Type Checking

Subtyping recap

Visit the official notes for a proper recap. A way to think of subtypes is with subsets. ColoredCircle <: Circle, and instances of ColoredCircles are a subset of all instances of Circles.

Here is a step-by-step guide on how to deal with type checking.

Summary of Steps

Type checking can be split into 2 main parts -- compile time and runtime check.

Compile Time

As the name suggests, we check whether this passes compilation or throws compilation error.

a = (C) b;

StepsIn other wordsWhat each step is doingPossible casesImpossible

1. Compiler find compile-time type of rhs b i.e. CTT(b)
2. Check for possibility for run-time type of b to be subtype of C i.e. RTT(b) <: C
   • If impossible, exit with compilation error
3. Find CTT of variable lhs a i.e. CTT(a)
4. Check if C <: CTT(a)
   • If no, exit with compilation error
   • Else, add run-time check for RTT(b) <: C

1. Find CTT(b)
2. possible for RTT(b) <: C ? continue : compilation error
3. Find CTT(a)
4. Check if C <: CTT(a) ? runtime check : compilation error

Checks if typecast [(C) b] can potentially occur

1. Compiler find compile-time type of b i.e. CTT(b)
2. Check for possibility for run-time type of b to be subtype of C i.e. RTT(b) <: C
   • If impossible, exit with compilation error

Checks if assignment [a = expr] satisfies subtyping relationship

1. Find CTT of variable a i.e. CTT(a)
2. Check if C <: CTT(a)
   • If no, exit with compilation error
   • Else, add run-time check for RTT(b) <: C

1. CTT(b) <: C
   • widening, always allowed
   • explicit type cast not needed
2. C <: CTT(b)
   • narrowing, runtime check needed
   • If CTT(b) = B and RTT(b) = C (or subtype of C), allowed at runtime
   • If CTT(b) = B and RTT(b) = B (or other subtype of B that is not C), not allowed at runtime
   • Due to possibility, compiler adds code to check at runtime
3. C is interface
   • Let CTT(b) = B, may have subclass B1 where B1 <: C (B1 implements C)
   • If RTT(b) = B1, allowed at runtime

1. Let CTT(b) = B, B and C unrelated i.e. B </: C and C </: B
   • impossible for RTT(b) <: C (subclass of B extends B, cannot also extends C)
2. C interface and B has final modifier
   • case 3 not possible, class cannot be inherited from

Runtime

Once the compile time check pass, compiler does a runtime check (added from step 4).

a = (C) b;

1. Find runtime type of rhs i.e. RTT(b)
2. Check if RTT(b) <: C

Step-by-step

Success

We will look at the following successful example first

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

Step 1Step 2Step 3Step 4Runtime

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

CTT(lhs)	cast	CTT(rhs)
		Object

1. Find compile-time type of obj i.e. CTT(obj)

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

CTT(lhs)	cast	CTT(rhs)
	String	Object

1. Find compile-time type of obj i.e. CTT(obj)
2. Check for possibility for run-time type of obj to be subtype of String i.e. RTT(obj) <: String
   • Case 2: String <: Object, continue

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

CTT(lhs)	cast	CTT(rhs)
String	String	Object

1. Find compile-time type of obj i.e. CTT(obj)
2. Check for possibility for run-time type of obj to be subtype of String i.e. RTT(obj) <: String
   • Case 2: String <: Object, continue
3. Find CTT of str2 i.e. CTT(str2)

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

CTT(lhs)	cast	CTT(rhs)
String	String	Object

1. Find compile-time type of obj i.e. CTT(obj)
2. Check for possibility for run-time type of obj to be subtype of String i.e. RTT(obj) <: String
   • Case 2: String <: Object, continue
3. Find CTT of str2 i.e. CTT(str2)
4. Check if String <: str2
   • String <: String, compiles successfully. Runtime check for RTT(obj) <: String added.

