Concatenating null strings in Java

Question

Why does the following work   I would expect a NullPointerException to be thrown   String s   null  s   s    hello   System out println s      prints  nullhello

User · Answer

See section 5 4 and 15 18 of the Java Language specification      String conversion applies only to the   operands of the binary   operator when   one of the arguments is a String  In   this single special case  the other   argument to the   is converted to a   String  and a new String which is the   concatenation of the two strings is   the result of the    String conversion   is specified in detail within the   description of the string   concatenation   operator    and      If only one operand expression is of   type String  then string conversion is   performed on the other operand to   produce a string at run time  The   result is a reference to a String   object  newly created  unless the   expression is a compile-time constant   expression    15 28  that is the   concatenation of the two operand   strings  The characters of the   left-hand operand precede the   characters of the right-hand operand   in the newly created string  If an   operand of type String is null  then   the string  null  is used instead of   that operand

User · Answer

This is behavior specified in the Java API s String valueOf Object  method  When you do concatenation  valueOf is used to get the String representation  There is a special case if the Object is null  in which case the string  null  is used      public static String valueOf Object obj       Returns the string representation of the Object argument       Parameters        obj - an Object       Returns       if the argument is null  then a string equal to  null   otherwise  the value of obj toString   is returned

User · Answer

Why must it work   The JLS 5  Section 15 18 1 1 JLS 8    15 18 1  String Concatenation Operator     leading to JLS 8     5 1 11  String Conversion   requires this operation to succeed without failure         Now only reference values need to be considered  If the reference is null  it is converted to the string  null   four ASCII characters n  u  l  l   Otherwise  the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments  but if the result of invoking the toString method is null  then the string  null  is used instead     How does it work   Let s look at the bytecode   The compiler takes your code   String s   null  s   s    hello   System out println s      prints  nullhello    and compiles it into bytecode as if you had instead written this   String s   null  s   new StringBuilder String valueOf s   append  hello   toString    System out println s      prints  nullhello     You can do so yourself by using javap -c   The append methods of StringBuilder all handle null just fine   In this case because null is the first argument  String valueOf   is invoked instead since StringBuilder does not have a constructor that takes any arbitrary reference type     If you were to have done s    hello    s instead  the equivalent code would be   s   new StringBuilder  hello   append s  toString      where in this case the append method takes the null and then delegates it to String valueOf     Note  String concatenation is actually one of the rare places where the compiler gets to decide which optimization s  to perform  As such  the  exact equivalent  code may differ from compiler to compiler  This optimization is allowed by JLS  Section 15 18 1 2      To increase the performance of repeated string concatenation  a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression    The compiler I used to determine the  equivalent code  above was Eclipse s compiler  ecj

User · Answer

You are not using the  null  and therefore you don t get the exception   If you want the NullPointer  just do  String s   null  s   s toString      hello     And I think what you want to do is   String s       s   s    hello

User · Answer

The second line is transformed to the following code   s    new StringBuilder    append  String null  append  hello   toString      The append methods can handle null arguments

[java] Concatenating null strings in Java

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