Page History

...

This code uses == to compare two Integer objects. According to guideline EXP01-J. Do not compare String objects using equality or relational operators, for == to return true for two object references, they must point to the same underlying object. Results of using the == operator in this case will be misleading.

...

These comparisons generate the output sequence: true, false, false and true. The cache in the Integer class can only make the integers from -127 to 128 refer to the same object, which explains the output of the above code. To avoid making such mistakes, use the equals() method instead of == to compare wrapper classes (See guideline EXP03-J for further details.).

Compliant Solution

Using object1.equals(object2) only compares the values of the objects. Now, the results will be true, as expected.

...

public class Wrapper {
 public static void main(String[] args) {
   // Create an array list of integers, where each element
   // is greater than 127
   ArrayList<Integer> list1 = new ArrayList<Integer>();

   for(int i = 0; i < 10; i++) {
     list1.add(i + 1000);
   }

   // Create another array list of integers, where each element
   // is the same as the first one
   ArrayList<Integer> list2 = new ArrayList<Integer>();
   for(int i = 0; i < 10; i++) {
     list2.add(i + 1000);
   }
 
   int counter = 0;
   for(int i = 0; i < 10; i++) {
     if(list1.get(i).equals(list2.get(i))) { 
       counter++;
     }
   }
 
   System.out.println(counter);
 }
}

Exceptions

EXP-03-EX1: Boolean variables can be compared using relational operators, however, if instantiated as an object this is counterproductive.

...

Using the equal and not equal operators to compare boxed primitives can lead to erroneous comparisons.

...