New Integer vs valueOf

Question

I was using Sonar to make my code cleaner, and it pointed out that I'm using new Integer(1) instead of Integer.valueOf(1). Because it seems that valueOf does not instantiate a new object so is more memory-friendly. How can valueOf not instantiate a new object? How does it work? Is this true for all integers?

Answer 1

Integer.valueOf implements a cache for the values -128 to +127. See the last paragraph of the Java Language Specification, section 5.1.7, which explains the requirements for boxing (usually implemented in terms of the .valueOf methods).

http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.7

Answer 2

From the JavaDoc:

public static Integer valueOf(int i) Returns a Integer instance representing the specified int value. If a new Integer instance is not required, this method should generally be used in preference to the constructor Integer(int), as this method is likely to yield significantly better space and time performance by caching frequently requested values.

ValueOf is generaly used for autoboxing and therefore (when used for autoboxing) caches at least values from -128 to 127 to follow the autoboxing specification.

Here is the valueOf implementation for Sun JVM 1.5.? Have a look at the whole class to see how the cache is initialized.

public static Integer valueOf(int i) {     final int offset = 128;     if (i >= -128 && i <= 127) { // must cache          return IntegerCache.cache[i + offset];     }     return new Integer(i); }