AngularJS training in Noida

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Java Training In Noida Java Basics Topics in this section include: ฀ What makes Java programs portable secure and robust   ฀ The structure of Java applets and applications   ฀ How Java applications are executed   ฀ How applets are invoked and executed   ฀ The Java Language Part I   ฀ Comments   ฀ Declarations   ฀ Expressions   ฀ Statements   ฀ Garbage collection   ฀ Java Semantics  Portability Java programs are portable across operating systems and hardware environments. Portability is to your advantage because: ฀ You need only one version of your software to serve a broad market.    ฀ The Internet in effect becomes one giant dynamic library.    ฀ You are no longer limited by your particular computer platform.  Three features make Java String programs portable: 1. The language. The Java language is completely specified all data-type sizes and formats are defined as part of the language. By contrast C/C++ leaves these "details" up to the compiler implementor and many C/C++ programs therefore

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Java Basics are not portable. 2. The library. The Java class library is available on any machine with a Java runtime system because a portable program is of no use if you cannot use the same class library on every platform. Window-manager function calls in a Mac application written in C/C++ for example do not port well to a PC. 3. The byte code. The Java runtime system does not compile your source code directly into machine language an inflexible and nonportable representation of your program. Instead Java programs are translated into machine-independent byte code. The byte code is easily interpreted and therefore can be executed on any platform having a Java runtime system. The latest versions of the Netscape Navigator browser for example can run applets on virtually any platform. Security The Java language is secure in that it is very difficult to write incorrect code or viruses that can corrupt/steal your data or harm hardware such as hard disks. There are two main lines of defense: ฀ Interpreter level:   ฀ No pointer arithmetic   ฀ Garbage collection   ฀ Array bounds checking   ฀ No illegal data conversions   ฀ Browser level applies to applets only:   ฀ No local file I/O   ฀ Sockets back to host only   ฀ No calls to native methods  Robustness The Java language is robust. It has several features designed to avoid crashes during program execution including: ฀ No pointer arithmetic  Java Basics -2

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Java Basics ฀ Garbage collection--no bad addresses   ฀ Array and string bounds checking   ฀ No jumping to bad method addresses   ฀ Interfaces and exceptions  Java Program Structure A file containing Java source code is considered a compilation unit. Such a compilation unit contains a set of classes and optionally a package definition to group related classes together. Classes contain data and method members that specify the state and behavior of the objects in your program. Java programs come in two flavors: ฀ Standalone applications that have no initial context such as a pre-existing main window   ฀ Applets for WWW programming   The major differences between applications and applets are:   ฀ Applets are not allowed to use file I/O and sockets other than to the host platform. Applications do not have these restrictions.   ฀ An applet must be a subclass of the Java Applet class. Aplications do not need to subclass any particular class.   ฀ Unlike applets applications can have menus.   ฀ Unlike applications applets need to respond to predefined lifecycle messages from the WWW browser in which theyre running.  Java Program Execution The Java byte-code compiler translates a Java source file into machine- independent byte code. The byte code for each publicly visible class is placed in a separate file so that the Java runtime system can easily find it. If your program instantiates an object of class A for example the class loader searches the directories listed in your CLASSPATH environment variable for a file called A.class that contains the class definition and byte code for class A. There is no link phase for Java programs all linking is done dynamically at Java Basics -3

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Java Basics runtime. The following diagram shows an example of the Java compilation and execution sequence for a source file named containing public class A and non- public class B: Java programs are in effect distributed applications. You may think of them as a collection of DLLs dynamically loadable libraries that are linked on demand at runtime. When you write your own Java applications you will often integrate your program with already-existing portions of code that reside on other machines. A Simple Application Consider the following trivial application that prints "hi there" to standard output: Java Basics -4

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Java Basics public class TrivialApplication // args0 is first argument // args1 the second public static void mainString args System.out.println"hi there" The command java TrivialApplication tells the Java runtime system to begin with the class file TrivialApplication.class and to look in that file for a method with the signature: public static void mainString args The main method will always reside in one of your class files. The Java language does not allow methods outside of class definitions. The class in effect creates scoped symbol StartingClassName.main for your main method. Applet Execution An applet is a Java program that runs within a Java-compatible WWW browser or in an appletviewer. To execute your applet the browser: ฀ Creates an instance of your applet   ฀ Sends messages to your applet to automatically invoke predefined lifecycle methods   The predefined methods automatically invoked by the runtime system are:   ฀ init. This method takes the place of the Applet constructor and is only called once during applet creation. Instance variables should be initialized in this method. GUI components such as buttons and scrollbars should be added to the GUI in this method.   ฀ start. This method is called once after init and whenever your applet is revisited by your browser or when you deiconify your browser. This method should be used to start animations and other threads.   ฀ paintGraphics g. This method is called when the applet drawing area needs to be redrawn. Anything not drawn by contained components must be drawn in this method. Bitmaps for example are drawn here but buttons are not because they handle their own painting.   ฀ stop. This method is called when you leave an applet or when you iconify your browser. The method should be used to suspend animations and other  Java Basics -5

