=32 java.specification.vendor=Sun Microsystems Inc. java.version=1.6.0_30 java.vendor=Sun Microsystems Inc. sun.desktop=windows sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+mmx pentium i486 i386 i86 10cycles start:1363301137531 end:1363301137531 total:0 &lt;---- BenchmarkStringStuff$OriginalCode 10cycles start:1363301137531 end:1363301137531 total:0 &lt;---- BenchmarkStringStuff$TwoSearches 10cycles start:1363301137531 end:1363301137531 total:0 &lt;---- BenchmarkStringStuff$SearchOnceAndSubstring 30cycles start:1363301137531 end:1363301137531 total:0 &lt;---- overall combined runtime 1000cycles start:1363301137531 end:1363301137546 total:15 &lt;---- BenchmarkStringStuff$OriginalCode 1000cycles start:1363301137546 end:1363301137546 total:0 &lt;---- BenchmarkStringStuff$TwoSearches 1000cycles start:1363301137546 end:1363301137562 total:16 &lt;---- BenchmarkStringStuff$SearchOnceAndSubstring 3000cycles start:1363301137531 end:1363301137562 total:31 &lt;---- overall combined runtime 100000cycles start:1363301137562 end:1363301138108 total:546 &lt;---- BenchmarkStringStuff$OriginalCode 100000cycles start:1363301138108 end:1363301138451 total:343 &lt;---- BenchmarkStringStuff$TwoSearches 100000cycles start:1363301138451 end:1363301138499 total:48 &lt;---- BenchmarkStringStuff$SearchOnceAndSubstring 300000cycles start:1363301137562 end:1363301138499 total:937 &lt;---- overall combined runtime 1000000cycles start:1363301138499 end:1363301143663 total:5164 &lt;---- BenchmarkStringStuff$OriginalCode 1000000cycles start:1363301143663 end:1363301146784 total:3121 &lt;---- BenchmarkStringStuff$TwoSearches 1000000cycles start:1363301146784 end:1363301147190 total:406 &lt;---- BenchmarkStringStuff$SearchOnceAndSubstring 3000000cycles start:1363301138499 end:1363301147190 total:8691 &lt;---- overall combined runtime </code></pre> <p>The winner is SearchOnceAndSubstring, at a consistent order of magnitude faster than the others.</p> <p>There may be other optimizations by playing directly with the char arrays, but I suspect that SearchOnceAndSubstring would be very close, with most of the time lost on the string concatenation. I'll leave performance testing of that as an exercise for the reader.</p>
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    1. COthank you, i'll use searchOnceAndSubstring then.
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    2. COwill the algorithm work if in is containing [time] more than once? it doesn't seem like...
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    3. COCorrect, the code I posted will replace only the first instance of "[time]" - just like the code you posted, using replace() rather than replaceAll(). You'll need to modify the code if you want different behavior.
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