這里記錄下運(yùn)算符中重要的知識(shí)點(diǎn)。

二元運(yùn)算符

除0

進(jìn)行除法運(yùn)算時(shí)需要注意， 整數(shù)除以 0 會(huì)拋出異常，但是浮點(diǎn)數(shù)除以 0 會(huì)得到無窮大或者 NaN。

<pre mdtype="fences" cid="n7" lang="java" class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">System.out.println(0.0 / 0); // NaN
System.out.println(1.0 / 0); // Infinity
System.out.println(1 / 0); // Exception in thread "main" java.lang.ArithmeticException: / by zero</pre>

隱式類型轉(zhuǎn)換

<pre mdtype="fences" cid="n20" lang="java" class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">short s1 = 1;
s1 = s1 + 1; // error</pre>

Java 執(zhí)行二元操作時(shí)會(huì)先將操作數(shù)轉(zhuǎn)化為同一種類型，再進(jìn)行計(jì)算，轉(zhuǎn)化規(guī)則為兩個(gè)操作數(shù)中如果有一個(gè)是 double 或 float 或 long 時(shí)， 另一個(gè)數(shù)就轉(zhuǎn)化為相應(yīng)的double 或 float 或 long，如果沒有，則都轉(zhuǎn)化為 int 類型再進(jìn)行計(jì)算，上述 s1 + 1 先將 s1 轉(zhuǎn)化為 int 類型（隱式轉(zhuǎn)換），然后執(zhí)行 + 運(yùn)算，操作得出的結(jié)果也是 int 類型， 不能賦值給 short 類型，因此會(huì)出錯(cuò)。

<pre mdtype="fences" cid="n24" lang="java" class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">s1 += 1;
s1++;</pre>

但是上述語(yǔ)句是有效的，因?yàn)槠涞韧?s1 = (short) (s1 + 1);

位運(yùn)算

<pre mdtype="fences" cid="n54" lang="java" class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">~4 = -5;</pre>

上述代碼對(duì) 4 進(jìn)行~ 運(yùn)算，對(duì) 4 所有位進(jìn)行取反， 求值過程如下：

• 4 （0000 0000 0000 0000 0000 0000 0000 0100）按位取反：1111 1111 1111 1111 1111 1111 1111 1011（補(bǔ)碼）

• 求反碼（補(bǔ)碼 - 1）：1111 1111 1111 1111 1111 1111 1111 1010（反碼）

• 求原碼（最高位不變，其他位取反）：1000 0000 0000 0000 0000 0000 0000 0101（-5）

這道題的求值過程用到了原碼，反碼，補(bǔ)碼的知識(shí)，簡(jiǎn)單總結(jié)一下：

• 帶符號(hào)整數(shù)有原碼、反碼、補(bǔ)碼等幾種編碼方式。原碼即直接將真值轉(zhuǎn)換為其相應(yīng)的二進(jìn)制形式，而反碼和補(bǔ)碼是對(duì)原碼進(jìn)行某種轉(zhuǎn)換編碼方式。

• 計(jì)算機(jī)運(yùn)算是都是通過補(bǔ)碼進(jìn)行運(yùn)算，原因是為了只使用加法運(yùn)算而不使用減法運(yùn)算

• 求位運(yùn)算結(jié)果需要將最終的補(bǔ)碼轉(zhuǎn)換為原碼，再通過原碼求其真值

• 整數(shù)的原碼，反碼，補(bǔ)碼都相同

• 復(fù)數(shù)的反碼，補(bǔ)碼需要根據(jù)原碼進(jìn)行轉(zhuǎn)換計(jì)算

運(yùn)算符優(yōu)先級(jí)

Reference

1. Java核心技術(shù)·卷 I（原書第10版）

2. operators