Misconceptions

AbstractClassMustImplementAbstractMethod

Java

DRAFT

An abstract class must implement all abstract methods defined in its superclass

Java

DRAFT

An abstract class cannot contain implemented methods

Java

Set of class members can change at runtime

Java

DRAFT

All classes automatically get a no-argument constructor

Java

DRAFT

Any class can be an exception class

Java

Addition has higher precedence than string concatenation

Java

DRAFT

Parentheses are used to access an element in an array

Java

DRAFT

Arrays are created without the new keyword

Java

DRAFT

The number of brackets in an array type or an array initializer corresponds to the length of the array

Java

DRAFT

The type of a multi-dimensional array is written as T[] T[] T[]

Java

DRAFT

Elements of arrays are untyped

Java

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

Java

DRAFT

Array initializers list the elements in square brackets

Java

DRAFT

An array cannot have a length of 0 elements

Java

DRAFT

The length of an array is part of its type

Java

ArrayLists are arrays

Java

DRAFT

Array rank and array length are the same thing

Java

DRAFT

The rank of an array is not part of its type

Java

Arrays can grow dynamically

Java

= compares two values

Java

Assignment copies the object

Java

An assignment a=b is not an expression

Java

DRAFT

Recursive computations do not necessarily need a base case

Java

DRAFT

The base case of a structural recursion consists of a recursive self-call

Java

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

Java

DRAFT

One can invoke a method on primitive values

Java

DRAFT

One needs a variable to invoke a method

Java

DRAFT

A method invocation does not necessarily allocate a stack frame

Java

DRAFT

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

Java

Member accesses cannot be chained together

Java

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

Java

DRAFT

Catch blocks always get executed

Java

DRAFT

Only the part of a catch block necessary to fix the cause of an exception is executed

Java

DRAFT

Chained methods are all called on the object at the beginning of the chain

Java

Char is not a numeric type

Java

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

Java

DRAFT

Expressions that consist of multiple parts have no type

Java

DRAFT

A concrete class needs to implement all abstract methods and override all concrete methods declared in its abstract superclasses

Java

DRAFT

A concrete class only needs to implement those abstract methods it inherits from abstract superclasses

ConcreteClassOnlyImplementDirectAbstract

Java

DRAFT

A concrete class only needs to implement abstract methods declared in its direct supertypes

ConcreteClassOnlyImplementInterfaceAbstract

Java

DRAFT

A concrete class only needs to implement those abstract methods it inherits from interfaces

Java

If-else is equivalent to sequence of two ifs

Java

The constructor allocates the object

Java

DRAFT

Formal constructor parameters are equivalent to instance variables

Java

Constructors need to return objects

Java

DRAFT

One can write the constructor name, without new, to instantiate a class

Java

One can control access to local variables using access modifiers

Java

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

Java

DRAFT

There is an elsif keyword for multi-way conditional statements

Java

DRAFT

The else branch of an if-else statement always executes

Java

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

Java

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

Java

DRAFT

Evaluating an expression means outputting its result

Java

DRAFT

Exception is the top-most exception class

Java

DRAFT

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

Java

DRAFT

One has to evaluate an expression to determine its type

Java

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

Java

DRAFT

An enhanced for loop iterates over the indices of an array or a collection

Java

DRAFT

For-each loops know how to traverse any recursive data structure

Java

DRAFT

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

Java

DRAFT

The body of a for statement executes at most once

Java

DRAFT

The scope of variables declared in a for loop header extends beyond the loop

Java

DRAFT

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

Java

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

Java

DRAFT

Every if statement requires an else

Java

DRAFT

Immutable classes need final constructor/method parameters

Java

DRAFT

Java implicitly produces implementations of any methods a class inherits from the interfaces it implements

Java

DRAFT

Dividing two integers can produce a rational number

Java

DRAFT

An interface can extend a class

Java

Large integer numbers have type long

Java

DRAFT

A literal is not an expression

Java

DRAFT

When passing a literal string as argument to a method, no quotes are needed

Java

Local variables are automatically initialized

Java

DRAFT

Lifetime of variables declared in a loop body extends across all loop iterations

Java

DRAFT

For and while loops end when the condition becomes true

Java

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

Java

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

Java

DRAFT

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

Java

DRAFT

A method declaration does not need to include a return type

Java

DRAFT

Methods can be defined outside a class

Java

DRAFT

A reference variable can point to multiple objects

Java

DRAFT

A variable can contain more than one value

Java

DRAFT

A multi-dimensional array is one single array object

Java

DRAFT

A class can have multiple superclasses

Java

Constructors must assign values to all fields

Java

DRAFT

To instantiate a generic type, for each type parameter one has to specify both the name and the type

