types

Summary

Subtype

Primitive types

• stores a copy of its value

int i = 5;
int j = i;
// both i and j store their own copy of the value 5

Reference types

• stores a pointer to the object in memory
• is a reference the instance of the object

Object i = new Object();
Object j = i;
// both i and j store the same pointer to the intance of object in memory

Concept

Dynamically typed languages

• Same variable can hold data of any data type

Statically typed languages

• Variable can hold data of the type the variable was declared with and its subtypes

Strong typed

• Type-safety, type-checking to reduce runtime-errors

Weak typed

• No type-checking

Rules of subtype notation

Widening and narrowing

• Java allows for a variable of type T to hold a value fron type S, if S<:T

Application

Java primitive type hierachy

Compile time type checking

double d = 5.0;
int i = 5;
d = i; // ok, since int is a subtype of double
i = d; // error