Why Doesn\'t Concatenating a Null String in Java Throw a NullPointerException?

Concatenating Null Strings in Java

In Java, concatenating a null string with another string does not throw a NullPointerException, as one might expect. Instead, the result is a new string with "null" prepended.

Why is this the Expected Behavior?

According to the Java Language Specification (JLS):

"If the reference [to the null string] is null, it is converted to the string 'null' (four ASCII characters n, u, l, l)."

Therefore, the null string is automatically converted to "null" before the concatenation takes place.

How is this Behavior Implemented?

The compiler optimizes string concatenation to improve performance. It does this by using a StringBuilder object to accumulate the concatenation results.

When encountering a null string:

• If the null string is the first argument, it calls String.valueOf() to convert it to a non-null string ("null").
• If the null string is not the first argument, it appends the null string to the StringBuilder and then calls String.valueOf() on the result.

Example

Consider the following code:

String s = null;
s = s + "hello";
System.out.println(s); // prints "nullhello"

The compiler optimizes this code into the following equivalent:

s = new StringBuilder(String.valueOf(s)).append("hello").toString();

Since s is null, String.valueOf(s) returns "null", which is then appended to the StringBuilder and converted to a string.

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