Conceptual Self-Check

Uncover your misconceptions by answering conceptual questions

In Java:
An abstract class must implement all abstract methods defined in its superclass

In Java:
An abstract class does not need to implement all abstract methods defined in its superclass

In Java:
An abstract class cannot contain implemented methods

In Java:
An abstract class can also contain implemented methods

In JavaScript:
Accessing a non existent property on an object produces an error

In JavaScript:
Accessing a non existent property on an object evaluates to undefined

In Java:
Set of class members stays fixed at runtime

In Java:
Set of class members can change at runtime

In Java:
Only classes without constructor automatically get a no-argument constructor

In Java:
All classes automatically get a no-argument constructor

In Java:
Exceptions are subtypes of Throwable

In Java:
Any class can be an exception class

In Java:
Addition has higher precedence than string concatenation

In Java:
Addition and string concatenation have same precedence

In Java:
Parenthesis are used to access an element in an array

In Java:
Square brackets are used to access an element in an array

In Java:
Arrays are created without the new keyword

In Java:
All arrays are allocated on the heap with new

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

In Java:
The number of brackets in an array type or an array initializer corresponds to the rank of the array

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

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

In Java:
Elements of arrays are untyped

In Java:
The type of an array specifies the type of the elements

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

In Java:
To get the length of an array, one needs to read its length field

In Java:
Array initializers list the elements in square brackets

In Java:
Array initializers list the elements in curly braces

In Java:
An array cannot have a length of 0 elements

In Java:
An array can have a length of 0 elements

In Java:
The length of an array is not part of its type

In Java:
The length of an array is part of its type

In Java:
ArrayLists and arrays are different things

In Java:
ArrayLists are arrays

In Java:
Array rank and array length are different concepts

In Java:
Array rank and array length are the same thing

In Java:
The rank of an array is part of its type

In Java:
The rank of an array is not part of its type

In Java:
The length of an array is fixed and is determined at allocation

In Java:
Arrays can grow dynamically

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

In JavaScript:
When an arrow function consists just of an expression, the return keyword is implied

In JavaScript:
Arrow functions also require the keyword 'function'

In JavaScript:
Arrow functions do not require the keyword 'function'

In Java:
= compares two values

In Java:
= assigns a value to a variable

In Python:
= compares two values

In Python:
= assigns a value to a variable

In Java:
Assignment copies the object

In Java:
Assignment copies the reference pointing to the object

In JavaScript:
Assignment copies the object

In JavaScript:
Assignment copies the reference pointing to the object

In Python:
Assignment copies the object

In Python:
Assignment copies the reference to the object

In Java:
An assignment a=b is an expression and thus produces a value

In Java:
An assignment a=b is not an expression

In Java:
Recursive computations do not necessarily need a base case

In Java:
Recursive computations need a base case to terminate

In Scratch:
Recursive computations need a base case to terminate

In Scratch:
Recursive computations do not necessarily need a base case

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

In Java:
The base case of a structural recursion does not perform a recursive call

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

In Java:
Wrapper class constructors expect a value of a primitive type, and null is not such a value

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

In Java:
A method invocation on a reference of a type that does not have that method will compile

In Java:
?

In Java:
One can invoke a method on primitive values

In Java:
One needs a variable to invoke a method

In Java:
?

In Java:
?

In Java:
A method invocation does not necessarily allocate a stack frame

In JavaScript:
A function that expects a callback has that callback function as a parameter

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

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

In Java:
?

In Python:
Method calls, and attribute accesses in general, can be chained to a constructor invocation.

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

In Java:
Member accesses cannot be chained together

In Java:
Member accesses can be chained together

In Java:
Method calls or field accesses can be chained to a constructor invocation

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

In Java:
?

In Java:
Catch blocks always get executed

In Java:
Only the necessary part of a catch block executes

In Java:
?

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

In Java:
Chained methods are called on the object that results from the previous call in the chain

In Java:
Char is a numeric type

In Java:
Char is not a numeric type

In JavaScript:
A signle character is of type string

In JavaScript:
A single character is of type char

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

In JavaScript:
A class definition does not define a type; all objects have type object

In Scratch:
To test whether an expression evaluates to true or false, one can just use it

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

In Python:
To test whether an expression is True or False, one can just use it

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

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

In Java:
To test whether an expression is true or false, one can just use it

In Java:
Expressions that consist of multiple parts have no type

In Java:
Expressions that consist of multiple parts have a type

In Java:
A concrete class needs to implement all methods declared in its abstract superclasses

In Java:
?

In Java:
?

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

In Java:
?

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

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

In Java:
?

In Java:
A condition node in a control-flow graph can have a single outgoing edge

In Java:
?

