Java � Java Basics � Java Program Statements Java Review � Conditional statements � Repetition statements (loops) � Writing Classes in Java Selim Aksoy � Class definitions Bilkent University � Encapsulation and Java modifiers Department of Computer Engineering � Method declaration, invocation, and saksoy@cs.bilkent.edu.tr parameter passing � Method overloading Summer 2004 CS 111 2 Programming Rules of Thumb Introduction to Objects � Learn program patterns of general utility � An object represents something with which (branching, loops, etc.) and use relevant we can interact in a program patterns for the problem at hand � An object provides a collection of services � Seek inspiration by systematically working test that we can tell it to perform for us data by hand and ask yourself: “what am I doing?” � The services are defined by methods in a � Declare variables for each piece of information class that defines the object you maintain when working problem by hand � A class represents a concept, and an object � Decompose problem into manageable tasks represents the embodiment of a class � Remember the problem’s boundary conditions � Validate your program by tracing it on test � A class can be used to create multiple objects data with known output Summer 2004 CS 111 3 Summer 2004 CS 111 4 Java Program Structure Java Program Structure � In the Java programming language: // comments about the class � A program is made up of one or more classes public class MyProgram { � A class contains one or more methods c l as s he ade r � A method contains program statements // comments about the method � Attributes/properties correspond to fields (or public static void main (String[] args) variables) { c l as s body m e t hod he ade r � Behaviors/operations correspond to methods m e t hod body � A Java application always contains a method } called main } Summer 2004 CS 111 5 Summer 2004 CS 111 6
Variables Primitive Data � There are exactly eight primitive data types in � A variable is a name for a location in Java memory � Four of them represent integers: � A variable must be declared by � byte , short , int , long specifying the variable's name and the � Two of them represent floating point type of information that it will hold numbers: � float , double dat a t ype var i abl e nam e � One of them represents characters: int total; � char int count, temp, result; � And one of them represents boolean values: � boolean M ul t i pl e var i abl e s c an be c r e at e d i n one de c l ar at i on Summer 2004 CS 111 7 Summer 2004 CS 111 8 Numeric Primitive Data Arithmetic Expressions � The difference between the various � An expression is a combination of one numeric primitive types is their size, or more operands and their operators and therefore the values they can store: � Arithmetic expressions use the operators: Addition + Type St or age M i n Val ue M ax Val ue Subtraction - byte Multiplication * 8 bi t s - 128 127 short Division / 16 bi t s - 32, 768 32, 767 int (no ^ operator) Remainder % 32 bi t s - 2, 147, 483, 648 2, 147, 483, 647 long < - 9 x 10 1 8 > 9 x 10 1 8 64 bi t s � If either or both operands associated 3. 4 x 10 3 8 wi t h 7 s i gni f i c ant di gi t s float with an arithmetic operator are floating 32 bi t s +/ - double 1. 7 x 10 3 0 8 wi t h 15 s i gni f i c ant di gi t s 64 bi t s +/ - point, the result is a floating point Summer 2004 CS 111 9 Summer 2004 CS 111 10 Division and Remainder String Concatenation � The string concatenation operator (+ ) is used � If both operands to the division operator to append one string to the end of another ( / ) are integers, the result is an integer � The plus operator (+ ) is also used for (the fractional part is discarded) arithmetic addition � The function that the + operator performs 14 / 3 4 e qual s ? depends on the type of the information on 8 / 12 e qual s ? 0 which it operates � If at least one operand is a string, it performs � The remainder operator (% ) returns the string concatenation remainder after dividing the second � If both operands are numeric, it adds them operand into the first � The + operator is evaluated left to right 14 % 3 e qual s ? 2 � Parentheses can be used to force the operation order 8 % 12 e qual s ? 8 Summer 2004 CS 111 11 Summer 2004 CS 111 12
