Date: 14 Oct 2021

A place dedicated to questions related to Autoboxing ;-)

Q. How does autoboxing handle the following code fragment?

Integer a = null;
int b = a;

A. It results in a run-time error. Primitive type can store every value of their corresponding wrapper type except null.

Q Why does the first group of statements print true , but the second false?

Integer a1 = 100;
Integer a2 = 100;
System.out.println(a1 == a2);   // true

Integer b1 = new Integer(100);
Integer b2 = new Integer(100);
System.out.println(b1 == b2);   // false

Integer c1 = 150;
Integer c2 = 150;
System.out.println(c1 == c2);   // false

A. The second prints false because b1 and b2 are references to different Integer objects. The first and third code fragments rely on autoboxing. Surprisingly the first prints true because vlaues between -128 and 127 apper to refer to the same immutable Integer objects (Java’s implementation of valueOf() retrives a cached values if the integer is between -128 and 127), while Java constructs new objects for each integer outside this range.