Java 简明教程

Java - Variable Types

What is a Java Variable?

变量为我们的程序提供命名存储，以便其进行操作。Java 中的每个变量都有一个特定的类型，它决定了变量内存的大小和布局；可以在该内存中存储的值的范围；以及可以应用于该变量的操作集。

A variable provides us with named storage that our programs can manipulate. Each variable in Java has a specific type, which determines the size and layout of the variable’s memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

Variable Declaration and Initialization

在你使用所有变量之前，都必须声明它们。以下是变量声明的基本形式 −

You must declare all variables before they can be used. Following is the basic form of a variable declaration −

data type variable [ = value][, variable [ = value] ...] ;

此处数据类型是 Java’s data types 之一，变量是变量的名称。要声明指定类型的多个变量，可以使用以逗号分隔的列表。

Here data type is one of Java’s data types and variable is the name of the variable. To declare more than one variable of the specified type, you can use a comma-separated list.

Example of Valid Variables Declarations and Initializations

以下是在 Java 中变量声明和初始化的有效示例 −

Following are valid examples of variable declaration and initialization in Java −

int a, b, c;         // Declares three ints, a, b, and c.
int a = 10, b = 10;  // Example of initialization
byte B = 22;         // initializes a byte type variable B.
double pi = 3.14159; // declares and assigns a value of PI.
char a = 'a';        // the char variable a iis initialized with value 'a'

Java Variables Types

以下是三种类型的 Java 变量：

The following are the three types of Java variables:

  1. Local variables

  2. Instance variables

  3. Class/Static variables

1. Java Local Variables

  1. Local variables are declared in methods, constructors, or blocks.

  2. Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor, or block.

  3. Access modifiers cannot be used for local variables.

  4. Local variables are visible only within the declared method, constructor, or block.

  5. Local variables are implemented at stack level internally.

  6. There is no default value for local variables, so local variables should be declared and an initial value should be assigned before the first use.

这里，age 是一个局部变量。它在 pupAge() 方法中定义，并且其范围仅限于此方法。

Here, age is a local variable. This is defined inside pupAge() method and its scope is limited to only this method.

public class Test {
   public void pupAge() {
      int age = 0;
      age = age + 7;
      System.out.println("Puppy age is : " + age);
   }

   public static void main(String args[]) {
      Test test = new Test();
      test.pupAge();
   }
}

Output

Puppy age is: 7

以下示例在未初始化的情况下使用了 age，因此它将在编译时给出错误。

Following example uses age without initializing it, so it would give an error at the time of compilation.

public class Test {
   public void pupAge() {
      int age;
      age = age + 7;
      System.out.println("Puppy age is : " + age);
   }

   public static void main(String args[]) {
      Test test = new Test();
      test.pupAge();
   }
}

Output

Test.java:4:variable number might not have been initialized
age = age + 7;
         ^
1 error

2. Java Instance Variables

  1. Instance variables are declared in a class, but outside a method, constructor or any block.

  2. When a space is allocated for an object in the heap, a slot for each instance variable value is created.

  3. Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.

  4. Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object’s state that must be present throughout the class.

  5. Instance variables can be declared in class level before or after use.

  6. Access modifiers can be given for instance variables.

  7. The instance variables are visible for all methods, constructors and block in the class. Normally, it is recommended to make these variables private (access level). However, visibility for subclasses can be given for these variables with the use of access modifiers.

  8. Instance variables have default values. For numbers, the default value is 0, for Booleans it is false, and for object references it is null. Values can be assigned during the declaration or within the constructor.

  9. Instance variables can be accessed directly by calling the variable name inside the class. However, within static methods (when instance variables are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.

import java.io.*;

public class Employee {

   // this instance variable is visible for any child class.
   public String name;

   // salary  variable is visible in Employee class only.
   private double salary;

   // The name variable is assigned in the constructor.
   public Employee (String empName) {
      name = empName;
   }

   // The salary variable is assigned a value.
   public void setSalary(double empSal) {
      salary = empSal;
   }

   // This method prints the employee details.
   public void printEmp() {
      System.out.println("name  : " + name );
      System.out.println("salary :" + salary);
   }

   public static void main(String args[]) {
      Employee empOne = new Employee("Ransika");
      empOne.setSalary(1000);
      empOne.printEmp();
   }
}

Output

name  : Ransika
salary :1000.0

3. Java Class/Static Variables

  1. Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.

  2. There would only be one copy of each class variable per class, regardless of how many objects are created from it.

  3. Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final, and static. Constant variables never change from their initial value.

  4. Static variables are stored in the static memory. It is rare to use static variables other than declared final and used as either public or private constants.

  5. Static variables are created when the program starts and destroyed when the program stops.

  6. Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.

  7. Default values are same as instance variables. For numbers, the default value is 0; for Booleans, it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor. Additionally, values can be assigned in special static initializer blocks.

  8. Static variables can be accessed by calling with the class name ClassName.VariableName.

  9. When declaring class variables as public static final, then variable names (constants) are all in upper case. If the static variables are not public and final, the naming syntax is the same as instance and local variables.

import java.io.*;

public class Employee {

   // salary  variable is a private static variable
   private static double salary;

   // DEPARTMENT is a constant
   public static final String DEPARTMENT = "Development ";

   public static void main(String args[]) {
      salary = 1000;
      System.out.println(DEPARTMENT + "average salary:" + salary);
   }
}

Output

Development average salary:1000

Note − 如果变量是从外部类访问的，则常量应作为 Employee.DEPARTMENT 访问

Note − If the variables are accessed from an outside class, the constant should be accessed as Employee.DEPARTMENT

What is Next?

您已经在本章中使用过访问修饰符（public 和 private）。下一章将详细说明访问修饰符和非访问修饰符。

You already have used access modifiers (public & private) in this chapter. The next chapter will explain Access Modifiers and Non-Access Modifiers in detail.