Misconceptions

These are the misconceptions we collected so far in:

There is no connection between the number of misconceptions listed here and the difficulty of the corresponding programming language. The effort we spent on collecting misconceptions is very different for different languages. It would be inappropriate to reason about the quality of languages based on the number of misconceptions in this collection.

AddMemberAtRuntime

AddMemberAtRuntime

Set of class members can change at runtime

ArithmeticPlusPrecedes

ArithmeticPlusPrecedes

Addition has higher precedence than string concatenation

ArrayHasLengthMethod

ArrayHasLengthMethod

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

ArrayListIsArray

ArrayListIsArray

ArrayLists are arrays

ArraysGrow

ArraysGrow

Arrays can grow dynamically

AssignCompares

AssignCompares

= compares two values

AssignmentCopiesObject

AssignmentCopiesObject

Assignment copies the object

AssignmentNotExpression

AssignmentNotExpression

An assignment a=b is not an expression

CallNotStaticallyChecked

CallNotStaticallyChecked

A method invocation on a reference of a type that does not have that method won't compile

CannotChainMemberAccesses

CannotChainMemberAccesses

Member accesses cannot be chained together

CannotChainMemberToConstructor

CannotChainMemberToConstructor

Method calls or field accesses cannot be chained to a constructor invocation

CharNotNumeric

CharNotNumeric

Char is not a numeric type

ComparisonWithBooleanLiteral

ComparisonWithBooleanLiteral

To test whether an expression is true or false, one must compare it to true or to false

ConditionalIsSequence

ConditionalIsSequence

If-else is equivalent to sequence of two ifs

ConstructorAllocates

ConstructorAllocates

The constructor allocates the object

ConstructorReturnsObject

ConstructorReturnsObject

Constructors need to return objects

ControlledLocalAccess

ControlledLocalAccess

One can control access to local variables using access modifiers

DeferredReturn

DeferredReturn

A return statement in the middle of a method doesn't return immediately

EqualityOperatorComparesObjectsValues

EqualityOperatorComparesObjectsValues

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

EqualsComparesReferences

EqualsComparesReferences

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

FinalReferenceImpliesImmutability

FinalReferenceImpliesImmutability

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

IfIsLoop

IfIsLoop

The body of an if statement executes repeatedly, as long as the condition holds

LargeIntegerLong

LargeIntegerLong

Large integer numbers have type long

LocalVariablesAutoInitialized

LocalVariablesAutoInitialized

Local variables are automatically initialized

MapToBooleanWithConditionalOperator

MapToBooleanWithConditionalOperator

To map a boolean expression to a boolean, a conditional operator is necessary

MapToBooleanWithIf

MapToBooleanWithIf

To map a boolean expression to a boolean, an if statement is necessary

MustInitializeFieldInConstructor

MustInitializeFieldInConstructor

Constructors must assign values to all fields

NoAtomicExpression

NoAtomicExpression

Expressions must consist of more than one piece

NoEmptyConstructor

NoEmptyConstructor

A constructor must do something

NoFloatLiterals

NoFloatLiterals

There are no float literals

NoLongLiterals

NoLongLiterals

There are no long literals

NoReservedWords

NoReservedWords

Every sequence of letters and digits starting with a letter can be used as an identifier

