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"
This question is related to
java
string
concatenation
string-concatenation
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.
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";
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.
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 (ยง15.28))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.
Source: Stackoverflow.com