Java

DRAFT

If objects are part of a containment hierarchy, their classes are nested, too

Java

DRAFT

Packages can contain other packages which have access to members of the containing package

Java

Expressions must consist of more than one piece

Java

DRAFT

One cannot invoke methods on String literals

Java

DRAFT

If a variable is at least as big (bit-width) as a value, then no cast is needed to a assign the value to the variable

Java

DRAFT

\ is a normal character in char and String literals

Java

A constructor must do something

Java

DRAFT

An object contains only the fields declared in its class

Java

DRAFT

Objects without instance variables can't exist

Java

There are no float literals

Java

DRAFT

Smaller types are never automatically converted into bigger ones without an explicit cast

Java

DRAFT

It is foirbidden to call other methods on the same object while inside its constructor

Java

DRAFT

Multi-dimensional arrays have a rectangular shape

Java

DRAFT

There are no local variables

Java

There are no long literals

Java

DRAFT

Subclasses inherit fields but not methods

Java

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

Java

&& and || always evaluate both operands

Java

& is only a bitwise AND

Java

DRAFT

One cannot invoke toString() on a String

Java

null is an object

Java

DRAFT

NullPointerExceptions are detected at compile time

Java

DRAFT

Numeric types can be coerced to boolean

Java

A variable is needed to instantiate an object

Java

DRAFT

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

Java

DRAFT

Out-of-bounds array elements are null

Java

Nested method calls are invoked outside in

Java

() are optional for method calls without arguments

Java

DRAFT

Pre-increment in update part of for loop means increment before loop body

Java

DRAFT

Type parameters of generic types can be instantiated with primitive types

Java

DRAFT

The type of a primitive variable depends on its value

Java

DRAFT

Private members of a superclass are accessible from a subclass

Java

DRAFT

A class where all fields are private is immutable

Java

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

Java

Static methods cannot access private members of instances of same class

Java

DRAFT

A private field cannot be changed

Java

DRAFT

Rational fractions are literals

Java

Recursive calls of a method share a stack frame

Java

DRAFT

Tail-recursive call sites of non-void methods need no return statement

Java

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

Java

A recursive method needs to contain an if-else statement

Java

DRAFT

References can point into the stack

Java

DRAFT

Every reference type can be coerced to boolean

Java

DRAFT

One can cast between references and ints

Java

References can point to variables

ReferringToRecursiveStructureMakesRecursive

Java

DRAFT

A class referring to a recursive data structure is (indirectly) part of that recursion as well

Java

Return statements need () around the return value

Java

A return statement can unwind multiple call stack frames

Java

Chained accesses are invoked from right to left

Java

DRAFT

RuntimeExceptions are checked exceptions

Java

DRAFT

String literals can be in single quotes

Java

DRAFT

A stack trace is the sequence of previously called methods

Java

DRAFT

A static method call is dispatched polymorphically at runtime based on the argument types

Java

DRAFT

The method to be called is determined by the static type

Java

DRAFT

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

Java

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

Java

String concatenation stringifies non-String operand expressions

Java

DRAFT

The multiplication operator can repeat a String a number of times

Java

DRAFT

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

Java

DRAFT

To call a method on a superclass, parentheses are needed after the keyword super

Java

DRAFT

super() can be called anywhere in the constructor of a subclass

Java

DRAFT

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

Java

DRAFT

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

Java

DRAFT

The type of a numerical expression depends on the type expected by the surrounding context

Java

DRAFT

this is a special field in the object

Java

One can assign to this

Java

this can be null

Java

this is a local variable, also in static methods

Java

DRAFT

In a constructor, this is null

Java

The name this is not an expression

Java

DRAFT

Invoking toString() prints something

Java

DRAFT

When a piece of code can throw an exception, it must be surrounded with a try/catch block

