What is a member variable in java

In Java, member variables refer to variables defined in the variable part of the class body, also called attributes. Member variables are declared in the class, outside the method body, and are instantiated when the object is created; member variables can be accessed by methods, constructors in the class, and statement blocks of a specific class. If a member variable is not initialized when it is declared, the system will initialize the member variable with a default value.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

Member variables: Variables defined in the variable part of the class body, also called attributes.

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

public class  ClassName{
    成员变量
    public void printNumber（）{
        // 其他代码
    }
}

In Java, the member variables of a class define the attributes of the class. For example, a student class generally needs to have attributes such as name, gender, and age. In this case, it is necessary to define three attributes: name, gender, and age.

The syntax for declaring member variables is as follows:

[public|protected|private][static][final]<type><variable_name>

The meaning of each parameter is as follows.

• public, protected, private: used to indicate the access rights of member variables.

• #static: Indicates that the member variable is a class variable, also called a static variable.

• #final: Indicates that the member variable is declared as a constant and its value cannot be changed.

• type: Indicates the type of the variable.

• variable_name: Indicates the variable name.

You can initialize a member variable while declaring it. If the member variable is not initialized when declaring it, the system will initialize the member variable with a default value.

The default value of initialization is as follows:

• The default value of basic type variables of integer type (byte, short, int and long) is 0.

• The default value of the basic type variable of single-precision floating point type (float) is 0.0f.

• The default value of the basic type variable of double precision floating point type (double) is 0.0d.

• The default value of the basic type variable of character type (char) is "\u0000".

• The default value of Boolean basic type variables is false.

• The default value of a variable of array reference type is null. If you create an instance of an array variable but do not explicitly assign a value to each element, the initialization value of the elements in the array uses the default value corresponding to the array data type.

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An example of defining member variables of a class is as follows:

public class Student
{
    public String name;    //姓名
    final int sex=0;    //性别：0表示女孩，1表示男孩
    private int age;    //年龄
}

The above example Three member variables are defined in the Student class: name of String type, sex of int type and age of int type. Among them, the access modifier of name is public, and the initialization value is null; the access modifier of sex is friendly (default), and the initialization value is 0, indicating that the gender is female, and its value cannot be changed; the access modifier of age is private, The initial value is 0.

The following is a simple example to introduce the initial value of member variables. The code is as follows.

public class Counter {
    static int sum;
    public static void main(String[] args) {
        System.out.println(sum);
    }
}

Here, a static method is used to modify the variable sum, and the output result is the initial value of the int type, that is: 0.

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