Java objects and classes


Java as an object-oriented language. Supports the following basic concepts:

  •             Polymorphism

  •           Inherit

  •           Encapsulation

  •           Abstract

  •           kind

  • Object

  •           Example

  •           method

  • Overloading

In this section we focus on the concepts of objects and classes.

  • Object: The object is an instance of the class (The object is not to find a girlfriend), with state and behavior. For example, a dog is an object. Its status includes: color, name, and breed; its behaviors include: wagging its tail, barking, eating, etc.

  • Class: A class is a template that describes the behavior and status of a type of object.

In the picture below, boys and girls are classes, and each specific person is the object of the class:

object-class.jpg

Objects in Java


#Now let us understand in depth what is an object. If you look at the real world around you, you will find that there are many objects around you, such as cars, dogs, people, etc. All these objects have their own state and behavior.

Take a dog as an example. Its status includes: name, breed, and color. Its behaviors include: barking, wagging its tail, and running.

Comparing real objects and software objects, they are very similar.

Software objects also have state and behavior. The state of a software object is its attribute, and its behavior is reflected through methods.

In software development, methods operate on changes in the internal state of objects, and mutual calls between objects are also completed through methods.

Classes in Java

Classes can be seen as templates for creating Java objects.

Let’s understand the definition of a class in Java through the following simple class: 

public class Dog{
   String breed;
   int age;
   String color;
   void barking(){
   }
   
   void hungry(){
   }
   
   void sleeping(){
   }
}

A class can contain the following types of variables:

  • Local variables: Variables defined in methods, constructors or statement blocks are called local variables. Variable declaration and initialization are all in methods. After the method ends, the variables will be automatically destroyed.

  • Member variables: Member variables are variables defined in the class and outside the method body. This type of variable is instantiated when the object is created. Member variables can be accessed by class methods, constructors, and class-specific statement blocks.

  • Class variables: Class variables are also declared in the class, outside the method body, but must be declared as static type.

A class can have multiple methods. In the above example: barking(), hungry() and sleeping() are all methods of the Dog class.

Constructor method

Every class has a constructor method. If no constructor is explicitly defined for a class, the Java compiler will provide a default constructor for the class.

When creating an object, at least one constructor must be called. The name of the constructor must be the same as the class. A class can have multiple constructors.

The following is an example of a constructor method: 

public class Puppy{
   public Puppy(){
   }

   public Puppy(String name){
      // 这个构造器仅有一个参数：name
   }
}

Creating objects

Objects are created based on classes. In Java, use the keyword new to create a new object. Creating an object requires the following three steps:

  • Declaration: Declare an object, including the object name and object type.

  • Instantiation: Use the keyword new to create an object.

  • Initialization: When using new to create an object, the constructor method will be called to initialize the object.

The following is an example of creating an object: 

public class Puppy{
   public Puppy(String name){
      //这个构造器仅有一个参数：name
      System.out.println("Passed Name is :" + name ); 
   }
   public static void main(String []args){
      // 下面的语句将创建一个Puppy对象
      Puppy myPuppy = new Puppy( "tommy" );
   }
}

Compile and run the above program, the following results will be printed: 

Passed Name is :tommy

Access instance variables and methods

Access member variables and member methods through the created object, as shown below: 

/* 实例化对象 */
ObjectReference = new Constructor();
/* 访问其中的变量 */
ObjectReference.variableName;
/* 访问类中的方法 */
ObjectReference.MethodName();

Instance

The following example shows How to access instance variables and call member methods: 

public class Puppy{
   int puppyAge;
   public Puppy(String name){
      // 这个构造器仅有一个参数：name
      System.out.println("Passed Name is :" + name ); 
   }

   public void setAge( int age ){
       puppyAge = age;
   }

   public int getAge( ){
       System.out.println("Puppy's age is :" + puppyAge ); 
       return puppyAge;
   }

   public static void main(String []args){
      /* 创建对象 */
      Puppy myPuppy = new Puppy( "tommy" );
      /* 通过方法来设定age */
      myPuppy.setAge( 2 );
      /* 调用另一个方法获取age */
      myPuppy.getAge( );
      /*你也可以像下面这样访问成员变量 */
      System.out.println("Variable Value :" + myPuppy.puppyAge ); 
   }
}

