`null` 被视为字符串? [英] `null` is treated as String?

问题描述

String s = null;
s = s + "hai";
System.out.println(s);

输出:

nullhai

认为这会让我 NPE.

Thought this would throw me NPE.

背后的基本逻辑是什么

• 在使用 +(连接)时不抛出 NPE
• 在使用 时抛出 NPE.

• not throwing NPE while using + (concatenation)
• throwing NPE while using .

推荐答案

For , s = s + "hai";在内部，将调用 String.valueOf(null).该方法会将 null 转换为 "null".

For , s = s + "hai"; Internally, String.valueOf(null) will be called. That method will convert null to "null".

字节码:

public static void main(java.lang.String[])   throws java.lang.Exception;
    0:   aconst_null
  //some code
   10:  invokestatic    #27; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;// check this line
// some other code
       27:  return

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