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Static 關鍵字：解碼 Java 中的靜態變數

Susan Sarandon

Oct 21, 2024 pm 08:12 PM

Static Keyword: Decoding Static Variables in Java

歡迎回到Java 關鍵字基礎系列！這篇文章主要是關於靜態變數——Java 中的一個強大功能，可讓您在同一類別的多個物件之間共享資料。

我們將透過實踐範例和見解來介紹靜態變數的目的、行為和關鍵用例。在單獨的貼文中，我們將深入研究靜態方法以避免過多的內容讓您不知所措。

這篇文章建立在先前文章中討論的概念的基礎上。我建議查看最終關鍵字和靜態區塊，以更深入地了解此處涵蓋的主題。

什麼是靜態變數？

靜態變數屬於類別而不是單一實例。它在類別的所有物件之間共享，並且在這些物件之間保留相同的值。

主要特徵

類別級變數：由類別的所有實例共用。
初始化一次：當類別被 JVM 載入時。
記憶體位置：儲存在方法區中。
存取：可以使用類別名稱和物件引用（儘管首選類別名稱）來存取。

程式碼範例：動作中的靜態變數

package keywords.static_keyword;

public class StaticVariables {

    // Static variable: Shared across all instances of the class
    // Automatically initialized to default value on class loading
    static int idCounter; // int default value -> 0

    // Static final variables
    // Must be initialized at declaration or in a static block
    static final String COMPANY_NAME = "TechCorp";
    static final String OFFICE_CODE;

    // Instance variables: Unique to each object
    int employeeId;
    String employeeName;

    // Static final variable Initialized in a static block
    static {
       // Default region: US
       String region = System.getProperty("user.region", "US");
        switch (region) {
            case "EU":
                regionalOfficeCode = "EU-01";
                break;
            case "APAC":
                regionalOfficeCode = "AP-11";
                break;
            default:
                regionalOfficeCode = "US-00";
        }
        System.out.println("Static Block Invoked: Office Code set to " +
        regionalOfficeCode);
    }

    // Constructor: Assigns a unique ID to each object
    public StaticVariables(String name) {
        this.employeeName = name;
        this.employeeId = ++idCounter; // Incrementing the shared counter
    }

    // Instance method
    // Displays instance details along with shared data(static variables)
    void displayEmployeeDetails() {
        System.out.println("Employee ID: " + employeeId + 
            ", Name: " + employeeName + 
            ", Company: " + COMPANY_NAME +
            ", Office Code: " + OFFICE_CODE);
    }

    public static void main(String[] args) {
        // Creating instances to observe static variable behavior
        StaticVariables emp1 = new StaticVariables("Alice");
        StaticVariables emp2 = new StaticVariables("Bob");

        emp1.displayEmployeeDetails();
        emp2.displayEmployeeDetails();

        // Accessing the static variable directly using the class name
        System.out.println("Total Employees: " + StaticVariables.idCounter);
    }
}

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代碼說明

輸出：

Static Block Invoked: Office Code set to US-00
Employee ID: 1, Name: Alice, Company: TechCorp, Office Code: US-00
Employee ID: 2, Name: Bob, Company: TechCorp, Office Code: US-00
Total Employees: 2

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示範的關鍵概念：

靜態變數:
- 在所有實例之間共用並在實例之間保留其值。
- 範例：idCounter 追蹤已建立的員工數量。
靜態最終變數：
- 其值被分配一次且永不改變的常數。
- 範例：COMPANY_NAME 和 OFFICE_CODE。
靜態區塊：
- 載入類別時執行一次。
- 用於根據環境配置初始化OFFICE_CODE等複雜的靜態變數。
實例變數與方法:
- 每個實例都有唯一的employeeId 和employeeName 值。

靜態變數的類別級存取：

使用類別名稱存取 idCounter：

package keywords.static_keyword;

public class StaticVariables {

    // Static variable: Shared across all instances of the class
    // Automatically initialized to default value on class loading
    static int idCounter; // int default value -> 0

    // Static final variables
    // Must be initialized at declaration or in a static block
    static final String COMPANY_NAME = "TechCorp";
    static final String OFFICE_CODE;

    // Instance variables: Unique to each object
    int employeeId;
    String employeeName;

    // Static final variable Initialized in a static block
    static {
       // Default region: US
       String region = System.getProperty("user.region", "US");
        switch (region) {
            case "EU":
                regionalOfficeCode = "EU-01";
                break;
            case "APAC":
                regionalOfficeCode = "AP-11";
                break;
            default:
                regionalOfficeCode = "US-00";
        }
        System.out.println("Static Block Invoked: Office Code set to " +
        regionalOfficeCode);
    }

    // Constructor: Assigns a unique ID to each object
    public StaticVariables(String name) {
        this.employeeName = name;
        this.employeeId = ++idCounter; // Incrementing the shared counter
    }

    // Instance method
    // Displays instance details along with shared data(static variables)
    void displayEmployeeDetails() {
        System.out.println("Employee ID: " + employeeId + 
            ", Name: " + employeeName + 
            ", Company: " + COMPANY_NAME +
            ", Office Code: " + OFFICE_CODE);
    }

    public static void main(String[] args) {
        // Creating instances to observe static variable behavior
        StaticVariables emp1 = new StaticVariables("Alice");
        StaticVariables emp2 = new StaticVariables("Bob");

        emp1.displayEmployeeDetails();
        emp2.displayEmployeeDetails();

        // Accessing the static variable directly using the class name
        System.out.println("Total Employees: " + StaticVariables.idCounter);
    }
}

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