why do we use properties file in java
.properties Each parameter is stored as a pair of strings, one storing the name of the parameter (called the key), and the other storing the value.
Java package In Java 9, "modules", a kind of collection of packages, are planned as part of Project Jigsaw; these were earlier called "superpackages" and originally planned for Java 7.
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Java virtual machine The aim is binary compatibility. Each particular host operating system needs its own implementation of the JVM and runtime. These JVMs interpret the bytecode semantically the same way, but the actual implementation may be different. More complex than just emulating bytecode is compatibly and efficiently implementing the Java core API that must be mapped to each host operating system.