Compile and run the above program, producing the following results: 

Passed Name is :tommy
Puppy's age is :2
Variable Value :2

Source file declaration rules

In this section In the final part, we'll look at the declaration rules for source files. Pay special attention to these rules when defining multiple classes in a source file and when there are import statements and package statements.

  •             There can only be one public class in a source file

  • A source file can have multiple non-public classes

  • The name of the source file should be consistent with the class name of the public class. For example: the class name of the public class in the source file is Employee, then the source file should be named Employee.java.

  •             If a class is defined in a package, the package statement should be on the first line of the source file.

  •             If the source file contains an import statement, it should be placed between the package statement and the class definition. If there is no package statement, the import statement should be first in the source file.

  •             The import statement and package statement are valid for all classes defined in the source file. In the same source file, different package declarations cannot be given to different classes.

Classes have several access levels, and classes are also divided into different types: abstract classes, final classes, etc. These will be introduced in the access control chapter.

In addition to the types mentioned above, Java also has some special classes, such as inner classes and anonymous classes.

Java package

Package is mainly used to classify classes and interfaces. When developing Java programs, you may write hundreds or thousands of classes, so it is necessary to classify classes and interfaces.

Import statement

In Java, if a complete qualified name is given, including the package name and class name, the Java compiler can easily locate the source code or class. The Import statement is used to provide a reasonable path so that the compiler can find a certain class.

For example, the following command line will instruct the compiler to load all classes in the java_installation/java/io path

import java.io.*;

A simple example

In this In the example, we create two classes: Employee and EmployeeTest.

First open the text editor and paste the following code into it. Note that the file is saved as Employee.java.

The Employee class has four member variables: name, age, designation and salary. The class explicitly declares a constructor method with only one parameter. 

import java.io.*;
public class Employee{
   String name;
   int age;
   String designation;
   double salary;
   // Employee 类的构造器
   public Employee(String name){
      this.name = name;
   }
   // 设置age的值
   public void empAge(int empAge){
      age =  empAge;
   }
   /* 设置designation的值*/
   public void empDesignation(String empDesig){
      designation = empDesig;
   }
   /* 设置salary的值*/
   public void empSalary(double empSalary){
      salary = empSalary;
   }
   /* 打印信息 */
   public void printEmployee(){
      System.out.println("Name:"+ name );
      System.out.println("Age:" + age );
      System.out.println("Designation:" + designation );
      System.out.println("Salary:" + salary);
   }
}

The program starts execution from the main method. In order to run this program, the main method must be included and an instance object must be created.

The EmployeeTest class is given below, which instantiates two instances of the Employee class and calls methods to set the values ​​of variables.

Save the following code in the EmployeeTest.java file. 

import java.io.*;
public class EmployeeTest{

   public static void main(String args[]){
      /* 使用构造器创建两个对象 */
      Employee empOne = new Employee("James Smith");
      Employee empTwo = new Employee("Mary Anne");

      // 调用这两个对象的成员方法
      empOne.empAge(26);
      empOne.empDesignation("Senior Software Engineer");
      empOne.empSalary(1000);
      empOne.printEmployee();

      empTwo.empAge(21);
      empTwo.empDesignation("Software Engineer");
      empTwo.empSalary(500);
      empTwo.printEmployee();
   }
}

Compile these two files and run the EmployeeTest class. You can see the following results: 

C :> javac Employee.java
C :> vi EmployeeTest.java
C :> javac  EmployeeTest.java
C :> java EmployeeTest
Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0