Issues
Expressions, Administrative Objects, and Class Methods
Lecture 4 Due Today
Homework 0
New Assignments
Program 1 Assigned
First real programming assignment WILL BE TIME CONSUMING! WORK OVER THE LONG WEEKEND!
From Last Time
Types, Variables, Assignment Example:
float testAvg; testAvg = 98.43;
Assignment Identifier
Why Expressions?
How do we find the test average? Example:
int test1, test2, test3; float testAvg; test1 = 95; test2 = 84; test3 = 90; testAvg = WHAT GOES HERE?
=
Expression Expressions are much more than just literals!
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Expressions
Expression
Combination of one or more operands and operators
Common Arithmetic Operators
Operation
Addition Subtraction Multiplication Division Remainder
Operands
Data to be operated on Constants, Variables, Literals, Objects,
Java Numeric Example Symbol (Not Java)
+ * / % 17 + 2 = 19 10 5 = 5 3 * 4 = 12 12 / 4 = 3 10 % 4 = 2
Operators
The operation to be performed Addition, Subtraction, Division,
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Operands and Data Types
Type of operands determines type of expression result Operand Types Both Integers Both Reals Mix of Reals/Integers Result of Expression Type Integer Real Real
Example Expressions
Expression
4+3 4.0 3 4.0 * 3.0 4 / 3.0 4/3 4.0 / 3.0
Result
7 1.0 12.0 1.33 1 1.33
Longer Expressions
Combine multiple operations to form long expressions int test1, test2, test3, sum; float testAvg; test1 = 95; test2 = 84; test3 = 90; sum = test1 + test2 + test3; testAvg = sum / 3.0;
Order of Operations
int num1; num1 = 10 + 10 / 2; What is the value of num1? If addition is first, 10 If division first, 15
Order of Operations
Each operator has a precedence level Operations performed in order of level
Like algebra, * before +
Javas Precedence Levels
Precedence Level 1 Operator + * / % + + = Operation Unary Plus (Positive) Unary Minus (Neg) Multiplication Division Remainder Addition Subtraction String Concatenation Assignment
On same level, operations performed left-to-right Parenthesis used to change order
Example: (5+5)*2 5+5*2
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3 4
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Test Average
int test1, test2, test3; float testAvg; test1 = 95; test2 = 84; test3 = 90; testAvg = (test1+test2+test3)/3.0;
2 Minute Exercise
What value is returned from each expression?
5+7/2 (5+7)/2 (5+7)/2.0 5.0+7/2 5.0+7.0/2.0 (5+4+3+2+1)/2 6%5+4%3+2%1
Back to Data Types
Recall, not all integers are the same
byte, short, int, long
Data Type Conversion
Values can be converted from one type to another Three ways
Assignment Conversion Arithmetic Promotion Casting
Not all real numbers are the same
float, double
Difference is number of bytes used to store data More bytes more possible values
Assignment Conversion
Occurs when a value of one type is assigned to a variable of another Example:
float money; int dollars = 5; money = dollars;
Value of dollars becomes a float
Arithmetic Promotion
Occurs automatically needed to perform an operation Example:
float result, sum = 24.0; int count = 5; result = sum / count;
Value of count becomes a float
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Casting
Programmer specifically requests conversion Casting requested with type name in parentheses Example:
float avg; int total, count; avg = (float)total / count;
Value of total becomes a float, so avg doesnt use integer division!
Widening / Narrowing
Conversions can change precision of data Widening: Added Precision
Example (8 bit byte to 32 bit int): byte grade = 100; int widerGrade = (int)grade;
Narrowing: Lower Precision
Example (16 bit short to 8 bit byte): short roadTripMiles = 129; byte miles = (byte)roadTripMiles; CAUTION: Narrowing is dangerous! byte has range [-128,127]. 129 becomes -127!
Review
So far
Using objects Variable declarations Assignment statements Expressions Type Conversions
Creating Objects
Variables can hold one of two things
A primitive value A reference to an object
Variables declared with a type or class
int String
Next?
Creating Objects
Examples:
int myOffice; String myName;
Variable Initialization
Variable values need to be initialized Unitialized Examples:
int myOffice; String myName;
new Operator
Objects are instantiated via the new operator An object is an instance of a class
My car is an instance of a Honda Civic
Initialized Examples:
int myOffice = 365; String myName = new String(Dave);
After the new operator, a constructor initializes the object
Constructor has same name as the class String myName = new String(Dave);
Constructor
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Instantiated Objects
After instantiation, dot notation to call methods Remember System.out.println()? Example for String:
int size; String myName = new String(Dave); size = myName.length();
The String Class
String(String str);
Constructor to create new String objects String myName = new String(Dave);
char charAt(int index);
Returns the character at a given position char lastLetter = myName.charAt(3); Index of letters starts with 0! If n letters in a string, last letter is at position n-1.
