Logging
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        Do Not use System.out for logging 
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        Use a logging framework 
How:
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        acquire logger: private static final LOG = Log.getLogger(Blabla.class); 
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        LOG.debug("bla"); 
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        LOG.info("bla"); 
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        logger can be configured for different log levels, and separately for each package / subpackage / class 
log4j
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          Commonly used 
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          Only use in new projects if you have previous experience with it! 
commons-logging
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          apache project 
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          auto-detects logger from classpath 
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          use in libraries 
java.util.log
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          built-in since Java 1.4 
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          Use in all user tools / gui projects