Java Basics

Java

A famous programming language.

JVM

Short for Java Virtual Memory. The virtual machine, on top of the operating system, which runs the compiled Java code.

JDK

Short for Java Development Kit. The set of tools to compile the Java programs.

Memory is allocated implicitly (variable declaration or new operator) and deallocated implicitly (by the runtime).

Class

A template for a object, giving information about state and actions. State is represented by variables and actions are represented by methods.

Objects can be instantiated using new operator, a unary operator that expects a class.

Object overhead

In a 32-bit system, int type is stored using 4 bytes. But Integer object will require 16 bytes of memory. The reason for this is, to accomodate for the object metadata (such as class pointer, flags, lock, size, etc.)

Common Classes

Vector

Can grow but not shrink.

Hashtable

An array with 2 parts: a hashcode and the actual data. Hashcode is generated from the content of the data. Data is accessed using the hashcode.

Collection

A group of objects, manipulated as a single entity.