Variables

• defaults for the variable types

  • byte, short, int, long = 0
  • float, double = 0.0
  • char = \u0000
  • boolean = false
  • reference = null

• All primitive local variables (stack allocated memory space) need to be initialized explicitly, during declaring or seperately before use, otherwise they will raise compile-time error when used.

• All primitive array of local variables (heap allocated memory space) need to be allocated memory space using new operator, otherwise the data elements will not be default to the default values and no allocation of the same will raise to compile-time error.

Variable Type	Category	Example	Remarks
Local Variable	Primitive	int i;	Not initialized, throws compile-time error
Local Variable	Array of Primitives	int[] i;	Not initialized, throws compile-time error(no memory allocated)
Local Variable	Array of Primitives	int[] i = new int[1];	defaulted elements to 0
Local Variable	Wrapper	Integer i;	Not initialized, throws compile-time error
Local Variable	Array of Wrapper	Integer[] i;	Not initialized, throws compile-time error(no memory allocated)
Local Variable	Array of Wrapper	Integer[] i = new Integer[1];	No Error, defaulted elements to null
Local Variable	objects	CustomObject i;	Not initialized, throws compile-time error
Local Variable	Array of objects	CustomObject[] i;	Not initialized, throws compile-time error(no memory allocated)
Local Variable	Array of objects	CustomObject[] i = new CustomObject[1];	No Error, defaulted elements to null

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Variable Type	Category	Example	Remarks
Instance/Static Variable	Primitive	int i;	Not initialized, no Error, Default value to 0
Instance/Static Variable	Array of Primitives	int[] i;	Not initialized, no memory allocated, default value to null
Instance/Static Variable	Array of Primitives	int[] i = new int[1];	No Error, defaulted elements to 0
Instance/Static Variable	Wrapper	Integer i;	Not initialized, no memory allocated, default value to null
Instance/Static Variable	Array of Wrapper	Integer[] i;	Not initialized, no memory allocated, default value to null
Instance/Static Variable	Array of Wrapper	Integer[] i = new Integer[1];	memory allocated, default elements value to null
Instance/Static Variable	objects	CustomObject i;	Not initialized, no memory allocated, default value to null
Instance/Static Variable	Array of objects	CustomObject[] i;	Not initialized, no memory allocated, default value to null
Instance/Static Variable	Array of objects	CustomObject[] i = new CustomObject[1];	memory allocated, default elements value to null

Types

• Local Variables
  • MUST be initialized before use, otherwise compile-time error
  • created in the stack memory area, for references objects, the object lives in heap but the type lives in stack
  • final is the ONLY modifier which can be used with local variables
• Instance Variables
  • eligible for GC, once they go out of scope.
  • scoped to object
  • ONLY accessed/modified within a non-static block
  • automatically initialized to default values
  • if declared as final, MUST be initialized in decleration or constructor or instance initializer block
• Static Variables
  • Global var, available to all programs running in the JVM instance
  • loaded into memory, when the class loads, even before the object creation
  • NOT participate or eligible for GC
  • automatically initialized to default values
  • if declared as final, MUST be initialized in decleration or static initializer block
• Order of Initialization : Static variable > Static Initializer block > Instance Variable > Instance Initializer block > Instance Constructor

Reference

• https://vivek-dev.firebaseapp.com/posts/local-variable-initialization/
• https://www.baeldung.com/java-primitives
• https://www.baeldung.com/java-stack-heap