1. CS2011 Introduction to Programming I Elementary Programming
Chengyu Sun
California State University, Los Angeles

2. Overview Variables and Constants More operators
Data types
More operators
And Math.pow()
More about literals (i.e. values)
Type mismatch and conversion
Input using Scanner
Put everything together

3. Example: Circle Measurements
Write a program that displays the diameter, perimeter, and area of a circle that has a radius of 6.5

4. CircleMeasurements.java public class CircleMeasurements {
  public static void main(String args[]) {
    System.out.println("Diameter is " * 2);
    System.out.println("Perimeter is " + 2 * * 6.5);
    System.out.println("Area is " * 6.5 * 6.5); }

5. Problem with CircleMeasurements.java
public class CircleMeasurements {
  public static void main(String args[]) {
    System.out.println("Diameter is " * 2);
    System.out.println("Perimeter is " + 2 * * 6.5);
    System.out.println("Area is " * 6.5 * 6.5); }
Repeating the same value multiple times is bad
Tedious to type
Easy to make mistake (i.e. typo)

6. Use a Variable double radius = 6.5;
Type
Variable
Name
Variable
Value
double radius = 6.5;
System.out.println("Diameter is " + 2*radius);
System.out.println("Perimeter is " + 2* * radius);
System.out.println("Area is " * radius*radius);

7. Why Repeating Variables Is Better
Variable name is more descriptive, which makes the code more readable, thereby easier to understand and debug
Variable is more reusable, e.g. set radius to a different value and use the same formula
Compiler/IDE can help finding typos

8. Create A Variable double radius = 6.5; double radius; radius = 6.5;
// Declare a variable
// Assign a value to a variable
Variables can be declared anywhere in a method
A variable can only be used after it’s declared

9. Variable Declaration double radius; Data Type Name Computer Memory
Compiler allocates a space in computer memory based on the size required by the data type
Name will be used to refer to whatever value stored in the space

10. Data Types in Java Primitive types Class types
E.g. int, float, double, boolean
Type name starts with a lowercase letter
Class types
E.g. String
Type name starts with a uppercase letter

11. Primitive Numerical Types
Name
Range
Size
byte
-27 to 27-1
1 byte
short
-215 to 215-1
2 bytes
int
-231 to 231-1
4 bytes
long
-263 to 263-1
8 bytes
float
Very large
double
For integer
numbers like
-2, 100
For numbers with
a fraction, e.g.
-2.5,
When unsure, use int and double

12. About float and double float: single-precision floating point numbers
double: double-precision floating point numbers
Because they use limited storage space to represent infinite possible values, some values may not be represented exactly (though usually they are accurate enough)
double is more accurate than float

13. Assignment Statement radius = 6.5;
Assignment operator
Assign the value on the right to the variable on the left
Right-hand side can be an expression
= is assignment, not equals

14. Some Other Forms of Variable Declaration/Assignment
int a, b, c, d;
double x, y=2.5, z;
a = b = c = 10;
d = a + b + c;
Assignment operator = is right associative (i.e. evaluates from right to left)
Arithmetic operators like + are left associative (i.e. evaluates from left to right)

15. Constant final double PI = 3.14159;
A constant is basically a variable whose value cannot be changed
Usually used to represent some value that should remain unchanged throughout a program
Naming convention
All capital letters
Use _ to concatenate multiple words

16. More About Numerical Operators
+, -, *, /
+ is also used for string concatenation
The remainder (a.k.a. modulus) operator %
The result is the remainder (i.e. the amount left over) after a division, e.g. 9%2=1
Example: PrintDigits.java

17. Associativity and Precedence
Precedence determines the order of evaluation of different operators
Associativity determines the order of evaluation of operators with the same precedence
For example, what's the result of the following expression??
1 + 2 * * 5 % 6 - 7

18. Convenience Operators …
Certain operations are used so often that they get their own operators
Use the value of a variable, does some calculation, then assign the result back to the variable
Increment and decrement by 1

19. … Convenience Operators
n = n + 10
n = n – 10
n = n * 10
n = n / 10
n = n % 10
n += 10
n -= 10
n *= 10
n /= 10
n %= 10
++n
n++
Pre-increment
n = n + 1
Post-increment
--n
n--
Pre-decrement
n = n - 1
Post-decrement

20. Exercise: Pre/Post Increment/Decrement
int a = 1, b = 1;
int c1 = a * 10;
int c2 = 10 + b++ * 10;
a = b = 1;
c1 = a * 10;
c2 = 10 + b-- * 10;

21. Math.pow(x,y) It is a method, not an operator
Used for calculating x to the power of y
For example: 23
Math.pow(2,3) 8

22. Types of Literals "Welcome to Java!" String 10
byte, short, int, or long ??
10.5
float or double??

23. Other Forms of Literals
Type
Notes
10l or 10L
long
10.5f or 10.5F
float
1.1e2 or 1.1E2
double
scientific notation 10
int
decimal (i.e. base 10)
0b10 or 0B10
binary (i.e. base 2)
0710
octal (i.e. base 8)
0x10 or 0X10
hexadecimal (i.e. base 16)

24. Type Mismatch short a; int b; float c; double d; a = 1000000; // ??

25. Type Mismatch short a; int b; float c; double d;
a = ; // error. overflow
b = ; // ok
c = ; // ok
d = ; // ok
a = 3.6; // error. Loss of data
b = 3.6; // error. Loss of data
c = 3.6; // error. Loss of precision
d = 3.6; // ok

26. Implicit Type Conversion
A.K.A. Coercion
Happens when converting a type with a smaller range to a type with a larger range, e.g.
short  int
int  long
float  double

27. Explicit Type Conversion
A.K.A. Casting
Needed when converting a type with a larger range to a type with a smaller range
Must be explicitly specified by the programmer, e.g.
int n = (int) 3.6;

28. Result Types When Mixing Types in an Expression
"The result is " ??
"The result is " + (10 + 2) ??
10 / 3 * 3 ??
10.0 / 3 * 3 ??
10 / 3.0 * 3 ??
10 / 3 * 3.0 ??

29. Circle Measurements with Input
Modify the Circle Measurements example so that it lets a user to enter the value of the radius

30. Console Output and Input
System.out
Console output
System.in
Console input
Unfortunately, unlike System.out which has convenient methods like println(), System.in is pretty difficult to use

31. Use Scanner
So the common practice is use a Scanner to wrap around System.in
Type
Variable
Create a new scanner
Scanner in = new Scanner( System.in );
// Read user input
...
// Close the scanner
in.close();

32. About Import
Scanner is a not class included by the compiler by default, so it has to be imported:
import java.util.Scanner;
It's easy to let Eclipse do the import
Click on the light bulb next to the error mark

33. Commonly Used Methods in Scanner
nextByte()
nextShort()
nextInt()
nextLong()
nextFloat()
nextDouble()
nextLine()
Use Eclipse's autocomplete function

34. Example: Compute Average
Read three values from console and output the average

35. So Where Are We? Comments Project Packages Classes Variables Methods
Statements
Statements
Expressions
Literals
Variables
Operators

36. Put Everything Together – Mortgage Calculator
See the mortgage calculator at bankrate.com
Given
Home price
Down payment percentage
Mortgage term (in years)
Annual interests rate
Display monthly payment

37. How Do We Do It?
Find the monthly payment formula (or in Computer Science terms, the algorithm)
Outline the steps in comments (i.e. pseudo-code)
Implement each step
Verify the result (i.e. test and debug)

38. Readings
Chapter 2 of the textbook (there will be a quiz next week)