What would you think if I said "We can run the following code snippet 5 times faster after changing just 1 character"?

long t = System.currentTimeMillis();
Long sum = 0L;
for (long i = 0; i < Integer.MAX_VALUE; i++) {
sum += i;
}
System.out.println("total:" + sum);
System.out.println("processing time: " + (System.currentTimeMillis() - t) + " ms") ;

Output:

total:2305843005992468481
processing time: 6756 ms

After some pondering, you can consider the following, even faster code snippet:

long t = System.currentTimeMillis();
//Long sum = 0L;
long sum = 0L;
for (long i = 0; i < Integer.MAX_VALUE; i++) {
sum += i;
}
System.out.println("total:" + sum);
System.out.println("processing time: " + (System.currentTimeMillis() - t) + " ms") ;

Output:

total:2305843005992468481

processing time: 1248 ms

We can explain this difference with the careless usage of the "Autoboxing" feature, which has been in our lives since Java 1.5. 

Before continuing the cause of the difference, let's consider "Autoboxing" and "Unboxing" concepts in Java.

Variables in Java are divided into two categories: primitive and reference.  There are 8 primitive types and 8 reference types (wrapper classes) for each primitive type.

"Autoboxing " and "Unboxing" examples can be seen in the following code snippet. In the code snippet, a "long" value is added to a List of "Long". In order to do this in Java 1.4, we must put our primitive variable into an appropriate reference type (boxing). Since Java 1.5, the compiler has done this operation for us. So we have been writing less code.

List<Long> longList = new ArrayList<>();      
long i = 4;
longList.add( i ); //autoboxing      
long j = longList.get( 0 ); //unboxing

Since Java 1.5, the compiler has been changing the above code snippet to the following  snippet automatically:

List<Long> longList = new ArrayList<>();      
long i = 4;
longList.add(Long.valueOf( i ) );      
long j = longList.get( 0 ).longValue();

Hence, we can say that, our first code snippet has been changed to the following one. So, we can explain the slower processing time with the code, creating 2147483647 unnecessary "Long" instances.

long t = System.currentTimeMillis();
Long sum = 0L;
for (long i = 0; i < Integer.MAX_VALUE; i++) {
sum += new Long(i);
}
System.out.println("total:" + sum);
System.out.println("processing time: " + (System.currentTimeMillis() - t) + " ms") ;

As a result, we need to consider "Autoboxing" and "Unboxing" concepts in order to write faster Java code.

Resources

Autoboxing and Unboxing
Autoboxing
Efective Java 2nd Edition, J. Bloch