출처

• Class System: https://docs.oracle.com/javase/6/docs/api/java/lang/System.html
• Class PrintStream: https://docs.oracle.com/javase/6/docs/api/java/io/PrintStream.html
• field out: https://docs.oracle.com/javase/6/docs/api/java/lang/System.html#out
• https://www.javatpoint.com/system-out-println-in-java