10:Language Fundamentals and Operators

Learning Objectives
After completing this session, you will be able to:
‰  Write Java programs by declaring and initializing variables
Declaring and Initializing Variables: 
 Sample Program
1 public class VariableSamples {
2 public static void main( String[] args ){
3 //declare a data type with variable name
4 //result and boolean data type
5 boolean result;
6
7 //declare a data type with variable name
8 // option and char data type
9 char option;
10 option = 'C'; //assign 'C' to option
11
12 //declare a data type with variable name
13 //grade, double data type and initialized
14 //to 0.0
15 double grade = 0.0;
16 }
17 }
Declaring and Initializing Variables: Coding Guidelines
‰  It is always good to initialize your variables as you declare them.
‰  Apply descriptive names for your variables.
‰  For example, for a student’s grade, the variable name can be given as “grade” or
“studentGrade” instead of giving any other names.
Declare one variable in each line of code.
For example, the variable declarations:
double exam = 0;
double quiz = 10;
double grade = 0;
is preferred over the declaration double exam=0, quiz=10, grade=0;
Try It Out
Problem Statement:
What will happen if you try to compile the code given in the next slide?
Code:
class Digit {
public void add() {
int k;
int s = k + 3;
}
}

How It Works:
The preceding code will not compile. You can declare k without a value, but as soon as you try to
use it, the compiler freaks out.
Tips and Tricks
Provide some important tips on conversion of primitives in Java
Solution:
‰  The three types of conversion are namely assignment conversion, method call
conversion, and arithmetic promotion.
‰  boolean may not be converted to or from any type that is not boolean.
‰  byte and short, cannot be converted to char and char cannot be converted to byte and
short.
Arithmetic promotion:
‰  Unary operators:
If the operand is byte, short or char {
convert it to int;
}
else {
do nothing; no conversion needed;
}
‰  Binary operators:
If the operand is double {
all double; convert the other operand to double;
}
else if one operand is float {
all double; convert the other operand to
double;
}
else if one operand is float {
all double; convert the other operand to double;
}
else {
all int; convert all to int;
}
Summary
‰  Local variables (method variables) live on the stack.
‰  Objects and their instance variables live on the heap.
‰  Scope of a variable refers to the lifetime of variable.
‰  There are four basic scopes:
‰  Static variables live basically as long as their class lives.
‰  Instance variables live as long as their object lives.
‰  Local variables live as long as their method is on the stack; however, if their method
invokes another method, they are temporarily unavailable.
‰  Block variables (e.g., in a for or an if) live until the block completes.