1. Variable is a name of memory location where data is stored. 2. The value of variable can vary, means value can be changed. There are three types of variables : 1. Local Variables : 1.1 DECLARED : inside methods, constructors or blocks. 1.2 SCOPE : within the methods, constructors or blocks but not outside of them. 1.3 MEMORY ALLOCATION : when method, constructor or block is invoked variable is allocated; when exits, variable gets destroyed. 1.4 STORED MEMORY : stack memory 1.5 DEFAULT VALUES : doesnt have any default value, so it should be assigned before use. 1.6 ACCESS MODIFIERS : public, private or protected access specifier cannot be used. 2. Instance Variable : 2.1 DECLARED : in a class but outside methods, constructors or blocks. 2.2 SCOPE : inside the class, within all methods, constructors or blocks. 2.3 MEMORY ALLOCATION : when object is created, variable allocated memory; when object destroyed, memory releases. 2.4 STORED MEMORY : heap memory 2.5 DEFAULT VALUES : have default values eg. int has 0 value, boolean false, String null etc. 2.6 ACCESS MODIFIERS : any access modifiers can be used. 2.7 HOW TO ACCESS : a) can be accessed directly by calling the variable name inside the class.       b) in static methods they should be called using the fully qualified name, ie objectRefName.variable_name. 3. static variable : 3.1 DECLARED : with static keyword in a class but outside methods, constructors or blocks. 3.2 SCOPE : similar to instance variable ie inside the class, within all methods, constructors or blocks including static part 3.3 MEMORY ALLOCATION : when we run program and .class file is loaded, variable allocated; when class file unload, variable gets destroyed. 3.4 STORED MEMORY : non-heap memory or static memory 3.5 DEFAULT VALUES : have default values eg. int has 0 value, boolean false, String null etc. 3.6 ACCESS MODIFIERS : any access modifiers can be used. 3.7 HOW TO ACCESS : a) directly       b) by using class name       b) by using object reference name