NoShortCircuit

NoShortCircuit

&& and || always evaluate both operands

NoSingleLogicAnd

NoSingleLogicAnd

& is only a bitwise AND

NullIsObject

NullIsObject

null is an object

ObjectsMustBeNamed

ObjectsMustBeNamed

A variable is needed to instantiate an object

OutsideInMethodNesting

OutsideInMethodNesting

Nested method calls are invoked outside in

ParenthesesOnlyIfArgument

ParenthesesOnlyIfArgument

() are optional for method calls without arguments

PrivateFromOtherInstance

PrivateFromOtherInstance

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

PrivateFromStatic

PrivateFromStatic

Static methods cannot access private members of instances of same class

RecursiveActivationsShareFrame

RecursiveActivationsShareFrame

Recursive calls of a method share a stack frame

RecursiveMethodImpliesRecursiveType

RecursiveMethodImpliesRecursiveType

A class with a recursive method represents part of a recursive data structure

RecursiveMethodNeedsIfElse

RecursiveMethodNeedsIfElse

A recursive method needs to contain an if-else statement

ReferenceToVariable

ReferenceToVariable

References can point to variables

ReturnCall

ReturnCall

Return statements need () around the return value

ReturnUnwindsMultipleFrames

ReturnUnwindsMultipleFrames

A return statement can unwind multiple call stack frames

RightToLeftChaining

RightToLeftChaining

Chained accesses are invoked from right to left

StringLiteralNoObject

StringLiteralNoObject

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

StringPlusStringifiesExpression

StringPlusStringifiesExpression

String concatenation stringifies non-String operand expressions

ThisAssignable

ThisAssignable

One can assign to this

ThisCanBeNull

ThisCanBeNull

this can be null

ThisExistsInStaticMethod

ThisExistsInStaticMethod

this is a local variable, also in static methods

ThisNoExpression

ThisNoExpression

The name this is not an expression

VariablesHoldExpressions

VariablesHoldExpressions

= stores an expression in a variable

VariablesHoldObjects

VariablesHoldObjects

A variable of a reference type contains a whole object

AssignmentCopiesObject

AssignmentCopiesObject

Assignment copies the object

ClassDefinesType

ClassDefinesType

The type of an object is equivalent to the type defined by its class definition

ConstReferenceImpliesImmutability

ConstReferenceImpliesImmutability

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

NoAtomicExpression

NoAtomicExpression

Expressions must consist of more than one piece

NullIsObject

NullIsObject

null is an object

ThisAssignable

ThisAssignable

One can assign to this

AssignCompares

AssignCompares

= compares two values

AssignmentCopiesObject

AssignmentCopiesObject

Assignment copies the object

CannotChainAttributeToObjectInstantiation

CannotChainAttributeToObjectInstantiation

Method calls, and attribute accesses in general, cannot be chained to a constructor invocation.

ComparisonWithBoolLiteral

ComparisonWithBoolLiteral

To test whether an expression is True or False, one must compare it to True or to False

ConditionalIsSequence

ConditionalIsSequence

If-else is equivalent to sequence of two ifs

DeferredReturn

DeferredReturn

A return statement in the middle of a function doesn't return immediately

IfIsLoop

IfIsLoop

The body of an if-statement executes repeatedly, as long as the condition holds

InitCreates

InitCreates

__init__ must create a new object

InitReturnsObject

InitReturnsObject

__init__ needs to return an object

MapToBooleanWithIf

MapToBooleanWithIf

To map a boolean expression to a bool, an if statement is necessary

MapToBooleanWithTernaryOperator

MapToBooleanWithTernaryOperator

To map a boolean expression to a bool, a ternary conditional operator is necessary

MultipleValuesReturn

MultipleValuesReturn

Functions can return multiple values

NoAtomicExpression

NoAtomicExpression

Expressions must consist of more than one piece

NoEmptyInit

NoEmptyInit

__init__ must do something

NoReservedWords

NoReservedWords

Every sequence of letters and digits starting with a letter or an underscore can be used as an identifier

NoShortCircuit

NoShortCircuit

and/or always evaluate both operands

ObjectsMustBeNamed

ObjectsMustBeNamed

A variable is needed to instantiate an object

OutsideInFunctionNesting

OutsideInFunctionNesting

Nested function calls are invoked outside in

ParenthesesOnlyIfArgument

ParenthesesOnlyIfArgument

() are optional for function calls without arguments

RecursiveFunctionNeedsIfElse

RecursiveFunctionNeedsIfElse

A recursive function needs to contain an if-else statement

ReturnCall

ReturnCall

Return statements need () around the return value

ReturnUnwindsMultipleFrames

ReturnUnwindsMultipleFrames

A return statement can unwind multiple call stack frames

RightToLeftChaining

RightToLeftChaining

Chained accesses are invoked from right to left

SelfNoExpression

SelfNoExpression

The name self is not an expression

VariablesHoldExpressions

VariablesHoldExpressions

= stores an expression: it stores a reference to the expression in a variable

VariablesHoldObjects

VariablesHoldObjects

A variable contains a whole object

BaseCaseNotNeeded

BaseCaseNotNeeded

Recursive computations do not necessarily need a base case

CompareBooleanToConstant

CompareBooleanToConstant

To test whether an expression evaluates to true or false, one must compare it to a constant

ConditionalIsSequence

ConditionalIsSequence

If-then-else block is equivalent to sequence of two if-then blocks

ElseAlwaysExecutes

ElseAlwaysExecutes

The else branch of an if-then-else block always executes

EqualityOperatorComparesListIdentities

EqualityOperatorComparesListIdentities

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

EqualityOperatorComparesOnlyTypes

EqualityOperatorComparesOnlyTypes

() = () compares only the types of its operands

ExpressionAssigns

ExpressionAssigns

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

ListLengthCannotBeZero

ListLengthCannotBeZero

A list cannot have a length of 0 items

ListsHomogeneous

ListsHomogeneous

All items in a list must have the same type

MissingElseTerminates

MissingElseTerminates

Blocks following an if without else will not execute if the condition is false

RepeatDistributes

RepeatDistributes

Each block in a loop is repeated individually

ResetStateEachLoopIteration

ResetStateEachLoopIteration

The computation of all loop iterations starts from the state before the loop

ResetStateEachProgramExecution

ResetStateEachProgramExecution

Running a Scratch program first resets the state of the world and then executes the program

Programming Language

Notional Machine

About This Site Contributors

Follow us on twitter to hear about new misconceptions.