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单选题
String a = null；  a.concat（“abc”）；  a.concat（“def”）；  System.out.println（a）；   What is the result？（）
A

 abc

B

 null

C

 abcdef

D

 Compilation fails.

E

 The code runs with no output.

F

 An exception is thrown at runtime.

参考答案

参考解析
解析： Exception in thread "main" java.lang.NullPointerException at X.main(X.java:12) 
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