java 与 Null 的字符串连接
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String concatenation with Null
提问by oshai
I have the following code
我有以下代码
System.out.println("" + null);
and the output is null.
How does Java do the trick in string concatenation?
并且输出是null。
Java 如何实现字符串连接？
回答by oxbow_lakes
Because Java converts the expression "A String" + xto something along the lines of "A String" + String.valueOf(x)
因为 Java 将表达式转换"A String" + x为类似于"A String" + String.valueOf(x)
In actual fact I think it probably uses StringBuilders, so that:
实际上，我认为它可能使用StringBuilders，因此：
"A String " + x + " and another " + y
resolves to the more efficient
解决更有效的问题
new StringBuilder("A String ")
.append(x)
.append(" and another ")
.append(y).toString()
This uses the appendmethods on String builder (for each type), which handle nullproperly
这使用appendString builder 上的方法（对于每种类型），这些方法可以null正确处理
回答by Alexander Pogrebnyak
Java uses StringBuilder.append( Object obj )behind the scenes.
JavaStringBuilder.append( Object obj )在幕后使用。
It is not hard to imagine its implementation.
不难想象它的实现。
public StringBuilder append( Object obj )
{
if ( obj == null )
{
append( "null" );
}
else
{
append( obj.toString( ) );
}
return this;
}
回答by Nathan Hughes
The code "" + nullis converted by the compiler to
代码"" + null由编译器转换为
new StringBuffer().append("").append(null);
and StringBuffer replaces null with the string "null". So the result is the string "null".
并且 StringBuffer 将 null 替换为字符串“null”。所以结果是字符串“null”。