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Java Basics threads so they do not burden system resources unnecessarily. It is guaranteed to be called before destroy. ฀ destroy. This method is called when an applet terminates for example when quitting the browser. Final clean-up operations such as freeing up system resources with dispose should be done here. The dispose method of Frame removes the menu bar. Therefore do not forget to call super.dispose if you override the default behavior.   The basic structure of an applet that uses each of these predefined methods is:  import java.applet.Applet // include all AWT class definitions import java.awt. public class AppletTemplate extends Applet public void init // create GUI initialize applet public void start // start threads animations etc... public void paintGraphics g // draw things in g public void stop // suspend threads stop animations etc... public void destroy // free up system resources stop threads All you have to do is fill in the appropriate methods to bring your applet to life. If you dont need to use one or more of these predefined methods simply leave them out of your applet. The applet will ignore messages from the browser attempting to invoke any of these methods that you dont use. A Simple Applet The following complete applet displays "Hello World Wide Web" in your browser window: import java.applet.Applet import java.awt.Graphics public class TrivialApplet extends Applet public void paintGraphics g // display a string at 2020 Java Basics -6 Java Basics

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// where 00 is the upper-left corner g.drawString"Hello World Wide Web" 20 20 An appletviewer may be used instead of a WWW browser to test applets. For example the output of TrivialApplet on an appletviewer looks like: HTML/Applet Interface The HTML applet tag is similar to the HTML img tag and has the form: applet codeAppletName.class widthw heighth parameters /applet where the optional parameters are a list of parameter definitions of the form: param namen valuev An example tag with parameter definitions is: applet codeAppletName.class width300 height200 param namep1 value34 param namep2 value"test" /applet where p1 and p2 are user-defined parameters. The code width and height parameters are mandatory. The parameters codebase alt archives align vspace and hspace are optional within the applet tag itself. Your applet can access any of these parameters by calling: Applet.getParameter"p" which returns the String value of the parameter. For example the applet: import java.applet.Applet public class ParamTest extends Applet public void init System.out.println"width is " + getParameter"width" System.out.println"p1 is " + getParameter"p1" Java Basics -7

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Java Basics System.out.println"p2 is " + getParameter"p2" prints the following to standard output: width is 300 p1 is 34 p2 is test Comments Java comments are the same as C++ comments i.e. / C-style block comments / where all text between the opening / and closing / is ignored and // C++ style single-line comments where all text from the opening // to the end of the line is ignored. Note that these two comments can make a very useful combination. C-style comments / ... / cannot be nested but can contain C++ style comments. This leads to the interesting observation that if you always use C++-style comments // ... you can easily comment out a section of code by surrounding it with C-style comments. So try to use C++ style comments for your "normal" code commentary and reserve C-style comments for commenting out sections of code. The Java language also has a document comment: / document comment / These comments are processed by the javadoc program to generate documentation from your source code. For example / This class does blah blah blah / class Blah / This method does nothing / This is a multiple line comment. The leading is not placed in documentation. / public void nothing Java Basics -8

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Java Basics Declarations A Java variable may refer to an object an array or an item of primitive type. Variables are defined using the following simple syntax: TypeName variableName For example int a // defines an integer int b // defines a reference to array of ints Vector v // reference to a Vector object Primitive Types The Java language has the following primitive types: Primitive Types Primitive Type Description boolean true/false byte 8 bits char 16 bits UNICODE short 16 bits int 32 bits long 64 bits float 32 bits IEEE 754-1985 double 64 bits IEEE 754-1985 Java int types may not be used as boolean types and are always signed. Objects A simple C++ object or C struct definition such as "Button b" allocates memory on the stack for a Button object and makes b refer to it. By contrast you must specifically instantiate Java objects with the new operator. For example Java Basics -9

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Java Basics // Java code void foo // define a reference to a Button init to null Button b // allocate space for a Button b points to it b new Button"OK" int i 2 As the accompanying figure shows this code places a reference b to the Button object on the stack and allocates memory for the new object on the heap. The equivalent C++ and C statements that would allocate memory on the heap would be: // C++ code Button b NULL // declare a new Button pointer b new Button"OK" // point it to a new Button / C code / Button b NULL / declare a new Button pointer / b calloc1 sizeofButton / allocate space for a Button / initb "OK" / something like this to init b / All Java objects reside on the heap there are no objects stored on the stack. Storing objects on the heap does not cause potential memory leakage problems because of garbage collection. Each Java primitive type has an equivalent object type e.g. Integer Byte Float Double. These primitive types are provided in addition to object types purely for efficiency. An int is much more efficient than an Integer. Java Basics -10

