Before starting with the post, try to find the output for below code snippet:
Integer a = 100;
Integer b = 100;
System.out.println(a == b);
Integer c = 1000;
Integer d = 1000;
System.out.println(c == d);
Note the answer somewhere, we will come back to it after looking at some concepts which will help in answering this.
Auto-boxing and Auto-unboxing in Java
Integer i = 2 doesn’t make sense as i is an object reference and you are assigning it a literal. But this works because of auto-boxing in Java.
During compilation its converted to Integer i = Integer.valueOf(2).
Let’s look at unboxing now,
Integer i = new Integer(2);
int a = i;
See in above code snippet, object reference i is assigned to the literal a, here unboxing is done. i’s integer value is assigned to a. Compiled code snippet looks like below.
Integer i = new Integer(2);
int a = i.intValue();
This covers auto-boxing and auto-unboxing, similar techniques are extended to other primitives like long, double, etc.
Integer Cache
Integer class keeps a cache of Integer objects within some range. Cache class is IntegerCache which is a static member of Integer class. Let’s decompile and have a look at Integer.valueOf(int i) implementation.
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
From above, we can say that if integer i is in range [IntegerCache.low, IntegerCache.high], then the Integer object is returned from the cache otherwise a new Integer object is created.
Default values of low and high are [-128,127].
Below is IntegerCache class definition. We can override the cache size to be more or less by setting the java.lang.Integer.IntegerCache.high parameter.
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
Conclusion
Now that you know auto-boxing, auto-unboxing and IntegerCache, you should be able to tell what is the output for the code snippet that we saw in the beginning of the post.
Answer is true and false.
NOTE: the output would vary if you change the IntegerCache size explicitly.
How I stumbled upon this
I was solving this leetcode problem. And wrote below code to solve it
public boolean canArrange(int[] arr, int k) {
int n = arr.length;
if(n%2!=0) return false;
Map<Integer,Integer> map = new HashMap<>();
for(int i=0;i<n;i++) {
int rem = (arr[i]%k+k)%k;
map.put(rem,map.getOrDefault(rem,0)+1);
}
for(int i=0;i<n;i++) {
int rem = (arr[i]%k+k)%k;
if(rem == 0) {
if(map.get(rem)%2 !=0) return false;
} else if(map.get(rem) != map.get(k-rem)) return false; // buggy line
}
return true;
}
I am doing Integer comparison using !=, this works fine if the value of the integer is in the cache range i.e [-128,127], because then both LHS and RHS would point to the same object from the cache. Changing it to !map.get(rem).equals(map.get(k-rem)) fixed the issue. Integer comparison should be done with compareTo() method or use unboxed values like integer.intValue().