Today will finally write system out. Println()

I've been sorting out IO When streaming notes , We found such a problem ······

understand System.out.println()

stay Java.lang There is a System class , It defines a class ( static state ) Member variables ：
public final static PrintStream out = null;
thus it can be seen ,out nothing but PrintStream An object of . because out stay System Class is statically decorated , So you can call... Directly through the class name System.out.（System.in It's the same thing , I won't go into details here .）
Now? , We got PrintStream An object of class System.out, Calling methods through objects （println） It's better to understand , So you get this statement System.out.println.

// Generally, no one will write like this , It's just easy to understand .
	PrintStream Myout = System.out;
    Myout.println("Sivan");

Null pointer out

here out As an object but declared as null. But why not throw NullPointerException？
Because in System Class has a static initialization block ：

/* register the natives via the static initializer. * * VM will invoke the initializeSystemClass method to complete * the initialization for this class separated from clinit. * Note that to use properties set by the VM, see the constraints * described in the initializeSystemClass method. */
    private static native void registerNatives();
    static {
    
        registerNatives();
    }

JVM When loading a class , It is through registerNatives() This static method gives out Given the initial value , As for the specific operation , You can refer to this article ：https://zhidao.baidu.com/question/38660871.html.

thus it can be seen ,Java All things are objects .

The most important thing is coming ~~~
It's not easy to code words , Ask for attention
Order a collection and don't get lost ~