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Basic units of action in Java – classes and objects

Keywords: class, property, behavior, instantiation, object \

The article directories

• Basic units of action in Java – classes and objects
• • I. Brief analysis of concepts
  • • 1. The concept of classes
    • 2. What are objects
    • 3. The relationship between classes and objects
  • Second, the class
  • • 1. The attribute
    • 2. Behavior
    • 3. Executable classes
  • Three, objects,
  • • 1. An object is an instance of a class
    • 2. Relationships between objects

I. Brief analysis of concepts

Speaking of class and object, these are two abstract words, and if you use them in a program, you don’t know what they mean. In fact, we can first do not contact the procedure, we are in a lot of things when the classification of nature is a standard, meet some characteristics divided into one category, meet some other characteristics divided into another category. In each category, everything has the same characteristics, but there are some differences. Each thing is like an object, which is a real thing, while the classified category is like a standard or description, which is more abstract.

1. The concept of classes

// The class used to describe the student
public class Student{
    String sno;/ / student id
    String name;/ / name
    String college;/ / college
    String major;/ / professional. }Copy the code

2. What are objects

Once you understand the concept and representation of classes in your program, objects are easier to understand. To put it simply, an object is a specific thing that can be operated according to the standards of a class, that is, a specific student. Each student has the same characteristics, but can have different name, school, major, and so on their own information. To create a new object, use the following code:

public class Test{
    public static void main(String[] args){
        // Variable type Variable name = new class constructor ();
        Student student = new Student();
        // The uppercase Student is the type of Student we want to use, and the names must be exactly the same
        // Student in lowercase is the name of the object created by the student class
        // Is also the name we use directly when manipulating objects
        // New is the key used to create the object, and the corresponding constructor needs to be called
        // The use of constructors will be explained in a later article. For starters, remember: class name ();}}Copy the code

3. The relationship between classes and objects

Second, the class

How should we describe and define a class? Wasn’t the HelloWorld we originally wrote a class? Why aren’t there so many confusing concepts? Let’s go further.

From the perspective of the objective world, for example, Xiao Ming went to university, which is a specific student. He has his student number, name, corresponding school, major and so on, which are all his basic personal information. In addition, he can also carry out various activities, such as choosing courses, taking exams and modifying part of his information.



So how do we define a class from a program point of view? It depends on our needs, for a teaching management system, we only care about and students learning related information, students can choose courses, exams and so on. For a logistics management system, we are concerned about the students’ meal card consumption, access to the dormitory time, students can recharge, consumption, swipe cards and so on. So how to define and describe a class depends on the needs of our application system, or the requirements of the problem.

1. The attribute

2. Behavior

Behavior refers to the current class created by the object can carry out what kind of behavior, in the program performance is the method, can be called by the specific object, in the method body with code to achieve specific functions.

// The class used to describe the student
public class Student{
    // Student class attributes
    String sno;/ / student id
    String name;/ / name
    String college;/ / college
    String major;/ / professional.// Student behavior
    public void study(a){
        / / to learn
        System.out.println("Go to class");
    }
    public void exam(a){
        / / to the exam
        System.out.println("Go to the exam"); }}Copy the code

3. Executable classes

public class HelloWorld{
    public static void main(String[] args){
        System.out.println("Hello World!"); }}Copy the code

// Call both classes in the same directory (in the same package)
public class Test{
    public static void main(String[] args){
        // Create a student object
        Student student = new Student();
        // Assign a value to an attribute of the object
        student.sno = "1130050152";
        student.name = "Xiao Ming";
        student.college = "School of Mathematical Sciences";
        student.major = "Information and Computing Science";
        // Displays the property values of the object
        System.out.println("The student's name is :" + student.name);
        // Call the method through the object, the behavior occursstudent.study(); student.exam(); }}Copy the code

Three, objects,

Above we have seen the concept of objects and how to create and use an object. Now let’s clarify the relationships between classes and objects, and between objects.

1. An object is an instance of a class

2. Relationships between objects

An important feature is that objects have something in common and do not interact with each other except for the use of statically declared properties. When we use different objects to call methods and output attribute information, although the structure of the class is completely the same, but because the state of the object (attribute value) may be different, so the result of method execution will be different.

// The class used to describe the student
public class Student{
    // Student class attributes
    String sno;/ / student id
    String name;/ / name
    double balance;/ / the balance.// Student behavior
    public void recharge(double money){
        // Recharge your student card
        balance += money;
        System.out.println("Current balance is" + balance);
    }
    public void getBalance(a){
        // Display the card balance
        System.out.println("Current balance is"+ balance); }}Copy the code

// Test classes: Put two classes in the same directory (in the same package)
public class Test{
    public static void main(String[] args){
        // Create the first student object
        Student student1 = new Student();
        // Assign a value to an attribute of the object
        student1.balance = 23.70;
        // Call the method
        student1.getBalance();
        student1.recharge(100.0);
        Create the second student object
        Student student2 = new Student();
        // Assign a value to an attribute of the object
        student2.balance = 15.60;
        // Call the method
        student2.getBalance();
        student2.recharge(50.0); }}Copy the code

After the execution of the program can be output again to view the property values of the two objects, will not affect each other.