Java Wrapper's Comparision

Question

public static void main(String[] args) {    
    System.out.println((Integer.valueOf("5000") <= Integer.valueOf("5000")));
    System.out.println((Integer.valueOf("5000") == Integer.valueOf("5000")));       
}

The above code prints true and false respectively. This is understandable, but why it prints false when we use ==.

But, when using <= (less than or equal to) why is the answer true?

Answer 1

To explain why the code prints such output, we might need to look into lower level:

I decompiled your code to bytecode level.

For the 1st line:

System.out.println((Integer.valueOf("5000") <= Integer.valueOf("5000")));

The bytecode is(useless information is removed):

LDC "5000"
INVOKESTATIC java/lang/Integer.valueOf (Ljava/lang/String;)Ljava/lang/Integer;  
INVOKEVIRTUAL java/lang/Integer.intValue ()I
LDC "5000"
INVOKESTATIC java/lang/Integer.valueOf (Ljava/lang/String;)Ljava/lang/Integer;
INVOKEVIRTUAL java/lang/Integer.intValue ()I
IF_ICMPGT L1

You can see for the left part of <= , JVM use Integer.valueOf function to convert the string to a Integer object. Then use Integer.intValue function to extract the inner value of this object(also called auto-unboxing). So, for the left part, we get an int value.

The right part of <= is the same as left part.

The last line IF_ICMPGT, is to compare these two int values. So, the conclusion is if you are using <=, Java compiler will do auto-unboxing for you and compare the inner int values.

For the 2nd line:

System.out.println((Integer.valueOf("5000") == Integer.valueOf("5000")));

The bytecode is(useless information is removed):

LDC "5000"
INVOKESTATIC java/lang/Integer.valueOf (Ljava/lang/String;)Ljava/lang/Integer;
LDC "5000"
INVOKESTATIC java/lang/Integer.valueOf (Ljava/lang/String;)Ljava/lang/Integer;
IF_ACMPNE L4

You can see the bytecode is different from the 1st line. It just convert string to Integer objects, but NOT auto-unboxing them. And because they are two individual objects, they must have different addresses(In the memory).

The last line IF_ACMPNE is going to compare the addresses of these two Integer objects. So, the conclusion is if you are using ==, Java compiler will do NOTauto-unboxing for you and compare the object addresses.

What's more

The Integer class caches the Integer objects for range -128~127. It means if you pass a string with in this range, you will get the exactly same Integer object. Below code will print true:

System.out.println((Integer.valueOf("127") == Integer.valueOf("127"))); // true

Answer 2

When you compare wrapper classes using <=, >= Java performs unboxing and compares actual int (or double, long, etc) values between each other. So Integer.valueOf("5000") <= Integer.valueOf("5000") will work like this Integer.valueOf("5000").intValue() <= Integer.valueOf("5000").intValue() which is equivalent to 5000 <= 5000.