問(wèn)題描述
我不知道為什么這些代碼行返回不同的值:
I have no idea why these lines of code return different values:
System.out.println(Integer.valueOf("127")==Integer.valueOf("127"));
System.out.println(Integer.valueOf("128")==Integer.valueOf("128"));
System.out.println(Integer.parseInt("128")==Integer.valueOf("128"));
輸出是:
true
false
true
為什么第一個(gè)返回true，第二個(gè)返回false?127 和 128 之間有什么我不知道的不同之處嗎?(當(dāng)然我知道127 < 128.)
Why does the first one return true and the second one return false? Is there something different that I don't know between 127 and 128? (Of course I know that 127 < 128.)
還有，為什么第三個(gè)返回true?
Also, why does the third one return true?
我已閱讀這個(gè)問(wèn)題的答案，但我還是沒(méi)搞懂怎么返回true，為什么第二行的代碼返回false.
I have read the answer of this question, but I still didn't get how it can return true, and why the code in second line returns false.
推薦答案
這里有一個(gè)顯著的區(qū)別.
There's a striking difference here.
valueOf 正在返回一個(gè) Integer 對(duì)象，該對(duì)象的值可能緩存在 -128 和 127 之間.這就是第一個(gè)值返回 true - 它已緩存 - 第二個(gè)值返回 false - 128 不是緩存值，因此您將獲得兩個(gè)單獨(dú)的 Integer 實(shí)例.
valueOf is returning an Integer object, which may have its values cached between -128 and 127. This is why the first value returns true - it's cached - and the second value returns false - 128 isn't a cached value, so you're getting two separate Integer instances.
請(qǐng)務(wù)必注意，您將引用與 Integer#valueOf 進(jìn)行比較，如果您比較的值大于緩存支持的值，它將not 評(píng)估為 true，即使解析的值是等價(jià)的(例如:Integer.valueOf(128) == Integer.valueOf(128)).您必須改用 equals().
It is important to note that you are comparing references with Integer#valueOf, and if you are comparing a value that is larger than what the cache supports, it will not evaluate to true, even if the parsed values are equivalent (case in point: Integer.valueOf(128) == Integer.valueOf(128)). You must use equals() instead.
parseInt 正在返回一個(gè)原始 int.這就是為什么第三個(gè)值返回 true - 128 == 128 被評(píng)估，當(dāng)然是 true.
parseInt is returning a primitive int. This is why the third value returns true - 128 == 128 is evaluated, and is of course, true.
現(xiàn)在，恰好使第三個(gè)結(jié)果 true:
Now, a fair bit happens to make that third result true:
一個(gè)拆箱轉(zhuǎn)換發(fā)生在 您正在使用的等價(jià)運(yùn)算符和您擁有的數(shù)據(jù)類型 - 即
int和Integer.當(dāng)然，您會(huì)從右側(cè)的valueOf獲得一個(gè)Integer.
An unboxing conversion occurs with respect to the equivalence operator you're using and the datatypes you have - namely,
intandInteger. You're getting anIntegerfromvalueOfon the right hand side, of course.
轉(zhuǎn)換后，您將比較兩個(gè)原始 int 值.比較會(huì)按照您對(duì)原語(yǔ)的預(yù)期進(jìn)行，因此您最終會(huì)比較 128 和 128.
After the conversion, you're comparing two primitive int values. Comparison happens just as you would expect it to with respect to primitives, so you wind up comparing 128 and 128.
這篇關(guān)于為什么 == 與 Integer.valueOf(String) 的比較對(duì) 127 和 128 給出不同的結(jié)果?的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！