Variables that refer to objects are called reference variables. They differ from primitive variables in important ways.
It is useful to think of the value held by a reference variable as the address in memory where an object is stored, not the object itself.
Any variable declared with some class type is a kind of reference variable.
When an reference variable is copied to another reference variable using assignment (=) a reference to the object is copied, not the object itself.
It is useful to think of a reference as the memory address at which the object is stored.
The implication is that two or more reference variables may refer to one object in memory if they both hold the same reference.
To break the connection between a reference variable and an object, assign the variable to the special value null. After such an assignment the variable no longer refers to the object as it no longer holds the object’s reference in memory.
When Java is unable to find a reference to an object amongst all its accessible reference variables, Java reclaims the memory for another purpose. This process is called Garbage Collection.
To permit garbage collection to be performed on memory holding an object, assign null to all variables that reference the object.
The StringBuilder class defines a special kind of object that builds large Strings in a memory-efficient manner.
The StringBuilder class implements a number of methods, including append() and toString().
When variables are declared within the scope of a method, they come in to existence during execution of the method, and go out of existence when the method returns.
To ensure a variable persists for longer than the time during which a method is executing, declare that variable within a class but outside all methods.
Variables declared outside a method must be at the top of a class definition.
Variables declared outside a method are accessible from within all methods of a class while the method is executing.
A useful problem solving strategy is to use such variables as a kind of bulletin board to post values to be used by other methods. One method can write a value, while another method can read and use that value.
Never declare a variable outside the scope of a method unless it is absolutely necessary. Variables declared in this manner often lead to bugs that are difficult to find.
