为什么空引用打印为“null”？

问题描述

在println中，这里o.toString（）抛出NPE但是o1抛出NPE。为什么？

In println, here o.toString() throws NPE but o1, does not. Why?

public class RefTest {
    public static void main(String[] args) {
        Object o = null;
        Object o1 = null;
        System.out.println(o.toString()); //throws NPE
        System.out.print(o1); // does not throw NPE
    }
}

推荐答案

它可能有助于向您显示字节码。看一下你班级的以下 javap 输出：

It might help showing you the bytecode. Take a look at the following javap output of your class:

> javap -classpath target\test-classes -c RefTest

Compiled from "RefTest.java"
public class RefTest extends java.lang.Object{
public RefTest();
  Code:
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   aconst_null
   1:   astore_1
   2:   aconst_null
   3:   astore_2
   4:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   7:   aload_1
   8:   invokevirtual   #23; //Method java/lang/Object.toString:()Ljava/lang/String;
   11:  invokevirtual   #27; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   14:  getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   17:  aload_2
   18:  invokevirtual   #33; //Method java/io/PrintStream.print:(Ljava/lang/Object;)V
   21:  return

}

只需查看主要方法，您就可以看到感兴趣的行是代码是8和33。

Just looking at the main method, you can see the lines of interest are where Code is 8 and 33.

代码8显示了调用 o.toString（）的字节码。这里 o 是 null 所以在 null 结果为 NullPointerException 。

Code 8 shows the bytecode for you calling o.toString(). Here o is null and so any attempt on a method invocation on null results in a NullPointerException.

代码18显示您的 null 将对象作为参数传递给 PrintStream.print（）方法。查看此方法的源代码将显示为什么不会导致NPE中的结果：

Code 18 shows your null object being passed as a parameter to the PrintStream.print() method. Looking at the source code for this method will show you why this does not result in the NPE:

public void print(Object obj) {
    write(String.valueOf(obj));
}

和 String.valueOf（）将使用 null s执行此操作：

and String.valueOf() will do this with nulls:

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

所以你可以看到有一个测试处理 null ，并阻止NPE。

So you can see there is a test there which deals with null, and prevents an NPE.

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