We can know from the previous article that data types in java are divided into basic data types and reference data types .

#The object-oriented principle of Java is: data and operations on data must be bound together. This is a class, that is, a reference data type. Therefore, a class is also a type. Java does not need to set basic types. It only sets basic types to improve operating efficiency. The main difference between basic types and reference types is:

1. The variable name of the basic type is the variable itself.

2. #The name of a reference type variable is the storage location of complex data.

We know that there are three major categories of variable types supported by the Java language:

• Local variables

• Member variables

• Class variables (Static variable)

This chapter actually starts from the scope and explains the scope of the variable type. Why is it called a scope? Let’s first look at the code and concepts below.

Java local variables

• Local variables are declared in methods, constructors, or statement blocks;

• Local variables are created when methods, constructors, or statement blocks are executed, and when their execution is completed After that, the variable will be destroyed;

• Access modifiers cannot be used for local variables;

• Local variables are only declared in the method, Visible in the constructor or statement block;

• Local variables are allocated on the stack.

• Local variables have no default value, so after a local variable is declared, it must be initialized before it can be used.

Example 1

In the following example age is a local variable. It is defined in the pupAge() method, and its scope is limited to this method.

package com.dujinyang.immqy;
 
public class Test{ 
   public void getAge(){
      int age=1;
      age = age + 9;
      System.out.println("--小狗的年龄 : " + age);
   }
   
   public static void main(String args[]){
      Test test = new Test();
      test.getAge();
   }
}

The above example compilation and running results are as follows:

--小狗的年龄是:10

Instance 2

In the following example, the age variable is not initialized, so an error will occur during compilation:

package com.dujinyang.immqy;
 
public class Test{ 
   public void getAge(){
      int age;
      age = age + 9;
      System.out.println("--小狗的年龄 : " + age);
   }
   
   public static void main(String args[]){
      Test test = new Test();
      test.getAge();
   }
}

The compiler will report an error directly:

Test.java:4:variable number might not have been initialized
age = age + 9;
         ^1 error

实例变量

• 实例变量声明在一个类中，但在方法、构造方法和语句块之外；

• 当一个对象被实例化之后，每个实例变量的值就跟着确定；

• 实例变量在对象创建的时候创建，在对象被销毁的时候销毁；

• 实例变量的值应该至少被一个方法、构造方法或者语句块引用，使得外部能够通过这些方式获取实例变量信息；

• 实例变量可以声明在使用前或者使用后；

• 访问修饰符可以修饰实例变量；

• 实例变量对于类中的方法、构造方法或者语句块是可见的。一般情况下应该把实例变量设为私有。通过使用访问修饰符可以使实例变量对子类可见；

• 实例变量具有默认值。数值型变量的默认值是0，布尔型变量的默认值是false，引用类型变量的默认值是null。变量的值可以在声明时指定，也可以在构造方法中指定；

• 实例变量可以直接通过变量名访问。但在静态方法以及其他类中，就应该使用完全限定名：ObejectReference.VariableName。

实例

import java.io.*;
public class Employee{
   // 这个实例变量对子类可见
   public String name;
   // 私有变量，仅在该类可见
   private double salary;
   //在构造器中对name赋值
   public Employee (String empName){
      name = empName;
   }
   //设定salary的值
   public void setSalary(double empSal){
      salary = empSal;
   }  
   // 打印信息
   public void printEmp(){
      System.out.println("名字 : " + name );
      System.out.println("薪水 : " + salary);
   }
 
   public static void main(String args[]){
      Employee empOne = new Employee("KARL-dujinyang");
      empOne.setSalary(1000);
      empOne.printEmp();
   }
}

以上实例编译运行结果如下:

$ javac Employee.java 
$ java Employee名字 : KARL-dujinyang薪水 : 1000.0

类变量（静态变量）

• 类变量也称为静态变量，在类中以static关键字声明，但必须在方法构造方法和语句块之外。

• 无论一个类创建了多少个对象，类只拥有类变量的一份拷贝。

• 静态变量除了被声明为常量外很少使用。常量是指声明为public/private，final和static类型的变量。常量初始化后不可改变。

• 静态变量储存在静态存储区。经常被声明为常量，很少单独使用static声明变量。

• 静态变量在程序开始时创建，在程序结束时销毁。

• 与实例变量具有相似的可见性。但为了对类的使用者可见，大多数静态变量声明为public类型。

• 默认值和实例变量相似。数值型变量默认值是0，布尔型默认值是false，引用类型默认值是null。变量的值可以在声明的时候指定，也可以在构造方法中指定。此外，静态变量还可以在静态语句块中初始化。

• 静态变量可以通过：ClassName.VariableName的方式访问。

• 类变量被声明为public static final类型时，类变量名称一般建议使用大写字母。如果静态变量不是public和final类型，其命名方式与实例变量以及局部变量的命名方式一致。

实例

import java.io.*;
 
public class Employee {
    //salary是静态的私有变量
    private static double salary;
    // DEPARTMENT是一个常量
    public static final String DEPARTMENT = "深圳的";
    public static void main(String args[]){
    salary = 1000;
        System.out.println(DEPARTMENT+"平均工资:"+salary);
    }
}

以上实例编译运行结果如下:

深圳的平均工资:1000.0

注意：如果其他类想要访问该变量，可以这样访问：Employee.DEPARTMENT。因为它是静态的，static的关键字。

In this chapter we learned about Java variable types. In the next chapter we will introduce the use of Java modifiers.

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