JAVA Getting Started Tutorial | Chapter 3 Variable Types
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:
The variable name of the basic type is the variable itself.
#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.
The above is the content of JAVA Getting Started Tutorial | Chapter 3 Variable Types. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!
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