[英]Why doesn't a string based template variable change in Kotlin?
I know I can embed a var such as $aa
in the string k expression, so line 3 displays the result 10. 我知道我可以在字符串k表达式中嵌入
$aa
,因此第3行显示结果10。
I think the var k will change when I change the value of aa, so I think the line 5 will display the result 15, but in fact it displays 10. why? 我认为var k会在更改aa的值时发生变化,因此我认为第5行将显示结果15,但实际上它会显示10。为什么?
1 var aa=10
2 var k="$aa"
3 toast(k) //This is 10
4 aa=aa+5
5 toast(k) //This is 10, I think it should be 15
6 k="$aa"
7 toast(k) //This is 15
Added Content 新增内容
I think the system will recalculate line 5 in the following code, so I think the new value aa 15 will be embed into k, but in fact , I get the result "10 bb", why? 我认为系统将在以下代码中重新计算第5行,因此我认为新值aa 15将嵌入到k中,但是实际上,我得到的结果是“ 10 bb”,为什么?
1 var aa=10
2 var k="$aa"
3 toast(k) //This is 10
4 aa=aa+5
5 k=k+" bb" //I think system will recalculate, so I think new value 15 will be embed into k
6 toast(k) //This is "10 bb", I think it should be "15 bb"
Strings are immutable and the reference k
points to the specific string "10". 字符串是不可变的,引用
k
指向特定的字符串“ 10”。 That string is made up of the value of the content of your variable aa
. 该字符串由变量
aa
的内容值组成。
If k
were to change with mutation to aa
would demand a few things: 如果
k
因突变而改变为aa
则将需要一些注意事项:
That is a lot of computational overhead to display a string. 显示字符串的计算量很大。
To connect my two points, the last line of your example evaluates as you expect because your variable now points to a different string "15", because the value that the variable aa
holds is different than previously at string construction time. 为了连接我的两点,示例的最后一行将按您期望的方式进行评估,因为变量现在指向不同的字符串“ 15”,因为变量
aa
持有的值与字符串构建时的值不同。
Kotlin doesn't have "lazy evaluation", and var k = "$aa"
is not a function call, it's just an evaluation and instantly assignment. Kotlin没有“惰性求值”,并且
var k = "$aa"
不是函数调用,它只是求值并立即赋值。 Then when you ask var k = "$aa"
, "$aa"
is evaluated, and k
is assigned. 然后,当您要求
var k = "$aa"
,将评估"$aa"
并分配k
。
After that, k
will "forget" about how it was evaluated, so you do aa = aa + 5
will have no effect on k
. 之后,
k
将“忘记”它的评估方式,因此您执行aa = aa + 5
将对k
无效。
For the "Added Content", regarding the same thought of above, k
has been "forgot" how it was evaluated, assigning k = k + " bb"
will just directly add the existing k
(program level joining, not memory level concatenate) with string " bb", not re-evaluate k
with "$aa". 对于“添加的内容”,基于上述相同的思想,
k
被“忘记”了如何评估,分配k = k + " bb"
只会直接添加现有的 k
(程序级联接,而不是存储级串联)字符串“ bb”,而不用“ $ aa”重新评估k
。
When you write down this line: 当您写下这一行时:
var k = "$aa"
... the string template on the right side of the assignment is evaluated immediately, and what gets assigned to k
is the result of the template itself, the literal "10"
string. ...赋值右侧的字符串模板将立即求值,分配给
k
是模板本身的结果,即文字"10"
字符串。 That line is the same as if you had and of these in Java - and of course these won't change automatically later as aa
gets updated either: 该行与您在Java中拥有的相同,并且当然不会随着
aa
的更新而自动更改:
String k = "" + aa;
String k = String.valueOf(aa);
String k = new StringBuilder().append(aa).toString();
To recompute this value every time, you could introduce a local function that you call over and over to create new strings every time, with aa
's then current value inserted into the template: 为了每次都重新计算该值,可以引入一个局部函数,每次调用它一次来创建新的字符串,并将
aa
的然后当前值插入模板:
var aa = 10
fun k() = "$aa" // same as: fun k(): String { return "$aa" }
println(k()) // 10
aa += 5
println(k()) // 15
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