The following table lists the primitive types and their corresponding wrapper classes, which are used by the Java compiler for autoboxing and unboxing:

Autoboxing and Unboxing Examples

Autoboxing

Autoboxing is the automatic conversion that the Java compiler makes to change primitive types to their corresponding object wrapper classes. Here is an example for autoboxing.

List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2) {
    li.add(i);
}

According to the Java Docs, The Java compiler applies autoboxing when a primitive value is:

• Passed as a parameter to a method that expects an object of the corresponding wrapper class.
• Assigned to a variable of the corresponding wrapper class.

Unboxing

Unboxing is the opposite process of autoboxing.

Integer myWrapperInt = 13;
int myPrimitive = myWrapperInt;

The Java compiler applies unboxing when an object of a wrapper class is:

• Passed as a parameter to a method that expects a value of the corresponding primitive type.
• Assigned to a variable of the corresponding primitive type.

Caveats

Autoboxing and unboxing lets developers write cleaner code, making it easier to read, however there are some caveats we need to understand before utilizing them in production code.

Unnecessary Object creation due to Autoboxing

As shown in the example from Javarevisited, autoboxing could throw away object which gets created if autoboxing occurs in a loop. This could potentially slow down the system with frequent garbage collection.

Integer sum = 0;
for(int i=1000; i<5000; i++){
    sum+=i;
    // Integer sum = new Integer(sum.intValue() + i);
}

Refer to Javarevisited for more details.

Complicated method overloading

As discussed in Javarevisited, we need method overloading to distinguish value(int) and value(Integer) since autoboxing/unboxing will potentially introduce subtle bugs if only either method exists.

For example, ArrayList.remove() is overloaded by remove(index) and remove(Object), so that autoboxing/unboxing won't occur to confuse us by mixing removing object by index and removing object itself (e.g. especially when Integer is the object).

Tricky "==" operator

I would like to borrow the example again from Javarevisited. More details can be found on the original post.

public class AutoboxingTest {

    public static void main(String args[]) {

        // Example 1: == comparison pure primitive – no autoboxing
        int i1 = 1;
        int i2 = 1;
        System.out.println("i1==i2 : " + (i1 == i2)); // true

        // Example 2: equality operator mixing object and primitive
        Integer num1 = 1; // autoboxing
        int num2 = 1;
        System.out.println("num1 == num2 : " + (num1 == num2)); // true

        // Example 3: special case - arises due to autoboxing in Java
        Integer obj1 = 1; // autoboxing will call Integer.valueOf()
        Integer obj2 = 1; // same call to Integer.valueOf() will return same
                          // cached Object
        System.out.println("obj1 == obj2 : " + (obj1 == obj2)); // true

        // Example 4: equality operator - pure object comparison
        Integer one = new Integer(1); // no autoboxing
        Integer anotherOne = new Integer(1);
        System.out.println(
                "one == anotherOne : " + (one == anotherOne)); // false
    }
}

Here is the output.

i1==i2 : true
num1 == num2 : true
obj1 == obj2 : true
one == anotherOne : false

I would like to put the insightful explanation from the original post here.

In first example both argument of == operator is primitive int type so no autoboxing occurs and since 1==1 it prints true.

While in second example during assignment to num1, autoboxing occurs which converts primitive 1 into Integer(1) and when we compare num1==num2 unboxing occurs and Integer(1) is converted back to 1 by calling Integer.intValue() method and since 1==1 result is true.

In Third example which is a corner case in autoboxing, both Integer object are initialized automatically due to autoboxing and since Integer.valueOf() method is used to convert int to Integer and it caches object ranges from -128 to 127, it returns same object both time. In short obj1 and obj2 are pointing to same object and when we compare two object with == operator it returns true without any autoboxing.

In last example object are explicitly initialized and compared using equality operator , this time == return false because both one and anotherOne reference variables are pointing to different object.

Resource

Oracle Docs
Javarevisited Tutorial: What is Autoboxing and Unboxing in Java
Javarevisited Tutorial: Why Comparing Integer using == in Java 5 is Bad?