问题
Recenlty I saw code (Java) like this:
myMethod(new Integer(123));
I am currently refactoring some code, and there is a tip in Sonar tool, that it's more memory friendly to use sth like this:
myMethod(Integer.valueOf(123));
However in this case, I think that there is no difference if I would use:
myMethod(123);
I could understand that, if I would pass a variable to the method, but hard coded int? Or if there would be Long/Double etc and I want Long representation of number. But integer?
回答1:
new Integer(123) will create a new Object instance for each call.
According to the javadoc, Integer.valueOf(123) has the difference it caches Objects... so you may (or may not) end up with the same Object if you call it more than once.
For instance, the following code:
public static void main(String[] args) {
Integer a = new Integer(1);
Integer b = new Integer(1);
System.out.println("a==b? " + (a==b));
Integer c = Integer.valueOf(1);
Integer d = Integer.valueOf(1);
System.out.println("c==d? " + (c==d));
}
Has the following output:
a==b? false
c==d? true
As to using the int value, you are using the primitive type (considering your method also uses the primitive type on its signature) - it will use slightly less memory and might be faster, but you won't be ale to add it to collections, for instance.
Also take a look at Java's AutoBoxing if your method's signature uses Integer- when using it, the JVM will automatically call Integer.valueOf() for you (therefore using the cache aswell).
回答2:
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.
Parameters:
i- an int value.
Returns:aInteger instance representingi.
Since:
1.5
refer http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Integer.html#valueOf%28int%29
This variant of valueOf was added in JDK 5 to Byte, Short, Integer, and Long (it already existed in the trivial case in Boolean since JDK 1.4). All of these are, of course, immutable objects in Java. Used to be that if you needed an Integer object from an int, you’d construct a new Integer. But in JDK 5+, you should really use valueOf because Integer now caches Integer objects between -128 and 127 and can hand you back the same exact Integer(0) object every time instead of wasting an object construction on a brand new identical Integer object.
private static class IntegerCache {
private IntegerCache(){}
static final Integer cache[] = new Integer[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Integer(i - 128);
}
}
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);
}
refer Why YOU should use Integer.valueOf(int)
EDIT
autoboxing and object creation:
The important point we must consider is that autoboxing doesn't reduce object creation, but it reduces code complexity. A good rule of thumb is to use primitive types where there is no need for objects, for two reasons:
Primitive types will not be slower than their corresponding wrapper types, and may be a lot faster. There can be some unexpected behavior involving == (compare references) and .equals() (compare values).
Normally, when the primitive types are boxed into the wrapper types, the JVM allocates memory and creates a new object. But for some special cases, the JVM reuses the same object.
The following is the list of primitives stored as immutable objects:
boolean values true and false
All byte values
short values between -128 and 127
int values between -128 and 127
char in the range \u0000 to \u007F
refer http://today.java.net/pub/a/today/2005/03/24/autoboxing.html#performance_issue
回答3:
int is primitive type, not an object.
new Integer(123) and Integer.valueOf(123) both return Integer object representing value 123. As per javadoc for Integer.valueOf():
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.
回答4:
Does your method require an int or an Integer?
new Integer(int) and Integer.valueOf(int) both return Integer objects, but valueOf should be preferred as it is more efficient because it returns cached objects. If your method requires an Integer you should use Integer.valueOf.
If your method requires an int, you should use an int (e.g. 123).
However, it is not strictly necessary to match types in this way because of autoboxing, which automatically converts an int to an Integer and vice versa when the types don't match. This allows you to pass an int into a method requiring an Integer, and an Integer into a method requiring an int. But be aware that there are performance costs associated with autoboxing. The most common example of when you would use autoboxing is if you wanted to store primitives in a collection.
回答5:
only range between -128 to +127 implements in cache.
Integer a = new Integer(1);
Integer b = new Integer(1);
System.out.println("a==b? " + (a==b));
Integer c = Integer.valueOf(127);
Integer d = Integer.valueOf(127);
System.out.println("c==d? " + (c==d));
Integer e = Integer.valueOf(128);
Integer f = Integer.valueOf(128);
System.out.println("e==f? " + (e==f));
Refer this java specification:
http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.7
in JDK 5+, you should really use valueOf because Integer now caches Integer objects between -128 and 127 and can hand you back the same exact Integer(0) object every time instead of wasting an object construction on a brand new identical Integer object.
来源：https://stackoverflow.com/questions/9030817/differences-between-new-integer123-integer-valueof123-and-just-123