[Java]变量及其初始化 与 类对象的初始化

1 变量

1.1 变量的【定义】

1.2 变量的【作用域】

1.3 变量的【初始值】

1.4 补充：缓存变量

1.5 变量的【分类】与【未初始化情况】

2 类对象

2.1 类对象的初始化/构造过程

• 1 【对象】的初始化流程

• 2 实验

class Base{
    static {  System.out.println("[Base:static area]"); }//step1
    { System.out.println("Base:instance area"); }//step3
    Base(){ System.out.println("<Base:constructor>"); }//step4
}
public class Create extends Base{
    static {  System.out.println("[Create:static area]"); }//step2
    {  System.out.println("Create:instance area");  }//step5
    Create(){//step6
//        super();//默认：首先调用super()
        System.out.println("<Create:constructor>");
    }
    public static void main(String args[]){ Create create = new Create(); }
}

//Output
[Base:static area]
[Create:static area]
Base:instance area
<Base:constructor>
Create:instance area
<Create:constructor>

• 3 360公司2020春招技术C卷试题
  
  (题目小有改动)

class Clazz {
    public static int x;
    public boolean z;
    public static String printX(Clazz clazz) { return "x:" + clazz.x; }
    public static String printZ(Clazz clazz){ return "z:" + clazz.z; }
}

public class Singleton {
    private static Singleton singleton = new Singleton();
    public int counter0;//[值域/类的属性 →基本类型变量] 默认 初始化为0
    public static int counter1;//[值域/类的属性] 默认 初始化为0
    public static int counter2 = 0;//[值域/类的属性]
    public Clazz clazz;//[值域/类的属性 → 引用类型变量]
    private Singleton() {
        int x;//[局部变量]
//        System.out.println(x);//局部变量未初始化 ∴编译时报错 [局部变量]
        System.out.println(Clazz.printX(clazz));//x:0 /[值域/类的属性 → 引用类型变量 de 静态属性]
//        System.out.println(Clazz.printZ(clazz));//运行时报错 (ExceptionInInitializerError) [值域/类的属性 → 引用类型变量 de 非静态属性]
        counter0++;
        counter1++;
        counter2++;
    }
    public static Singleton getInstance(){ return singleton; }
    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();
        System.out.println(singleton.counter0);//1
        System.out.println(Singleton.counter1);//1
        System.out.println(Singleton.counter2);//0
    }
}

x:0
1
1
0