Autoboxing

Computer Science Level 3

What will be the output of the following Java code -

Integer a = 100, b = 100;
System.out.println(a == b);
Integer c = 200, d = 200;
System.out.println(c == d);

Details and Assumptions

Ignore the line break in the print statement.
Code is only tested for Java 5+.

False True True True False False True False Compilation Error Runtime Error

2 solutions

Arulx Z
Nov 24, 2015

Integer is not a primitive type and it shouldn't be compared using == . The == comparison only works when the references of both objects are same.

Java caches the values of integers from -128 to 127. Hence a == b have the same reference due to Java's mechanisms. So the answer returned is true .

However in case of 300 (which is outside the range of -128 to 127), the values are not cached. Hence two objects having the same value may* have different references. Hence c == d returns false.

1	`a.equals(b)`

must be used to compare two objects.

Note: two objects outside the range of -128 to 127 may have the same reference. However the probability is low.

Moderator note:

This is quite peculiar. I didn't know about this behavior.

Well, I learnt something new today. Nice! +1 :)

Prasun Biswas - 5 years, 6 months ago

Agnishom Chattopadhyay
Nov 23, 2015

[This is not a solution]

I've heard that Java cache's values from [-128,128] to prevent recomputation and etc. I have very little Java knowledge, but this feels like nonsense.

Please tell me how this works

This blog might be helpful.

Arulx Z - 5 years, 6 months ago

From what I've understood,

Integer is not a primitive type and == should not be used to compare them.

Integers from -128 to 127 are cached by Java and hence for values of i and j (Note: i = j ) within the range, i == j will always return the same reference. Integers outside of the limits may not do so. So for j and k (Note: j = k ) outside the range, j == k may not return true.

Arulx Z - 5 years, 6 months ago

It's important to note that i == k will only return true if i = k and they both have the same referance.

Arulx Z - 5 years, 6 months ago

@Arulx Z – If Integer isn't a primitive type, what is?

Agnishom Chattopadhyay - 5 years, 6 months ago

@Agnishom Chattopadhyay – It's a class that wraps int primitives into an object.

Arulx Z - 5 years, 6 months ago

Relevant SO post that might help you .

My feelings after I learn about all this stuff .

Prasun Biswas - 5 years, 6 months ago

Haha. C is the best after all.

Arulx Z - 5 years, 5 months ago

Autoboxing

2 solutions

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