Data Conversions Data Conversions � In Java, data conversions can occur in three � Casting is the most powerful, and dangerous, ways: technique for conversion � assignment conversion � Both widening and narrowing conversions can be � arithmetic promotion accomplished by explicitly casting a value � casting � To cast, the type is put in parentheses in front of � Assignment conversion occurs when a value the value being converted of one type is assigned to a variable of another � For example, if total and count are � Only widening conversions can happen via integers, but we want a floating point result assignment when dividing them, we can cast total : � Arithmetic promotion happens automatically result = (float) total / count; when operators in expressions convert their operands Summer 2004 CS 111 13 Summer 2004 CS 111 14 Creating Objects Creating Objects � Generally, we use the new operator to � A variable holds either a primitive type or a reference to an object create an object � A class name can be used as a type to title = new String ("Java Software Solutions"); declare an object reference variable String title; Thi s c al l s t he St r i ng cons t r uct or , whi c h i s � No object is created with this declaration a s pe c i al m e t hod t hat s e t s up t he obj e c t � An object reference variable holds the � Creating an object is called instantiation address of an object � An object is an instance of a particular � The object itself must be created separately class Summer 2004 CS 111 15 Summer 2004 CS 111 16 Conditional Statements The if Statement � A conditional statement lets us choose � The if statement has the following which statement will be executed next syntax: � Therefore they are sometimes called The condition m us t be a bool e an e xpr e s s i on. selection statements if i s a Java I t m us t e val uat e t o e i t he r t r ue or f al s e . r e s e r ve d wor d � Conditional statements give us the if ( condition ) power to make basic decisions statement1 ; � Java's conditional statements are else statement2 ; � the if statement � the if-else statement I f t he condition i s t r ue , statement1 i s e xe c ut e d. � the switch statement I f i t i s f al s e , statement2 i s e xe c ut e d. Summer 2004 CS 111 17 Summer 2004 CS 111 18
Boolean Expressions Logical Operators � A condition often uses one of Java's equality � Boolean expressions can use the following operators or relational operators , which all logical operators : return boolean results: ! Logical NOT && Logical AND == equal to || Logical OR != not equal to � They all take boolean operands and produce < less than > boolean results greater than <= less than or equal to � Logical NOT is a unary operator (it operates >= greater than or equal to on one operand) � Logical AND and logical OR are binary � Note the difference between the equality operators (each operates on two operands) operator ( == ) and the assignment operator ( = ) Summer 2004 CS 111 19 Summer 2004 CS 111 20 Repetition Statements The while Statement � Repetition statements allow us to execute a � The while statement has the following statement multiple times syntax: � Often they are referred to as loops while ( condition ) � Like conditional statements, they are while i s a statement ; controlled by boolean expressions r e s e r ve d wor d � Java has three kinds of repetition statements: � the while loop I f t he condition i s t r ue , t he statement i s e xe c ut e d. The n t he condition i s e val uat e d agai n. � the do loop � the for loop � The programmer should choose the right kind The statement i s e xe c ut e d r e pe at e dl y unt i l t he condition be c om e s f al s e . of loop for the situation Summer 2004 CS 111 21 Summer 2004 CS 111 22 Example The do Statement //******************************************************************** // Counter.java Author: Lewis/Loftus � The do statement has the following // // Demonstrates the use of a while loop. syntax: //******************************************************************** public class Counter do { //----------------------------------------------------------------- do and { // Prints integer values from 1 to a specific limit. while ar e //----------------------------------------------------------------- statement ; public static void main (String[] args) r e s e r ve d { } final int LIMIT = 5; wor ds while ( condition ) int count = 1; while (count <= LIMIT) { System.out.println (count); The statement i s e xe c ut e d onc e i ni t i al l y, count = count + 1; } and t he n t he condition i s e val uat e d System.out.println ("Done"); } } The statement i s e xe c ut e d r e pe at e dl y unt i l t he condition be c om e s f al s e Summer 2004 CS 111 23 Summer 2004 CS 111 24
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