代碼塊分類及執行順序

根據代碼塊定義的位置以及關鍵字，可以分爲以下四種：

• 普通代碼塊
• 構造代碼塊
• 靜態塊
• 同步代碼塊

一、普通代碼塊
定義在方法中的代碼塊，如：

//直接使用{}定義
public class Main {
    public static void main(String[] args) {
        {
            int x = 1;
            System.out.println("x = " + x);
        }
        {
            int y = 2;
            System.out.println("y = " + y);
        }
    }
}

二、構造代碼塊
定義在類中的代碼塊(不加修飾符)，也稱實例代碼塊，一般用於初始化實例成員變量。如：

class Student{
    private String name;
    private int age;
    //構造函數
    public Student(){
        System.out.println("we are winner");
    }
    //構造代碼塊
    {
        this.age = 20;
        this.name = "jack";
        System.out.println("I am the best");
    }

    public void show(){
        System.out.println("name:" + name + " age:" + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Student student = new Student();
        student.show();
    }
}

輸出：
I am the best
we are winner
name:jack age:20

注意：實例代碼塊優先於構造函數執行。
三、靜態代碼塊
使用static定義的代碼塊，一般用於初始化靜態成員屬性。如：

class Student{
    private String name;
    private int age;
    private static int count = 0;//靜態成員變量，存於共享區

    //構造函數
    public Student(){
        System.out.println("we are winner");
    }

    //構造代碼塊(實例代碼塊)
    {
        this.age = 20;
        this.name = "Run";
        System.out.println("I am the best");
    }

    //靜態代碼塊
    static {
        count = 10;
        System.out.println("I am static and count = " + count);
    }

    public void show(){
        System.out.println("name:" + name + " age:" + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Student student1 = new Student();
        Student student2 = new Student();
        student1.show();
        student2.show();
    }
}

輸出：
I am static and count = 10
I am the best
we are winner
I am the best
we are winner
name:Run age:20
name:Run age:20
注意事項：

• 不管生成多少個對象，靜態代碼塊只會執行一次，且是最先執行。
• 靜態代碼塊執行完畢後，實例代碼塊(構造塊)執行，再構造函數執行。