Null陷阱

public class Null {

        public static void g() {
            System.out.println("hello, world");
        }

        public static void main(String[] args) {
            Null x = null;
            x.g();
            ((Null) x).g();
            ((Null) null).g();
        }

    }

字节码

// class version 52.0 (52)
// access flags 0x21
public class Null {

  // compiled from: Null.java

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 1 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LNull; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static g()V
   L0
    LINENUMBER 4 L0
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "hello, world"
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L1
    LINENUMBER 5 L1
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 0

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 8 L0
    ACONST_NULL
    ASTORE 1
   L1
    LINENUMBER 9 L1
    ALOAD 1
    POP
    INVOKESTATIC Null.g ()V
   L2
    LINENUMBER 10 L2
    ALOAD 1
    POP
    INVOKESTATIC Null.g ()V
   L3
    LINENUMBER 11 L3
    ACONST_NULL
    CHECKCAST Null
    POP
    INVOKESTATIC Null.g ()V
   L4
    LINENUMBER 12 L4
    RETURN
   L5
    LOCALVARIABLE args [Ljava/lang/String; L0 L5 0
    LOCALVARIABLE x LNull; L1 L5 1
    MAXSTACK = 1
    MAXLOCALS = 2
}