- The values of
Primitive data types and
Reference data types are
stored differently
(in Java):
- Variables of
Primitive data types
store the actual value
- Variables of Reference data types store the reference (= address) of the memory cells where the actual values are stored
Fact:
- The different way to
store the values of
Primitive data types and
Reference data types give rise to:
- different ways to create variables of Primitive data types and Reference data types
- Variables of
Primitive data types
store the actual value
- Primitive data types:
- Defining (creating)
(and using) a variable of
a primitive data type:
int a; // Define (create) the variable a = 4; // Store value 4 in variableGraphically:
- Defining (creating)
(and using) a variable of
a primitive data type:
- Reference data types:
- Defining (creating)
(and using) a variable of
a reference data type:
int[] a; // Define (create) a REFERENCE variable a = new int[10]; // Create the actual "object" (big variable) a[2] = 4; // Store value 4 in some part of the objectGraphically:
- First we define (create) the
referencevariable:
- Then we create the
actual "object"
(large number of variables):
Notice that:
- The variable a now contains a valid address (of an "object") !!!
- Finally we use
some variable inside the actual object:
Notice that:
- The computer uses the
variable name a to
find the start of the object
- The computer uses the index 2 to locate the portion of the object that it needs to use.
- First we define (create) the
referencevariable:
- Defining (creating)
(and using) a variable of
a reference data type:
- A variable int x stores:
- A reference to an int
- An integer value (correct answer)
- The identifier "x"
- Lots of goodies for every good Java-slave
- A variable BankAccount x
stores:
- A reference to an object of the BakAccount class (correct answer)
- An object of the BakAccount class
- The identifier "x"
- Even more goodies than a mere int x
- Which of the following will always correctly check whether
an object variable obj
contains a
null
reference?
- obj.equals(null)
- This test if obj variable
and the
null variable
are equal
Since null is not a variable, there is an error
- This test if obj variable
and the
null variable
are equal
- null == obj
- Correct answer
This is the same as: obj == null
- Correct answer
- obj = null
- The = operation is
the assignment operation
We need to compare (==)
- The = operation is
the assignment operation
- null.equals(obj)
- This test if
null variable
and the
obj variable
are equal
Since null is not a variable, there is an error
- This test if
null variable
and the
obj variable
are equal
- obj.equals(null)