Stack Heap Global Diagram

Represent variables as boxes, organized in different memory areas

This notional machine represents the state of a program by showing three separate parts of memory: the stack with its stack frames and their local variables, the heap with heap objects and their instance variables, and the global area with static variables.

Every variable is shown as a rectangle containing its current value. Variables are annotated with their name. In statically typed languages, variables are also annotated with their type.

64 Misconceptions Expressible in this Notional Machine

A notional machine focuses on a subset of the syntax and semantics of a programming language. The following misconceptions are related to the language features expressible in this notional machine.

AddMemberAtRuntime

Set of class members can change at runtime

ArrayHasLengthMethod

To get the length of an array, one needs to call its length method

ArrayListIsArray

ArrayLists are arrays

ArrayRankIsLength

Array rank and array length are the same thing

ArraysGrow

Arrays can grow dynamically

AssignCompares

= compares two values

AssignCompares

= compares two values

AssignmentCopiesObject

Assignment copies the object

BoxedNull

Passing null to a wrapper class constructor creates an object representing the absence of a value

CallWithoutFrame

A method invocation does not necessarily allocate a stack frame

CallerFrameContainsCalleeFormal

Stack frame of caller includes variables for callee's formal parameters

ConstructorAllocates

The constructor allocates the object

ConstructorParameterIsField

Formal constructor parameters are equivalent to instance variables

EqualityOperatorComparesListIdentities

(list a) = (list b) compares the identities of list a and list b

EqualityOperatorComparesObjectsValues

o==p compares the objects referred to by variables o and p

EqualsComparesReferences

o.equals(p) compares the references stored in the variables o and p

ExpressionAssigns

An expression that reads a variable also updates its value after the evaluation

FinalReferenceImpliesImmutability

An object referred to by a final variable is an immutable object

ForEachVariableIsElement

One can assign to the variable of an enhanced for statement to store a value in the corresponding array or collection element

FrameIsClassInstance

A stack frame is the same as an instance of a class

ImmutableRequiresFinalParameters

Immutable classes need final constructor/method parameters

InitCreates

__init__ must create a new object

LocalVariablesAutoInitialized

Local variables are automatically initialized

MethodAsField

Each object contains its own special fields for all of its methods

MultiReferenceVariable

A reference variable can point to multiple objects

MultiValueVariable

A variable can contain more than one value

MultidimensionalArray

A multi-dimensional array is one thing

MustInitializeFieldInConstructor

Constructors must assign values to all fields

NoFieldInheritance

An object contains only the fields declared in its class

NoFieldlessObjects

Objects without instance variables can't exist

NoJaggedArrays

Multi-dimensional arrays have a rectangular shape

NoLocalVariables

There are no local variables

NullIsObject

null is an object

NullPointerExceptionCompileTime

NullPointerExceptions are detected at compile time

ObjectLabeledWithConstructorSignature

In stack and heap diagrams, an object on the heap is labeled like a stack frame of a constructor call

ObjectLabeledWithVariableName

In stack and heap diagrams, an object on the heap is labeled with the name of the variable that refers to it

ObjectsMustBeNamed

A variable is needed to instantiate an object

ObjectsMustBeNamed

A variable is needed to instantiate an object

OnlyInnermostArrayElements

Only the elements of the innermost array of a multi-dimensional array are accessible

PrimitiveIsObject

Primitive values are heap objects

PrimitiveVariablesDynamicallyTyped

The type of a primitive variable depends on its value

PrivateFieldsImplyImmutability

A class where all fields are private is immutable

PrivateFromOtherInstance

An object cannot access private members of other objects of the same class

PrivateFromStatic

Static methods cannot access private members of instances of same class

PrivateMeansFinal

A private field cannot be changed

RecursiveActivationsShareFrame

Recursive calls of a method share a stack frame

ReferenceIntoStack

References can point into the stack

ReferenceLabeledWithDynamicType

In stack and heap diagrams, reference variables are labeled with the types of the objects they refer to

ReferenceToVariable

References can point to variables

StackTraceIsCallHistory

A stack trace is the sequence of previously called methods

StaticFrameInGlobals

Stack frame of static method calls lives in the global memory area

StringLengthField

One can know the length of a String object by accessing its length field

StringLiteralNoObject

One needs to call the String constructor to get a String object from a literal

SubtypeCompatibleWithSupertype

A variable of a subtype can reference an object of a supertype

SuperclassObjectAllocated

When instantiating an object of a subclass, an object of a superclass is also allocated

SupertypeIncompatibleWithSubtype

A variable of a supertype cannot reference an object of a subtype

ThisAsField

this is a special field in the object

ThisAssignable

One can assign to this

ThisCanBeNull

this can be null

ThisExistsInStaticMethod

this is a local variable, also in static methods

ThisInConstructorIsNull

In a constructor, this is null

UntypedVariables

Variable declarations don't need a type

VariablesHoldExpressions

= stores an expression in a variable

VariablesHoldObjects

A variable of a reference type contains a whole object