JAVA Essentials

Java Development Kit – JDK

JDK is a collection of programming tools (JRE, JVM & Java API’s)
JDK is required on Eclipse IDE to develop java applications and applets
Its primary components are:

    • • javac – Compiler which converts source code to java byte code
      • java – Loader for java application & interprets the class files generated by javac compiler
      • appletviewer – Used to run & debug java applets
      • javadoc – Documentation generator
      • jdb – The Java Debugger
      • javap – To print the Java bytecodes
      • jar – Archiver which packages related class libraries into single jar file.
  • Java Virtual Machine – JVM

    • • A virtual machine capable of executing Java bytecode
      • Also, a piece of software that is implemented on non-virtual hardware and operating system
      • JVM is available for various hardware and operating system combinations, hence enabling the use of same bytecode on different platforms
      • The use of the same bytecode for all JVMs on all platforms allows Java to be described as a “compile once, run anywhere”programming language
      • The JVM verifies all bytecode before it is executed
      • The verification process consists primarily of three types of checks:

      • o Branches are always to valid locations
        o Data is always initialized and references are always type-safe
        o Access to “private” or “package private” data and methods is rigidly controlled
    • • The first two checks take place when a class is loaded and made eligible for use
      • The third step is performed dynamically, when data items or methods of a class are first accessed by another class

  • Java Run-time Environment – JRE

    • • Java’s execution environment
      • Interprets & executes .class files (bytecode)
      • JRE is required on Eclipse IDE to run java applications and applets
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