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Java Basics Strings Java string literals look the same as those in C/C++ but Java strings are real objects not pointers to memory. Java strings may or may not be null-terminated. Every string literal such as "a string literal" is interpreted by the Java compiler as new String"a string literal" Java strings are constant in length and content. For variable-length strings use StringBuffer objects. Strings may be concatenated by using the plus operator: String s "one" + "two" // s "onetwo" You may concatenate any object to a string. You use the toString method to convert objects to a String and primitive types are converted by the compiler. For example String s "1+1" + 2 // s "1+12" The length of a string may be obtained with String method length e.g. "abc".length has the value 3. To convert an int to a String use: String s String.valueOf4 To convert a String to an int use: int a Integer.parseInt"4" Array Objects In C and C++ arrays are pointers to data in memory. Java arrays are objects that know the number and type of their elements. The first element is index 0 as in C/C++. Generic Array Object elements Java Basics -11

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Java Basics element type element 0 element 1 ... element n-1 The syntax for creating an array object is: TypeName variableName This declaration defines the array object--it does not allocate memory for the array object nor does it allocate the elements of the array In addition you may not specify a size within the square brackets. To allocate an array use the new operator: int a new int5 // Java code: make array of 5 ints new int5 5 int 0 0 0 0 0 In C or C++ by contrast you would write either / C/C++ code: make array of 5 ints on the stack / int a5 or / C/C++ code: make array of 5 ints on the heap / Java Basics -12 .

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Java Basics int a new int5 An array of Java objects such as // Java code: make array of 5 references to Buttons Button a new Button5 creates the array object itself but not the elements: new Button5 5 Button null pointer null pointer null pointer null pointer null pointer You must use the new operator to create the elements: a0 new Button"OK" a3 new Button"QUIT" In C++ to make an array of pointers to objects you would write: // C++: make an array of 5 pointers to Buttons Button a new Button 5 // Create the array a0 new Button"OK" // create two new buttons a3 new Button"QUIT" In C code for the same task would look like: / C: make an array of 5 pointers to structs / / Allocate the array / Button a calloc5 sizeofButton / Allocate one button / a0 calloc1 sizeofButton / Init the first button / setTitlea0 "OK" / Allocate another button / a3 calloc1 sizeofButton / Init the second button / Java Basics -13

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Java Basics setTitlea3 "QUIT" Multi-dimensional Java arrays are created by making arrays of arrays just as in C/C++. For example T t new T105 makes a five-element array of ten arrays of references to objects of type T. This statement does not allocate memory for any T objects. Accessing an undefined array element causes a runtime exception called ArrayIndexOutOfBoundsException. Accessing a defined array element that has not yet been assigned to an object results in a runtime NullPointerException. Initializers Variables may be initialized as follows: ฀ Primitive types  int i 3 boolean g true ฀ Objects  Button b null Employee e new Employee ฀ Arrays  int i 1 2 3 4 or in Java 1.1 int i i new int 1 2 3 4 Constants Variables modified by the static final keywords are constants equivalent to the const keyword in C++ no equivalent in C. For example // same as "const int version1" in C++ static final int version 1 static final String Owner "Terence" Java Basics -14

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Java Basics Expressions Most Java expressions are similar to those in C/C++. Constant Expressions Item Examples or Description id i nameList qualified-id Integer.MAX_VALUE obj.member npackage.class package.obj idef...g ai b34 String literal "Jim" delimited by "" char literal a \t delimited by Unicode character constant \u00ae boolean literal true false not an int int constant 4 float constant 3.14f 2.7e6F f or F suffix double constant 3.14 2.7e6D default / d or D suffix hexadecimal constant 0x123 octal constant 077 null the null object note lowercase this the current object super the superclass view of this object General Expressions Item Examples or Description Java Basics -15

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Java Basics id i nameList obj.methodargs instance method call class.methodargs class method call expr 3+47 new Tconstructor-args instantiates a new object or class T new Tef...g allocates an array object Operators The Java language has added the zero-extend right-shift operator to the set of C++ operators. C++ operators include instanceof and new which are not present in C. Note that sizeof has been removed as memory allocation is handled for you. The operators in order of highest to lowest priority are: ฀ new   ฀ .   ฀ -- ++ + - TypeName   ฀ /   ฀ + -   ฀   ฀ instanceof   ฀   ฀   ฀   ฀ |   ฀   ฀ ||   ฀ :   ฀ / + - |  Note that the precedence of the new operator and the . operator bind Java Basics -16

