Question

Why is java.lang.Integer.valueOf a flyweight pattern?
I tried to find the reason but wasn’t able to.

Answer 1

If we look at the source for valueOf, we can get a hint:
Source of java.lang.Integer lines 638-643:

public static Integer valueOf(int i) {
        assert IntegerCache.high >= 127;
        if (i >= IntegerCache.low && i <= IntegerCache.high)
             return IntegerCache.cache[i + (-IntegerCache.low)];
         return new Integer(i);
}

It looks like the Integer class maintains a cache of Integer objects for common values. Rather than make a new one every time somebody asks for valueOf, it just returns a reference to one that already exists. Therefore, if you call Integer.valueOf(1) multiple times, you’ll end up getting the same object back every time (not just equivalent objects).