Member data in java

Declaring Member Variables

The Bicycle class uses the following lines of code to define its fields:

public int cadence; public int gear; public int speed;

Field declarations are composed of three components, in order:

1. Zero or more modifiers, such as public or private .
2. The field’s type.
3. The field’s name.

The fields of Bicycle are named cadence , gear , and speed and are all of data type integer ( int ). The public keyword identifies these fields as public members, accessible by any object that can access the class.

Access Modifiers

The first (left-most) modifier used lets you control what other classes have access to a member field. For the moment, consider only public and private . Other access modifiers will be discussed later.

• public modifier—the field is accessible from all classes.
• private modifier—the field is accessible only within its own class.

In the spirit of encapsulation, it is common to make fields private. This means that they can only be directly accessed from the Bicycle class. We still need access to these values, however. This can be done indirectly by adding public methods that obtain the field values for us:

public class Bicycle < private int cadence; private int gear; private int speed; public Bicycle(int startCadence, int startSpeed, int startGear) < gear = startGear; cadence = startCadence; speed = startSpeed; >public int getCadence() < return cadence; >public void setCadence(int newValue) < cadence = newValue; >public int getGear() < return gear; >public void setGear(int newValue) < gear = newValue; >public int getSpeed() < return speed; >public void applyBrake(int decrement) < speed -= decrement; >public void speedUp(int increment) < speed += increment; >>

Types

All variables must have a type. You can use primitive types such as int , float , boolean , etc. Or you can use reference types, such as strings, arrays, or objects.

Variable Names

All variables, whether they are fields, local variables, or parameters, follow the same naming rules and conventions that were covered in the Language Basics lesson, Variables—Naming.

In this lesson, be aware that the same naming rules and conventions are used for method and class names, except that

• the first letter of a class name should be capitalized, and
• the first (or only) word in a method name should be a verb.

Источник

Structure and Members of the Java Program

When we are writing any program in any language we need to follow a standard structure for writing the program which is recommended by the language experts. A java program may contain many classes of which only one class will have a main method. Class will contain data members and methods that operate on the data members of the class. To write a Java program, we first need to define classes and then put them together. Generally a standard java program consists of following blocks as shown in below figure.

Explanation:
1. Package is a collection of classes, interfaces and sub packages. In a java program if we are using any pre-defined classes and interfaces then it is the responsibility of the java programmer to import that particular package containing such specific classes and interface. In java by default java.lang.* package is imported by every program.
2. Class is a keyword used for developing user defined data types. Every java program must starts with a prototype of class. The class has been declared public, means all classes can access the class from all packages. Generally, however, we will declare classes in java without specifying a modifier.
3. Class name is the name given to that class. Every class name is treated as one kind of user defined data type.
4. Data Members represents either instance members or static members.
5. Constructor function is called when an object of the class is created. It is a block of code that initializes the newly created object. The constructor simply has the same name as the name of the class name. A constructor does not have a return type. A constructor is called automatically when a new instance of an object is created. In the following code, the constructor bird() prints a message.

When we create the object of the bird class as shown above:
bird b = new bird();
The new keyword here creates the object of class bird and invokes the constructor to initialize this newly created object.
Constructor and method are different because the constructor is used to initialize the object of a class while the method is used to perform a task by implementing java code. Constructors cannot be declared as abstract, final, static and synchronized while methods can be declared. Constructors do not have return types while methods do.
6. User-defined methods represent either instance (or) static and they will be selected depends on the class name and these methods are used for performing the operations either once (or) repeatedly. All the user-defined methods of a class contain logic for a specific problem. These methods are known as Business logic methods.
7. All java program starts its execution with main() method so main() method is known as the backbone of the program. The Java Virtual Machine starts running any java program by executing main() method first.
8. Java’s main() method is not returning any value so its return type must be void.
9. Also main() method executes only once throughout the life of the Java program and before the object creation so its nature must be static.
10. The main() method is accessed in all the java programs, its access specifier must be public (universal).
11. Each and every main() method of java must take an array of objects of String class as an argument.
12. The block of statements are set of executable statements written for calling user-defined methods of the class.
13. If we have multiple java files then the naming convention of class file in java is that, whichever class is containing main() method, that class name will be given as the file name with an extension (dot) .java.
Types of Data Members:
Java Class is a collection of data members and functions. Any java program may contain two types of data members. They are;
1. Instance or non-static data members
2. Static or class data members
The following table describes the difference between the two.

