What are the keywords used to define classes in java?

The keyword for defining a class in java is: "class". To define a class in Java, you need to use the "class" keyword, a custom class name, and a pair of braces to represent the program body.

Class is an important reference data type in Java and the basic element of Java programs, because all Java programs are based on classes.

To define a class in Java, you need to use the class keyword, a custom class name and a pair of braces indicating the program body. The complete syntax is as follows:

[public][abstract|final]class<class_name>[extends<class_name>][implements<interface_name>] {
    // 定义属性部分
    <property_type><property1>;
    <property_type><property2>;
    <property_type><property3>;
    …
    // 定义方法部分
    function1();
    function2();
    function3();
    …
}

Tip: In the above syntax, the part in the square brackets "[]" means that it can be omitted, and the vertical bar "|" means "or relationship", such as abstract|final, indicating that abstract can be used or final keyword, but the two keywords cannot appear at the same time.

The description of each keyword in the above syntax is as follows.

• public: means "shared". If public is used, it can be accessed by other classes and programs. The main class of every Java program must be a public class, and classes that serve as public tools for use by other classes and programs should be defined as public classes.

• abstract: If a class is modified by abstract, the class is an abstract class. Abstract classes cannot be instantiated, but abstract classes can have abstract methods (methods modified with abstract) and Specific methods (methods without abstract modification). All subclasses that inherit this abstract class must implement all abstract methods in the abstract class (unless the subclass is also an abstract class).

• final: If a class is modified with final, it is not allowed to be inherited.

• class: Keyword for declaring a class.

• class_name: The name of the class.

• extends: Indicates inheriting other classes.

• implements: Indicates the implementation of certain interfaces.

• property_type: Indicates the type of member variable.

• property: Indicates the name of the member variable.

• function(): Indicates the member method name.

Naming rules for Java class names:

• Class names should start with an underscore (_) or a letter, ending with It's better to start with a letter.

• It is best to capitalize the first letter. If the class name consists of multiple words, it is best to capitalize the first letter of each word.

• The class name cannot be a keyword in Java, such as boolean, this, int, etc.

• Class names cannot contain any embedded spaces or periods and special characters except the underscore (_) and dollar sign ($) characters.

Example:

Creating a new class means creating a new data type. Instantiating a class is getting an object of the class. Therefore, an object is a collection of variables and related methods, where variables indicate the state and properties of the object, and methods indicate the behavior of the object. The steps to define a class are described below.

(1) Declare the class. Write the outermost framework of the class and declare a class named Person.

public class Person {
    // 类的主体
}

(2) Write the attributes of the class. The data and methods in a class are collectively called class members. Among them, the attributes of the class are the data members of the class. The characteristics (properties) of a class are described by defining variables in the body of the class. The variables declared here are called member variables of the class.

(3) Write the method of the class. Class methods describe the behavior of the class and are method members of the class. Methods can be simply understood as unit modules that independently complete a certain function.

Let’s define a simple Person class.

public class Person {
    private String name;    // 姓名
    private int age;    // 年龄
    public void tell() {   
        // 定义说话的方法
        System.out.println(name+"今年"+age+"岁！");
    }
}

As shown in the above code, two attributes are first defined in the Person class, namely name and age, and then a method named tell() is defined.

Recommended tutorial: "java tutorial"

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