为 static 变量分配空间，设置默认值（准备阶段）

话不多说直接上代码：

package com.beyond.dhl;


class Singleton {
    private static Singleton instance;   // 懒汉式所以不会进行实例化对象

    private Singleton() {
        System.out.println("构造方法:" + Thread.currentThread().getName());
    }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }


}

public class Test {
    public static void main(String[] args) {
        new Thread(() -> Singleton.getInstance(), "线程A").start();
        new Thread(() -> Singleton.getInstance(), "线程B").start();
        new Thread(() -> Singleton.getInstance(), "线程C").start();
        new Thread(() -> Singleton.getInstance(), "线程D").start();
        new Thread(() -> Singleton.getInstance(), "线程E").start();
    }
}

public class Demo01 {
    public static void main(String[] args) {
        int i = 10;  //十进制
        int i2 = 010;  //八进制0
        int i3 = 0x10;  //十六进制0x
        int i4 = 0b10;  //二进制0b

        System.out.println(i);
        System.out.println(i2);
        System.out.println(i3);
        System.out.println(i4);
    }
}


public class Demo02 {
    public static void main(String[] args) {
        float f = 0.1f;
        double d = 1.0/10;

        System.out.println(f == d);
        System.out.println(f);
        System.out.println(d);

        System.out.println("========================================");

        float d1 = 233333333333333333F;
        float d2 = d1 + 1;
        System.out.println(d1 == d2);
    }
}

public class MyClass {
	static int num;
}
public class Test_MyClass {
	public static void main(String[] args) {
		//可直接通过  类名.属性名  进行访问
		MyClass.num=10;
		
		MyClass m1 = new MyClass();
		m1.num=20;
		System.out.println(m1.num);//num=20
		
		MyClass m2 = new MyClass();
		m2.num=30;
		System.out.println(m2.num);//num=30
		System.out.println(m1.num);//num=30
	}
}


package com.soder;
public class Soder1 extends Soders{
	
	public Soder1() {
		p--;
		System.out.println("子类构造方法\t"+p);
	}
	{
		System.out.println("子类代码块\t"+p);
	}
	
	static {
		System.out.println("子类静态代码块\t"+p);
	}
	public static void m1() {
		System.out.println("子类静态指令m1\t"+p);
	}
	public void m2() {
		System.out.println("子类指令m2\t"+p);
	}
}

tips：当类被加载时，类的所有信息（包名，类名，属性，方法等）都会被加载到方法区，而其中的静态属性方法又会被调进静态域，可通过类名直接调用出来