为什么在字符串输出中出现空值?

问题描述

当我执行以下代码时，输​​出为"nullHelloWorld". Java如何处理null?

When I execute the following code the output is "nullHelloWorld". How does Java treat null?

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        String str=null;
        str+="Hello World";
        System.out.println(str);
    }
}

推荐答案

您正在尝试将值连接到null.这由字符串转换"控制，该字符串转换在一个操作数为String时发生，并由 JLS，第5.1.11节:

You are attempting to concatenate a value to null. This is governed by "String Conversion", which occurs when one operand is a String, and that is covered by the JLS, Section 5.1.11:

• 如果引用为空，则将其转换为字符串"null". (四个ASCII字符n，u，l，l).

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