Types of Methods:
In java program generally we may define two types of methods apart from constructor. They are;
1. Instance or non –static methods
2. Static or class methods
The following table describes the difference between the two.

The following example named TestGVP.java demonstrates the use of different members of the java class.

Источник

Member variables in Java

Member variables are known as instance variables in java.

• Instance variables are declared in a class, but outside a method, constructor or any block.
• When space is allocated for an object in the heap, a slot for each instance variable value is created.
• Instance variables are created when an object is created with the use of the keyword ‘new’ and destroyed when the object is destroyed.
• Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object’s state that must be present throughout the class.
• Instance variables can be declared in a class level before or after use.
• Access modifiers can be given for instance variables.
• The instance variables are visible for all methods, constructors, and block in the class. Normally, it is recommended to make these variables private (access level). However, visibility for subclasses can be given for these variables with the use of access modifiers.
• Instance variables have default values. For numbers, the default value is 0, for Booleans it is false, and for object references it is null. Values can be assigned during the declaration or within the constructor.
• Instance variables can be accessed directly by calling the variable name inside the class. However, within static methods (when instance variables are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.

Example

import java.io.*; public class Employee < // this instance variable is visible for any child class. public String name; // salary variable is visible in Employee class only. private double salary; // The name variable is assigned in the constructor. public Employee (String empName) < name = empName; >// The salary variable is assigned a value. public void setSalary(double empSal) < salary = empSal; >// This method prints the employee details. public void printEmp() < System.out.println("name : " + name ); System.out.println("salary :" + salary); >public static void main(String args[]) < Employee empOne = new Employee("Ransika"); empOne.setSalary(1000); empOne.printEmp(); >>

Output

This will produce the following result −

name : Ransika salary :1000.0

Источник

Java — (Field|Member Variable)

A field is a variables in a class and they are used to contain state information. They are also known as member variable.

A member that is not declared as static is implicitly an instance member.

Articles Related

• Java — Annotations
• Java — Object (instance of a class)
• JSF — (Managed Bean|Controller)
• Java — Static Modifier
• Java — Final Modifier
• Java — Variable
• Java — Access Modifier (private, public, )
• Code — Refactoring
• JPA — Entity Annotations
• Java — Reflection (java.lang.Class)
• More .

Field

Initialization

Fields are initialized when class is loaded into memory. They are initialized from top to bottom in the order they are declared in Java source file.

Declaration

Field declarations are composed of three components, in order:

The public keyword identifies these fields as public members, accessible by any object that can access the class.

Modifiers

Access

The first (left-most) access modifier used lets you control what other classes have access to a member field. For the moment, consider only public and private.

In the spirit of encapsulation, it is common to make fields private. This means that they can only be directly accessed from their owne class. We still need access to these values, however. This can be done indirectly by adding public methods that obtain the field values

The process of modifying the code structure, without modifying its current behavior, is called Refactoring. Refactoring is a method that search to improve the code quality of a program. code duplication.

A key feature of EJB 3.0 and JPA is the ability to create entities that contain object-relational mappings by using (metadata) annotations rather than deployment descriptors (orm.xml) as in earlier versions.

Managed Beans (also known as model objects and controllers) are lightweight container-managed objects (POJOs) with minimal requirements. They support a small set of basic services, such as: resource.

Class (Initialization|Instantiation) in java. During an class initialization: Java’s class loader loads the main method Java’s byte code verifier verifies the class. The first initialization.

in Java. Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control: At the top level: public, or package-private.

Since Java version 1.5. An annotation is an element of the java programming language (modifier or Metadata tag) that can be associated with: Java classes, interfaces, constructors, methods.

A class variable is any field declared with the modifier static. A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number.

The names of constant fields are in upper-case letters. By convention, the names of constant values are spelled in uppercase letters. If the name is composed of more than one word, the words are separated.

Final is modifier prohibiting value modification. Declaring an entire class final prevents the class from being subclassed. This is particularly useful, for example, when creating an immutable.

The members of a class are the inherited components of the class body including: fields, methods, nested classes, interfaces, and enumerated types. Since constructors are not inherited.

Источник