More String Methods
boolean equals(String str);
Returns true if two strings have the same characters boolean result = myName.equals(aStr);
String Declarations
Standard declaration uses the constructor
String myName = new String(Dave);
int length()
Returns the number of characters in a string int size = myName.length();
String objects are so common, Java provides a shorter declaration
String = myName Dave;
More String methods in Figure 2.8 Full class reference in Appendix M
2 Minute Exercises
With more tools, problems get harder From here until end of semester, work in groups of 2 or 3 Each group should have a spokesperson After the exercise, Ill ask the spokesperson for each group for their answer
2 Minute Exercise
What output is produced by the following?
char result; int size; String myName = new String(Dave); size = myName.length(); result = myName.charAt(size%2); System.out.println(result); result = myName.charAt((size+1)%2); System.out.println(result);
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Class Libraries
Java programs use lots of classes A Class Library is a set of classes to support program development Class libraries can provide methods that are very helpful, but are not technically part of the Java language For example, the String class is not part of the Java language!
Class Libraries
The String class is part of the Java standard class library The standard class library contains classes that can be used in any Java IDE The System class we use for output is also in the standard class library
Packages
A library may contain several groups of classes Each group of classes is a package
Package Library Class
More Packages
The String class is in the java.lang package The System class is in java.lang as well You can create your own packages and libraries!
More Packages
Classes you write for your assignments will be in their own package
The import Declaration
Classes in java.lang are automatically available in Java programs
String, System, many more
To use classes in other packages, there are two options
Fully quality the class name Import the package or class
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The Random Class
The class Random is used to generate random numbers
Random numbers are needed in many programs Example: Card game to shuffle the deck
Importing Random
Avoids fully qualifying a class name Example:
import java.util.Random;
Random is in the java.util package
Not automatically available
After importing a class, you can reference it by its unqualified name (i.e. Random) To import a whole package:
import java.util.*;
Two options
You can reference the class as java.util.Random Or, you can use an import statement
Partial list of packages in standard library: page 94
The Random Class
Random();
Constructor to create new Random objects Random generator = new Random();
More Random Methods
int nextInt();
Returns a random integer Uniform distribution across all ints int val = generator.nextInt();
float nextFloat();
Returns a random float in range [0,1]
float val1 = generator.nextFloat();
int nextInt(int max);
Returns a random int in range [0,max-1] int val1 = generator.nextInt(5);
Examples: Random Class
Assuming weve imported the Random class: Random generator = new Random(); int num1, num2; float num3; // Generate a random integer num1 = generator.nextInt(); // Generate a random integer from 0 to 9 num2 = generator.nextInt(10); // Generate a float from [0-1] num3 = generator.nextFloat();
Invoking Class Methods
Some methods are invoked without instantiating an object Known as Static Methods or Class Methods Example classes with static methods:
Math: some basic math functions Keyboard: user input
Call class methods using dot notation from the class name
Math.cos(3.14159);
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Example: HondaCivic
Define a class of HondaCivic Create two objects
HondaCivic johnsCar = new HondaCivic(); HondaCivic janesCar = new HondaCivic();
Math Class
List of common methods in Figure 2.13 Full list in Appendix M Some provided functions
abs cos, sin, tan acos, asin, atan pow sqrt
All Civics have same gas tank size
Static method of class HondaCivic HondaCivic.getGasTankSize();
Each object has different amount in the tank
Regular method, called from a specific object johnsCar.getGasTankContents();
Math Class Examples
float val1, val2, a, b, c; // Checking for close numbers if (Math.abs(val1val2) < 0.1) then System.out.println(Close!); // Pythagorean theorem val1 = Math.pow(b,2)+Math.pow(c,2); a = Math.sqrt(val1);
Keyboard Class
Keyboard provides static methods to help read input from the keyboard Hides details of input: encapsulation Not part of the Standard Library! Written by book authors Lewis and Loftus Part of package cs1 Youll need this for Program 1!
Installation instructions on Blackboard
Keyboard Class Methods
No constructor! Static methods static static static static static static static static static boolean readBoolean() byte readByte() char readChar() double readDouble() float readFloat() int readInt() long readLong() short readShort() String readString()
Example Keyboard Program
import cs1.Keyboard; public class HelloUser { // Asks the user for their name, then // says hello public static void main(String[] args) { String userName; System.out.print(What is your name?); userName = Keyboard.readString(); System.out.println(Hello, + userName); } }
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2 Minute Exercise
Write a code fragment to do the following
Asks a user to enter two integers Prints out the value of: num1num2
JBuilder Demos
Expressions Using the Keyboard class Using the Debug feature of JBuilder
For Next Class
Reading: Sections 3.0-3.3 Get started on Program 1 and bring your questions! No class on Monday! Enjoy the long weekend. But use the weekend to work on Program 1!
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