Java

DRAFT

Exceptions get thrown at the end of the try block

Java

DRAFT

Variables can be used even without being declared beforehand

Java

DRAFT

The unqualified names of different classes must be different

Java

DRAFT

Variable declarations don't need a type

Java

DRAFT

If a class has a method that has a local variable, parameter, or return value with the class as its type, the class is a recursive type

Java

= stores an expression in a variable

Java

A variable of a reference type contains a whole object

Java

DRAFT

A method with void return type can't be recursive

Java

DRAFT

A method with void return type can return a value

Java

DRAFT

In integer numbers, decimal digits with value `0` take less storage than decimal digits with other values

JavaScript

DRAFT

Accessing a non existent property on an object produces an error

JavaScript

DRAFT

Even when an arrow function consists just of an expression, the return keyword must be explicitly written

JavaScript

DRAFT

Arrow functions also require the keyword 'function'

JavaScript

Assignment copies the object

JavaScript

DRAFT

Parameters of a callback function may be written as parameters of the caller function

JavaScript

DRAFT

A single character is of type char

JavaScript

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

JavaScript

DRAFT

The conditional operator is not an expression

JavaScript

DRAFT

Declarations of constants do not need to be immediately initialized

JavaScript

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

JavaScript

DRAFT

The equality operator compares two objects' values

JavaScript

DRAFT

The equality operator '==' compares only the types of the operands

JavaScript

DRAFT

To use a function as a value, one needs to have parentheses after its name

JavaScript

DRAFT

It is possible to create multiple functions with the same name but with different signatures

JavaScript

DRAFT

Functions cannot be called in the expression in which they are defined

JavaScript

DRAFT

Every function definition requires an associated name

JavaScript

DRAFT

An identifier used to access a property with the bracket notation is treated as a string

JavaScript

DRAFT

An expression must be assigned to have a valid statement

JavaScript

DRAFT

Map modifies the elements of the array on which it operates in place

JavaScript

Expressions must consist of more than one piece

JavaScript

DRAFT

Square brackets cannot be used to access properties of an object

JavaScript

DRAFT

It is not allowed to chain function calls

JavaScript

DRAFT

There isn't a global object

JavaScript

DRAFT

Functions without return statements return no value at all

JavaScript

DRAFT

The values null and undefined are the same

JavaScript

null is an object

JavaScript

DRAFT

Functions must be called with the same number of arguments as defined in their signature

JavaScript

DRAFT

Objects are passed by value

JavaScript

DRAFT

JavaScript is based on a class-based object model

JavaScript

DRAFT

One can repeat a String by multiplying it with a number

JavaScript

One can assign to this

JavaScript

DRAFT

The typeof operator applied on an array returns 'array'

JavaScript

DRAFT

The value null is of type 'null'

Python

= compares two values

Python

Assignment copies the object

Python

Attribute accesses cannot be chained together

CannotChainAttributeToObjectInstantiation

Python

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

Python

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

Python

If-else is equivalent to sequence of two ifs

Python

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

Python

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

Python

__init__ must create a new object

Python

__init__ needs to return an object

Python

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

Python

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

Python

Functions can return multiple values

Python

Expressions must consist of more than one piece

Python

__init__ must do something

Python

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

Python

DRAFT

There is no operator that repeats sequences

Python

and/or always evaluate both operands

Python

& is only a bitwise AND

Python

A variable is needed to instantiate an object

Python

Nested function calls are invoked outside in

Python

() are optional for function calls without arguments

Python

DRAFT

The plus operator can concatenate strings and numbers

Python

A recursive function needs to contain an if-else statement

Python

Return statements need () around the return value

Python

A return statement can unwind multiple call stack frames

Python

Chained accesses are invoked from right to left

Python

Reassigning self changes the object on which a method is called

Python

The name self is not an expression

Python

One needs to call str to instantiate a str object from a string literal

Python

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

Python

A variable contains a whole object

Scratch

Recursive computations do not necessarily need a base case

Scratch

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

Scratch

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

Scratch

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

Scratch

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

Scratch

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

Scratch

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

Scratch

A list cannot have a length of 0 items

Scratch

All items in a list must have the same type

Scratch

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

Scratch

Each block in a loop is repeated individually

Scratch

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

Scratch

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

Misconceptions

Find Misconceptions by...

About