Questions and Answers
What is a variable?
A variable provides us with named storage that our programs can manipulate.
What is a data type in Java?
It is one of Java's datatypes.
What is the name for the variable itself?
Variable
How many kinds of variables does Java have?
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What are local variables?
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Local variables are automatically initialized to a default value.
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When are instance variables created?
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What keyword is used to declare a class variable?
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Class variables have multiple copies per class.
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What happens to local variables when the method or block is exited?
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What is the default value for instance variables of type number?
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How can static variables be accessed?
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Study Notes
Variables in Java
- A variable is a named storage that programs can manipulate.
- Each variable has a specified type affecting its memory layout, value range, and applicable operations.
Data Types
- Java supports various data types crucial for defining the nature of variables.
Variable Naming
- The name given to each variable is referred to as the variable.
Types of Variables
- Java has three kinds of variables:
- Local variables
- Instance variables
- Class/static variables
Local Variables
- Declared within methods, constructors, or blocks, local variables exist only during execution of that method, constructor, or block.
- They cannot utilize access modifiers and have no default values, thus requiring initialization before use.
- Visibility is limited to the method, constructor, or block they are declared in.
- Local variables occupy stack memory.
Instance Variables
- Declared within a class but outside any methods, constructors, or blocks.
- Created when an object is instantiated using the
new
keyword, and are destroyed when the object is no longer in use. - Hold values referenced by multiple methods or represent the state of an object.
- May be declared before or after use and can have access modifiers.
- Typically designated as private but can be made visible to subclasses.
- Instance variables have default values (0 for numbers, false for Booleans, and null for object references) and can be initialized during declaration or in constructors.
Class/Static Variables
- Declared with the
static
keyword in a class, outside methods, constructors, or blocks, resulting in a single copy regardless of object instantiation. - Often declared as constants, which are public/private, final, and static — they maintain their initial value.
- Created upon program start and destroyed when the program terminates.
- Visibility follows similar rules to instance variables, with many being public for user access.
- Default values mirror those of instance variables; can also be initialized in static initializer blocks.
- Accessed through ClassName.VariableName if public; constant variable names are in uppercase when declared as public static final.
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