BM214E Object Oriented Programming Lecture 4

BM214E Object Oriented Programming Lecture 4

Computer Numbers Integers (byte, short, int, long) whole numbers exact relatively limited in magnitude (~10 19 ) Floating Point (float, double) fractional often approximations (0.33333) larger magnitude (~10 308 ) actually holds signed mantissa & exponent 6.023 x 10 23

List of Data Types Primitive Type Default Value boolean false char '\u0000' (null) byte (byte) 0 short (short) 0 int 0 long 0L float 0f double 0d void N/A Note: At times, the Java Language Specification refers to void as a primitive, though other parts (e.g., s. 14.7) say void is not a primitive as in C/C++. One cannot cast to a void type in Java.

Data Type Ranges Type Size Min Max Default boolean char 1 false* true* 16 false '\u0000' (null) byte 8-128 127 (byte) 0 short 16-32,768 32,767 (short) 0 int 32-2,147,483,648 2,147,483,647 0 long 64-9,223,372,036,854,775,808 9,223,372,036,854,775,807 float 32 Approx ±3.4E+38 with 7 significant digits 0.0F double 64 Approx ±1.7E+308 with 15 significant digits 0.0D void * Not truly min and max. 0L

Understanding Casting Last class, we covered some basics of casting. Sometimes an explicit cast is required, sometimes it s not. OK! Compiles ERROR Explicit cast needed int x = 10; float f1 = x; float f2 = 11.234f; int y = f2; Which one requires a cast to compile? Why?

Understanding Casting To understand when we need casting, let s look closely at an example. Let s work with these two types and identifiers: int x; char c; The real question is, How BIG are these data types in memory?

Sizes of Data Types A char is 2 bytes, and an int is 4 bytes. So when we code the following: int x; char c; We get: Symbol Picture of Memory x 0000 0000 0000 0000 0000 0000 0101 0110 c 0000 0000 0100 0001 Each block is one byte

So Will a char fit into an int? Will an int fit into a char? Symbol Picture of Memory x c

Explicit Casting: Intentional Loss of Precision int x = 45; char c = A ; c = (char) x; // cast needed Symbol Picture of Memory x c cast needed!

Testing Yourself What happens when we do this: int someint = 2000; long longnumber; longnumber = (long) someint; Is that cast legal? Is that cast required?

Another Example float f; int i, j; i = 9; j = 2; f = i / j; f = (float) i / j; f = i / (float) j; Returns 4 because of integer truncation! If that is not what was intended, Three possible solutions f = (float) i / (float) j;

Another example We also have to consider how Java preserves the sign of a value through casting. Consider the bytes used for an int. The highest bit is used for a sign (plus or minus, through a 0 or 1). int Sign bit 31 bits for data

Casting and Sign Casting is not just chopping one data type into another. The VM doesn t merely throw away half of the information. The sign is preserved. Preserves sign bit byte int

Another Example Suppose we had some very, very old data, where the date was expressed in a int (four byte) format, such as: 19231029 1923 10 (October) 29 This is very crude, but some old data source (e.g., tape archives) might have this format.

public class CutData { public static void main(string[] args) { int idate = 19231029; // this is how it was read in byte byday = (byte) (idate % 100); System.out.println ("The day was " + byday); idate = idate / 100; byte bymonth = (byte) (idate % 100); System.out.println ("The month was " + bymonth); idate = idate / 100; short syear = (short) idate; Explicit Cast needed System.out.print ("The year was = " + syear); } } // CutData

When working in the opposite direction (accumulating bytes and shorts into an int), no casting is needed, because we do not lose precision or information. public class PackDate { public static void main(string[] args) { byte month = 10; byte day = 29; short year = 1923; int date = year * 10000 + month * 100 + day; } System.out.println ("The date = " + date); } // PackDate

Shorthand Operators counter = counter + 1; OR counter++; counter = counter - 1; OR counter--; counter = counter + 2; OR counter = counter * 5; OR counter+=2; counter*=5; Last two examples: perform operation first (e.g. counter+2) then performs the assignment. Quick Review

Caution Before we can talk about methods in Java, we need to look at programming languages in general, from a very high level. Don t be alarmed by some of the syntax on the following slides. The big picture is what s important, not the coding details...

Programming Paradigms Procedural Programming Imperative assignment used to create state, and procedures manipulate state. e.g., C, Assembly, Pascal int y; int x = 3; y = manipulatedata(x); Functional Programming Functions (procedures that do not depend on outside data) are used to provided data. e.g., Lisp (defun check-member (input-item input-list) (cond ((null input-list) nil) ((equal input-item (first input-list)) T) (T (check-member input-item (rest input-list)))))

Programming Paradigms Object-Oriented Programming All data exists in objects; interaction occurs only between objects. Even numbers are objects that know how to add themselves. e.g., SmallTalk: array array := Array new: 5. rect := 0@0 corner: 8@9. 1 to: array size do: [ :item rect origin: item@item. array at: item put: rect copy ]. There are other paradigms, but these three help explain where Java comes from Where does Java fit? Object-oriented, but not 100% OO, since it contains primitives, and tolerates some (hopefully small) degree of procedural programming.

Java Methods There exists in Java a single construct, the method, for both procedures and functions: when a procedure is called for, specify the return type void before method name public void printhelloworld( ) { System.out.println( Hello World! ); } // printhelloworld Note the comment Note: All methods must have parentheses for parameters... even if no parameters!

