問題描述
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
為什么第一個返回true，第二個返回false?127 和 128 之間有什么我不知道的不同之處嗎?(當然我知道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，為什么第三個返回true?
Also, why does the third one return true?
我已閱讀這個問題的答案，但我還是沒搞懂怎么返回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
這里有一個顯著的區別.
There's a striking difference here.
valueOf 正在返回一個 Integer 對象，該對象的值可能緩存在 -128 和 127 之間.這就是第一個值返回 true - 它已緩存 - 第二個值返回 false - 128 不是緩存值，因此您將獲得兩個單獨的 Integer 實例.
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.
請務必注意，您將引用與 Integer#valueOf 進行比較，如果您比較的值大于緩存支持的值，它將not 評估為 true，即使解析的值是等價的(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 正在返回一個原始 int.這就是為什么第三個值返回 true - 128 == 128 被評估，當然是 true.
parseInt is returning a primitive int. This is why the third value returns true - 128 == 128 is evaluated, and is of course, true.
現在，恰好使第三個結果 true:
Now, a fair bit happens to make that third result true:
一個拆箱轉換發生在 您正在使用的等價運算符和您擁有的數據類型 - 即
int和Integer.當然，您會從右側的valueOf獲得一個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.
轉換后，您將比較兩個原始 int 值.比較會按照您對原語的預期進行，因此您最終會比較 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.
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