wrapper classes

Complete

Summary

Reference types for primitives

  • using primitive types as reference types
  • allows primitive types to have OOP properties
java
Integer i = Integer.valueOf(4); // reference type Integer
int j = i.intValue();           // primitive type int

Auto-boxing/unboxing

  • pre-compilation
java
Integer i = 4; // boxed into -> Integer.valueOf(4)
int j = i;     // unboxed from -> i.intValue()

Concept

A reference type equivalent of the java primitive types

  • primitive wrapper classes are final

Variance of complex types

Integer[] is covariant, but int[] is invariant

Arrays are covariant

java
Integer[] ints;
Object[] objects;
objects = ints; // Compiles as Integer[] <: Object[]
ints = objects; // Error

Dangers of covariance of arrays

  • runtime error
  • mismatch of types within arrays
java
Object[] objs = new Integer[] { 1, 2 }; // CTT(objs) -> Object[], RTT(objs) -> Integer[]
objs[0] = "String"; // compiler allows String to be added to Object[], type mismatch occurs in runtime

Application

Iterating with wrapper classes

java
for (Integer i = 2030; i <= 2040; i += 1) {
	// new instance of Integer is created for every iteration
}

Double and Integer

java
double a = 1.0; // works
double b = 1;   // casting int to double, works since int <: double

int c = 1;   // works
int d = 1.0; // casting double to int, fails since narrowing type conversion

Double e = 1.0; // works
Double f = 1;   // cannot autobox int to Double, additionally Integer </: Double