Java Methods Single construct (method) for both procedures and functions: when a function is called for, specify the appropriate return type before method name public float average (float num1, float num2, float num3) { float answer; answer = (num1 + num2 + num3) / 3; return (answer); } // of average

A Java requirement: Writing Methods: A Larger Look --Each method belongs to an object (or class). --It must be unambiguous which object or class that is when a method is called. --To run an application program, there must be a class whose name is that of the program and that class must have a method called main. for command line parameters visible to all nothing returned public static void main (String[ ] argv) a class method, not an instance method method name

class A { public static void main(... } class B { public static void main(... } class C { public static void main(... } Each class may have its own main method. You pick the one you wish to run when invoking the JVM. This fact becomes critical when we learn to write debug test mains.

Method Signatures The signature of a method consists of the name of the method and the number and types of formal parameters to the method. A class may not declare two methods with the same signature, or a compile time error occurs. --Java Language Specification s.8.4.2 Method overloading occurs where identically named methods have subtle variations in the method parameters. public int getcube(int num){ return num*num*num; } public int getcube(float num){ return (int)(num*num*num); } public int getcube(double num){ return (int) (num*num*num); } Java lets you overload instead.

Methods: Common Mistakes public float average (float num1, float num2, float num3); { float answer; answer = (num1 + num2 + num3)/ 3; return (answer); } // average Note ending semicolon -- could be viewed as abstract method (more on abstract methods later) -- results in unhelpful error message about abstract methods (more on this later) -- EASY mistake to make!

Where Do Methods Go? Just like the main method we saw last time, any method we create must appear in classes, as either class or instance members. More on creating classes and objects shortly... For now, just know that methods belong in a class. i.e. They are defined inside a class.

OO Programming Note Already, we ve covered one of the most important aspects of Object Oriented (OO) programming: Data and methods belong together in a class. Right now, it s sufficient that you know that variables and methods belong in classes. Later, we ll see how this enables us to encapsulate state (the variables) with behavior (the methods).

Questions?

Conditionals

Java: Decision Statements if (condition) single statement; else single statement; or: if (condition) { statements; } else { statements; }

Examples Java: What happens here? int testgrade = 65; boolean passing = true; if (testgrade < 60) passing = false; System.out.println( Is + testgrade + passing? +passing);

Boolean and Relational Operators Boolean: Java: AND && OR NOT! Relational: equal to == not equal to!= less than < less than or equal to <= greater than > greater than or equal to >= Note: Assignment =

Example if (enrolled && passing) { // etc. } Java also supports short-circuiting, where only part of a boolean will be evaluated, as necessary: if (enrolled(studentnum) getpassing(studentnum)) { // etc. }

A bit of code public static void main(string args[]) { int quiz1 = 42; int quiz2 = 99; if(ispassing(quiz1) && ispassing(quiz2)) System.out.println( Passed both quizzes ); } public static boolean ispassing(int testgrade) { boolean passing = true; if (testgrade < 60) passing = false; System.out.println( Is + testgrade + passing? + passing); return passing; }

Final Answer? Is 42 passing? false a Is 42 passing? false Is 99 passing? true b

Multiple Selections via switch Use if construct for one selection. Use if/else construct for double selection. Use switch construct for multiple selection. (e.g., situations appropriate for if-elseif-elseif-else) Note: Useful when making a selection among multiple values of the same variable. Not useful when selecting among values of different variables.

Switch switch (<variable>) { case <value>: // whatever code you want break; case <value>: // whatever code you want break; default: // whatever code you want break; }

Multiple Selections via switch Note the optional default case at the end of the switch statement. It is optional only in terms of syntax. switch (inumber) { case 1: System.out.println ( One ); This would break; work without case 2: the default, System.out.println ( Two ); but would break; be poor case 3: technique System.out.println ( Three ); break; default: System.out.println( Not 1, 2, or 3 ); break; // Needed??? } // switch In practice you should always include a default case statement.

How many days? if(month == 4 month == 6 month == 9 month == 11) numdays = 30; else if(month == 2) { numdays = 28; if(leap) numdays = 29; } else numdays = 31;

Switch switch (month) { case 4: case 6: case 9: case 11: numdays = 30; break; case 2: numdays = 28; if(leap) numdays = 29; break; default: /* Good idea? */ numdays = 31; }

Multiple Selections via switch--notes The switch statement can be used with the following types: int, char, short, byte String (Java SE 7 and later) The case values must all be of the same type. The case values must all be FINAL constants.

switch (grade) { case A : case a : countofagrades++; break; case B : case b : countofbgrades++; break; case C : case c : countofcgrades++; break; case D : case d : countofdgrades++; break; case F : case f : countoffgrades++; break; default: } same System.out.println( Invalid grade ); break; if (grade== A grade== a ) countofagrades++; else if (grade== B grade== b ) countofbgrades++; else if (grade== C grade== c ) countofcgrades++; else if (grade== D grade== d ) countofdgrades++; else if (grade== F grade== f ) countoffgrades++; else System.out.println ( Invalid grade ); Multiple Selections via switch (assume these variables exist and have value)

Before we go too far... We often take liberties with good coding practice just to fit material onto a slide. Your coding style should reflect clearly and unambiguously what the code is supposed to do. Keep in mind your two audiences The machine Other programmers

If Style 1 if(<boolean expression>) { // code if true } else { // code if false }

If Style 2 if(<boolean expression>) { // code if true } // comment else { // code if false } // comment

Conditional Assignment boolean b; int count; <boolean>? <true condition> : <false condition> b = checkcompletion(); count = (b)? 3 : 1; Must resolve to boolean If true...... if false Note: This is not any faster. It s less readable. It exists only for recovering C hackers

A Semi Partial Conditional Summary Control structures Two kinds of conditional (summary) if {... } else {... } branch on boolean expression switch (...) { case...; break; default:... } branch on constant value don t forget to break!!

Questions?