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Java Basics differently than in C++. A proper Java statement is: // Java code new T.method In C++ you would use: // C++ code new T-method Statements Java statements are similar to those in C/C++ as the following table shows. Forms of Common Statements Statement Examples if if boolean-expr stat1 if boolean-expr stat1 else stat2 switch int-expr case int-const-expr : stat1 switch case int-const-expr : stat2 default : stat3 for for int i0 i10 i++ stat while while boolean-expr stat do-while do stats while boolean-expr return return expr The Java break and continue statements may have labels. These labels refer to the specific loop that the break or continue apply to. Each loop can be preceded by a label. Java Semantics We say that the Java language has "reference semantics" and C/C++ have "copy semantics." This means that Java objects are passed to methods by reference in Java while objects are passed by value in C/C++. Java primitive types however are not treated in the same way as Java objects. Primitive types are assigned compared and passed as arguments using copy Java Basics -17

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Java Basics semantics just as in C/C++. For example i j for two int variables i and j performs a 32-bit integer copy. Assignment of Objects Assignment makes two variables refer to the same object. For example class Data public int data 0 public Dataint d data d I Data a new Data1 // is 1 IData b new Data2 // is 2 II b a // and are 1 III 3 // and are 3 IV a new Data4 // is 3 is 4 To copy objects define and use clone: class Data implements Cloneable public int data 0 public Dataint d data d public Object clone Data d Data super.clone data return d ... Data a new Data1 // is 1 Data b new Data2 // is 2 b a.clone // and are 1 3 // is 1 is 3 Java Basics -18

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Java Basics Note: The above class definition requires exception handling code. We however have not yet discussed exception handling. For now pretend that it is not necessary. Method Parameters and Return Values Arguments and return values for primitive types are passed by value to and from all Java methods because they are implied assignments as in C/C++. However all Java objects are passed by reference. For example the C/C++ code: // C++ code int fooint j return j + 34 Button bfooButton b if b NULL return b else return new Button or in C / C code / int fooint j return j + 34 Button bfooButton b if b NULL return b else return callocsizeofButton would be written in the Java language: // Java code int fooint j return j + 34 Button bfooButton b if b null return b else return new Button"OK" Equality Two Java primitive types are equal using the operator when they have the same value e.g. "3 3". However two object variables are equal if and only if they refer to the same instantiated object--a "shallow" comparison. For example void test Data a new Data1 Data b new Data2 Data c new Data1 // a b is FALSE // a c is FALSE in C++ thisd be TRUE Java Basics -19

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Java Basics Data d a Data e a // d e is TRUE // de are referring to same object To perform a "deep" comparison the convention is to define a method called equals. You would rewrite Data as: class Data public int data 0 public Dataint d data d boolean equalsData d return data ... Data a new Data1 Data b new Data1 // a.equalsb is true No Pointers The Java language does not have pointer types nor address arithmetic. Java variables are either primitive types or references to objects. To illustrate the difference between C/C++ and Java semantics consider the following equivalent code fragments. // C++ code C code would be similar Stack s new Stack // point to a new Stack s-push... // dereference and access method push The equivalent Java code is: // Java code // internally consider s to be a Stack Stack s new Stack // dereference s automatically s.push... Garbage Collection An automatic garbage collector deallocates memory for objects that are no longer needed by your program thereby relieving you from the tedious and error-prone Java Basics -20

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Java Basics task of deallocating your own memory. As a consequence of automatic garbage collection and lack of pointers a Java object is either null or valid--there is no way to refer to an invalid or stale object one that has been deallocated. To illustrate the effect of a garbage collector consider the following C++ function that allocates 1000 objects on the heap via the new operator a similar C function would allocate memory using calloc/malloc: // C++ code void f T t for int i 1 i 1000 i++ t new T // ack Every time the loop body is executed a new instance of class T is instantiated and t is pointed to it. But what happens to the instance that t used to point to Its still allocated but nothing points to it and therefore its inaccessible. Memory in this state is referred to as "leaked" memory. In the Java language memory leaks are not an issue. The following Java method causes no ill effects: // Java code void f T t for int i 1 i 1000 i++ t new T In Java each time t is assigned a new reference the old reference is now available for garbage collection. Note that it isnt immediately freed it remains allocated until the garbage collector thread is next executed and notices that it can be freed. Put simply automatic garbage collection reduces programming effort programming errors and program complexity. MML: 0.995a Version: Id: //depot/main/src/edu/modules/JavaBasics/javaBasics.mml3

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JAVA training in Noida Java Basics -21

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Java Basics PHP training in Noida

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Java Basics -22 © 1996-2003 All Rights Reserved.

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Java Basics This page intentionally left blank CakePHP training in noida

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Address Techavera Solutions Pvt. Ltd. Noida 3rd Floor Om Complex Naya Bans Sector-15 Noida. +91-8506-888-288/ +91-8826-953-776 © 1996-2003 All Rights Reserved. Java Basics -23

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