Java中null或空值的判斷處理

1、 null或空值的判斷處理-java 1，錯(cuò)誤用法一： if (n ame = ) /do something 2，錯(cuò)誤用法二： if (n ame.equals() /do something 3，錯(cuò)誤用法三： if (! name.equals() /do something 我們來(lái)解說(shuō)一下： 上述錯(cuò)誤用法1是初學(xué)者最容易犯，也最不容易被發(fā)現(xiàn)的錯(cuò)誤，因?yàn)樗鼈兊恼Z(yǔ)法本身沒(méi)問(wèn)題，Java編譯器編譯 時(shí)不報(bào)錯(cuò)。但這種條件可能在運(yùn)行時(shí)導(dǎo)致程序出現(xiàn)bug，永遠(yuǎn)也不會(huì)為true，也就是時(shí)說(shuō)，if塊里的語(yǔ)句永遠(yuǎn)也 不會(huì)被執(zhí)行。 上述用法二，用法三 的寫(xiě)法，是包括很多Java熟手也很容易犯的錯(cuò)誤，為什么是

2、錯(cuò)誤的呢？也許你會(huì)感到納悶。 對(duì)，它們的寫(xiě)法本身沒(méi)錯(cuò)，但是，少了一個(gè)null判斷的條件，試想，如果 name=null的情況下，會(huì)發(fā)生什么后果 呢？后果是，你的程序?qū)伋鯪ullPointerException 異常，系統(tǒng)將被掛起，不再提供正常服務(wù)。 當(dāng)然，如果之前已經(jīng)對(duì)name作了 null判斷的情況例外。 正確的寫(xiě)法應(yīng)該先加上name != null的條件，如例： if (name != null testNullOrEmpty(value); value =; testNullOrEmpty(value); value =; testNullOrEmpty(value); value =

3、 hello me!; testNullOrEmpty(value); static void testNullOrEmpty(String value) if (value = null ) /正確的寫(xiě)法 System.out.println(” value is null.); else if (.equals(value) /正確的寫(xiě)法 System.out.println(” value is blank but not null. ); else System.out.println(” value is / + value + /); if (value = ) /NG 錯(cuò)誤的寫(xiě)法

4、 /別用這種寫(xiě)法 編譯執(zhí)行: c:/javac TestNullOrEmpty.java c:/java TestNullOrEmpty value is null. value is blank but not null. value is value is hello me! When you are old and grey and full of sleep, And no ddi ng by the fire, take dow n this book, And slowly read, and dream of the soft look Your eyes had once, a

5、nd of their shadows deep; How many loved your mome nts of glad grace, And loved your beauty with love false or true, But one man loved the pilgrim soul in you, And loved the sorrows of your cha nging face; And bending dow n beside the glow ing bars, Murmur, a little sadly, how love fled And paced up

6、on the mountains overhead And hid his face amid a crowd of stars. The furthest dista nee in the world Is not betwee n life and death But whe n I sta nd in front of you Yet you dont know that I love you. The furthest dista nee in the world Is not whe n I sta nd in front of you Yet you cant see my lov

7、e But whe n un doubtedly knowing the love from both Yet cannot be together. The furthest dista nee in the world Is not being apart while being in love But whe n I pla inly cannot resist the year ning Yet prete nding you have n ever bee n in my heart. The furthest dista nee in the world Is not struggli ng