Java Variable Types
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Instance variables are declared in a class but outside any method or constructor. They are created when an object is instantiated and store that object's unique state. Instance variables can be accessed from any method, constructor or block of the class using the object reference. They have default values of 0, false or null which can be assigned during declaration or in the constructor. Local variables are declared within a method, constructor or block and are only visible within that block. They have no default value so must be initialized before use.
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There are three types of variables in Java: local variables, instance variables, and class/static variables. Local variables are declared within methods, constructors, or blocks and exist only within their scope. Instance variables are declared within a class but outside of methods and constructors, and each object instance has its own copy. Class/static variables are declared with the static keyword, and there is only one copy per class regardless of instances. Each variable type has different scopes, lifetimes, and ways of accessing them.
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Java Variable Types
- 1. In Java, all variables must be declared before they can be used.
- 2. Few examples
- 3. Instance variables Technically speaking, objects store their individual states in “non-static fields”, that is, fields declared without the static keyword.
- 4. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent fromthe currentSpeed of another.
- 5. Instance variables are declared in a class, but outside a method, constructor or any block.
- 6. Instance variables are created when an object is created and destroyed when the object is destroyed.
- 7. Instance variables hold object's state, available to all methods, constructors or blocks of a class, and accessible by using references or objects out the class.
- 8. Instance variables have default values ● numbers : 0 ● Booleans : false ● Object references : null Values can be assigned during the declaration or within the constructor.
- 10. Class Variables (Static Fields) A class variable is any field declared with the static modifier;
- 11. there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated.
- 12. Example : A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number of gears will apply to all instances. static int numGears =6
- 13. Local variables Similar to how an object stores its state in fields, a method will often store its temporary state in local variables.
- 14. The syntax for declaring a local variable is similar to declaring a field
- 15. There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method.
- 16. Access modifiers cannot be used for local variables.
- 17. Local variables are visible only within the declared method, constructor or block.
- 18. There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.
- 20. How we can fix this error? Error: variable mmight not have been initialized m++; 1 error
- 21. Parameters args variable is the parameter to main method. public static void main ( String[ ] args )
- 22. Parameters are always classified as “variables”not “fields”. This applies to parameter-accepting constructs as well.