String str1 = "CS2030S";
Object obj = str1; // String <: Object
String str2 = (String) obj;

cast	RTT(rhs)
String	String

1. RTT(obj) is String
2. Since String <: String, no error occurs

Interface

This example shows how it could work with interface

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

Step 1Step 2Step 3Step 4Runtime

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

CTT(lhs)	cast	CTT(rhs)
		Animal

1. Find compile-time type of a i.e. CTT(a)

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

CTT(lhs)	cast	CTT(rhs)
	Runnable	Animal

1. Find compile-time type of a i.e. CTT(a)
2. Check for possibility for run-time type of a to be subtype of Runnable i.e. RTT(a) <: Runnable
   • Case 3: Runnable is interface, no final modifier on Animal, continue

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

CTT(lhs)	cast	CTT(rhs)
Moveable	Runnable	Animal

1. Find compile-time type of a i.e. CTT(a)
2. Check for possibility for run-time type of a to be subtype of Runnable i.e. RTT(a) <: Runnable
   • Case 3: Runnable is interface, no final modifier on Animal, continue
3. Find CTT of m i.e. CTT(m)

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

CTT(lhs)	cast	CTT(rhs)
Moveable	Runnable	Animal

1. Find compile-time type of a i.e. CTT(a)
2. Check for possibility for run-time type of a to be subtype of Runnable i.e. RTT(a) <: Runnable
   • Case 3: Runnable is interface, no final modifier on Animal, continue
3. Find CTT of m i.e. CTT(m)
4. Check if Runnable <: m
   • Runnable <: Moveable, compiles successfully. Runtime check for RTT(a) <: Runnable added.

interface Moveable{}
interface Runnable extends Moveable{}
class Animal implements Moveable{}
class Cheetah extends Animal implements Runnable{}

Cheetah c = new Cheetah();
Animal a = c; // Cheetah <: Animal
Moveable m = (Runnable) a;

cast	RTT(rhs)
Runnable	Cheetah

1. RTT(a) is Cheetah
2. Since Cheetah implements Runnable, Cheetah <: Runnable, no error occurs

Fail step 4

Here is an example where a compilation error occurs as it does not satisfy subtyping relationship

ColoredCircle cc = new ColoredCircle();
Circle c1 = cc; // ColoredCircle <: Circle
Circle c2 = (Shape) c1;

Step 1Step 2Step 3Step 4

ColoredCircle cc = new ColoredCircle();
Circle c1 = cc; // ColoredCircle <: Circle
Circle c2 = (Shape) c1;

CTT(lhs)	cast	CTT(rhs)
		Circle

1. Find compile-time type of c1 i.e. CTT(c1)

ColoredCircle cc = new ColoredCircle();
Circle c1 = cc; // ColoredCircle <: Circle
Circle c2 = (Shape) c1;

CTT(lhs)	cast	CTT(rhs)
	Shape	Circle

1. Find compile-time type of c1 i.e. CTT(c1)
2. Check for possibility for run-time type of c1 to be subtype of Shape i.e. RTT(c1) <: Shape
   • Case 1: Circle <: Shape, continue

ColoredCircle cc = new ColoredCircle();
Circle c1 = cc; // ColoredCircle <: Circle
Circle c2 = (Shape) c1;

CTT(lhs)	cast	CTT(rhs)
	Shape	Circle

1. Find compile-time type of c1 i.e. CTT(c1)
2. Check for possibility for run-time type of c1 to be subtype of Shape i.e. RTT(c1) <: Shape
   • Case 1: Circle <: Shape, continue
3. Find CTT of c2 i.e. CTT(c2)

ColoredCircle cc = new ColoredCircle();
Circle c1 = cc; // ColoredCircle <: Circle
Circle c2 = (Shape) c1;

CTT(lhs)	cast	CTT(rhs)
Circle	Shape	Circle

1. Find compile-time type of obj i.e. CTT(obj)
2. Check for possibility for run-time type of obj to be subtype of String i.e. RTT(b) <: C
   • Case 1: Circle <: Shape, continue
3. Find CTT of c2 i.e. CTT(c2)
4. Check if Shape <: c2
   • Shape </: Circle, compilation error