COMP 110 Introduction to Programming Mr. Joshua Stough September 10, 2007.
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Transcript of COMP 110 Introduction to Programming Mr. Joshua Stough September 10, 2007.
Announcements
• Tutoring Tuesdays 6-9 Dey Hall 2nd floor
• Feedback on programs, options:– Paper copies due in class.
Review
System.out.println ("I said, \"Hi There!\"");I said "Hi There!"
System.out.print ("Hello\n");System.out.println ("World");HelloWorld
System.out.println ("Hello\rWorld");HelloWorld
System.out.println ("Hi\b\bHello");Hello
System.out.println ("Hello\tWorld");Hello World
Review• In the Java programming language:
– a program is made up of one or more classes– a class contains one or more methods– a method contains program statements
• A Java application always contains a method called main
• Source code saved in a file with the extension .java
• File must have the same name as the class with the main method
• Use the dot (.) operator to call methods:Math.round
Reviewimport statements if any
public class ClassName{
declare named constants and/or stream objects
public static void main (String[] args) throws IOException
{variable declarationsexecutable statements
}}
Review• Output
System.out.print (stringExp);System.out.println (stringExp);– print vs. println
• Comments// this is a one-line comment– “comments out” the rest of the line after ///* this is a multi-linecomment */– “comments out” everything between /* and */
Review Questions
What is stored in num?int num = (int) Math.round (12.7);
What is printed to the screen? Assume the user enters 10 and that keyboard is already defined.System.out.print ("Enter a number: ");int num = Integer.parseInt
(keyboard.readLine());System.out.println (num * num);
13
Enter a number: 10100
Assignment Operators
• Just for convenience
count += 5; // count = count + 5;
count -= 5; // count = count - 5;
count *= 5; // count = count * 5;
count /= 5; // count = count / 5;
Assignment Operators• The right hand side of an assignment operator
can be a complex expression
• The entire right-hand expression is evaluated first, then the result is combined with the original variable
• Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);
Questions
• What is stored in total and count in the following statements?
int total = 10, count = 5;total += count++;
int total = 20, count = 3;total /= --count;
String str = "COMP";str += 110;
total count15 6
total count10 2
strCOMP110
Java Variables
• Primitive Variables– primitive data types (int, double, ...)– stores the data in the memory
location
• Reference Variables– stores an address in the memory
location– "points to" another memory location
Objects
• More complex data type than a primitive data type
• Stored in the "other" memory location– is "pointed to" by a reference variable
• Is operated on by special operators called methods
Objects and Classes
• An object's data type is a class• The class contains the data types
that make up the object and what methods can operate on the object
• Examples:– String– Integer– Double
Primitive Variables
int x = 45;
• When the computer sees x, it knows which memory location to look up the value in
Reference Variables
Integer num;
• When the computer sees num, it knows which memory location to look for the address in
• It will read the address in num and look up a value in that memory location
Creating Objects
• We use the new operator to create objects, called instantiation Integer num;num = new Integer(78);
parameter
Review the Terms
• We declare a reference variable of a class type.
• We use the new operator to instantiate an object of that class type.
• We store the address of that object in the reference variable.
Changing the Reference Var
num = new Integer (50);• The address of the newly-created object
is stored in the already-created reference variable num
Garbage Collection
• What happened to the memory space that held the value 78?
• If no other reference variable points to that object, Java will "throw it away"
System.out.println (”Hello World!”);
objectmethod
information provided to the method
(parameters)
Using Objects
• System.out object – represents a destination to which we
can send output
• Example:– println method
dot operator
Questions1. True or False. A primitive variable is a variable that
stores the address of a memory space.
2. The operator is used to create a class object.
3. In Java, the operator is used to access members of a class. It separates the class (or object) name from the method name.
4. True or False. Class objects are instances of that class.
new
dot (.)
False
True
The class String
• String variables are reference variables
• Given String name; – Equivalent Statements:
name = new String("Lisa Simpson");name = "Lisa Simpson";
The class String
• The String object is an instance of class string
• The value “Lisa Simpson” is instantiated• The address of the value is stored in
name• The new operator is unnecessary when
instantiating Java strings• String methods are called using the dot
operator
Common String Methods• String(String str)
– constructor– creates and initializes the object
• char charAt(int index)– returns char at the position specified by index
(starts at 0)
• int indexOf(char ch)– returns the index of the first occurrence of ch
• int compareTo(String str)– returns negative if this string is less than str– returns 0 if this string is the same as str– returns positive if this string is greater than str
Common String Methods• boolean equals(String str)
– returns true if this string equals str
• int length()– returns the length of the string
• String replace(char toBeReplaced, char replacedWith)
– returns the string in which every occurrence of toBeReplaced is replaced with replacedWith
• String toLowerCase()– returns the string that is the the same as this string,
but all lower case
• String toUpperCase()– returns the string that is the same as this string, but
all upper case
String Examples
String str = "Go Panthers!";
System.out.println (str.length());
System.out.println (str.charAt(3));
System.out.println (str.indexOf('!');
System.out.println (str.toLowerCase());
P
12
11
go panthers!
Example Program
• MathStats.java
• Ask the user for 3 integers.• Output the following:
– the 3 numbers– the sum– the average– the sum squared– the square root of the sum
Force Errors
Try the following in MathStats.java to see what error message is displayed:– comment out import statement– comment out throws clause– make sumSquare an int– don't initialize sum– don't cast average to double
(semantic error)
Summary
• assignment operators (+=, /=, ...)• primitive variables vs. reference
variables• objects• dot (.) operator• String class