I am a java novice and so confused by the following example. Is it okay to think that "==" sign will compare the values between Integers and "autoboxed" Integers from int, and compare reference address between Integers?
What about doubles and 0/0?
import edu.princeton.cs.introcs.*;
public class Autoboxing {
public static void cmp(Integer first, Integer second) {
if (first < second)
StdOut.printf("%d < %d\n", first, second);
else if (first == second)
StdOut.printf("%d == %d\n", first, second);
else if (first > second)
StdOut.printf("%d > %d\n", first, second);
else
StdOut.printf("%d and %d are incomparable\n", first, second);
}
public static void main(String[] args) {
cmp(new Integer(42), 43);
cmp(new Integer(42), new Integer(42));
cmp(43, 43);
cmp(142, 142);
Integer a0 = 1000;
int b0 = 1000;
Integer c0 = 1000;
StdOut.println("a0==b0?" + (a0==b0));
StdOut.println("a0==c0?" + (a0==c0));
StdOut.println("b0==c0?" + (b0==c0));
double x1 = 0.0, y1 = -0.0;
Double a1 = x1, b1 = y1;
StdOut.println(x1 == y1);
StdOut.println(a1.equals(b1));
double x2 = 0.0/0.0, y2 = 0.0/0.0;
Double a2 = x2, b2 = y2;
StdOut.println(x2 != y2);
StdOut.println(!a2.equals(b2));
}
}
The result is:
42 < 43
42 and 42 are incomparable
43 == 43
142 and 142 are incomparable
=====
a0==b0?true
a0==c0?false
b0==c0?true
=====
true
false
=====
true
false
Answer
Unboxing will be happing when arithmetic operators, comparison operators appear.
eg:
Integer a = 10;
a = a+10; //1.unboxing a to int 2.calculate a+10 3.boxing 20 to Integer.
System.out.print(a > 10); //1.unboxing a to int 2. compare
But when == appear, it depends.
If boxing type appear on both side, it will compare the reference.But if base type appear on one side, and the other side is a boxing type, the boxing type will unboxing to base type.
eg:
Integer a = new Integer(129);
Integer b = new Integer(129);
System.out.println(a == b); // compare reference return false
System.out.println(a == 129); // a will unboxing and compare 129 == 129 return true
PS: In Java.lang.Integer Cache to support the object identity semantics of autoboxing for values between -128 and 127 (inclusive) as required by JLS.
See source code
So:
Integer a = 127;
Integer b = 127; //cached, the same as b a==b return ture
Integer c = 129;
Integer d = 129; // not cached, c==d return false