In Java:
If-else is equivalent to sequence of two ifs

In Java:
If-else can behave differently from sequence of two ifs

In Python:
If-else can behave differently from sequence of two ifs

In Python:
If-else is equivalent to sequence of two ifs

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

In Scratch:
If-then-else block can behave differently from sequence of two if-then blocks

In JavaScript:
The conditional operator is not an expression

In JavaScript:
The conditional operator is not an expression

In JavaScript:
Declarations of constants do not need to be immediately initialized

In JavaScript:
Declarations of constants need to be immediately initialized

In JavaScript:
An object referred to by a const variable is an immutable object

In JavaScript:
An object referred to by a const variable can be a mutable object

In Java:
The constructor allocates the object

In Java:
The constructor does not allocate the object, it just initializes it

In Java:
Formal constructor parameters are equivalent to instance variables

In Java:
Formal constructor parameters are completely separate from instance variables

In Java:
Constructors need to return objects

In Java:
Constructors cannot return anything

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

In Java:
The constructor name needs to come after the keyword new to instantiate a class

In Java:
One cannot control access to local variables

In Java:
One can control access to local variables using access modifiers

In Java:
A return statement immediately returns from the method

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

In Python:
A return statement immediately returns from the function

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

In Java:
There is an elsif keyword for multi-way conditional statements

In Java:
?

In Java:
The else branch of an if-else statement always executes

In Java:
The else branch of an if-else statement only executes if the condition evaluates to false

In Scratch:
The else branch of an if-then-else block only executes if the condition evaluates to false

In Scratch:
The else branch of an if-then-else block always executes

In Scratch:
(list a) = (list b) compares the contents of list a and list b

In Scratch:
(list a) = (list b) compares the identities of list a and list b

In Java:
o==p compares the objects referred to by variables o and p

In Java:
o==p compares the references stored in the variables o and p

In JavaScript:
The equality operator compares two objects' references

In JavaScript:
The equality operator compares two objects' values

In JavaScript:
The equality operator '==' compares only the values of the operands

In JavaScript:
The equality operator '==' compares only the types of the operands

In Scratch:
() = () compares the values of its operands

In Scratch:
() = () compares only the types of its operands

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

In Java:
o.equals(p) compares the objects referred to by variables o and p

In Java:
?

In Java:
Evaluating an expression means outputting its result

In Java:
Exception is the top-most exception class

In Java:
?

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

In Java:
A variable is only written using an assignment

In Scratch:
A variable is only written using an assignment

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

In Java:
One has to evaluate an expression to determine its type

In Java:
The type of expressions is determined, without evaluation, at compile time

In Java:
An object referred to by a final variable can be a mutable object

In Java:
An object referred to by a final variable is an immutable object

In Java:
An enhanced for loop iterates over the elements of an array or a collection

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

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

In Java:
For-each loops only know how to traverse arrays and Iterables

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

In Java:
The variable of an enhanced for statement contains a copy of the value of the corresponding array or collection element

In Java:
The body of a for statement executes repeatedly, as long as the condition holds

In Java:
The body of a for statement executes at most once

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

In Java:
The scope of variables declared in a for loop header is limited to the loop

In Java:
?

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

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

In JavaScript:
To use a function as a value, one just needs to write its name

In JavaScript:
It not is possible to create multiple functions with the same name but with different signatures

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

In JavaScript:
Functions cannot be called in the expression in which they are defined

In JavaScript:
Functions can be immediately called in the expression in which they are defined

In JavaScript:
Every function definition requires an associated name

In JavaScript:
One can define a function without associating a name to it

In JavaScript:
An identifier used to access a property with the bracket notation is firstly evaluated

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

In Java:
The body of an if statement executes at most once

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

In Python:
The body of an if-statement executes at most once

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

In Java:
?

In Java:
Every if statement requires an else

In Java:
Immutable classes can have constructor/method parameters that are not final

In Java:
Immutable classes need final constructor/method parameters

In Java:
?

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

In Python:
__init__ must create a new object

In Python:
__init__ does not create the object, it just initializes it

In Python:
__init__ cannot return values other than None

In Python:
__init__ needs to return an object

In Java:
The expression tree of an expression involving a call inlines the call's computation of the returned value

In Java:
The expression tree of an expression involving a call shows a node for the call, with children for each argument

In Java:
The expression tree of an expression involving a variable inlines the variable's definition

In Java:
The expression tree of an expression involving a variable shows a node for that variable

In Java:
?

In Java:
Dividing two integers can produce a rational number

In Java:
An interface can extend a class

In Java:
?

In Java:
In an intra-procedural control-flow graph, method call nodes point to the called method

In Java:
?

In Java:
Large integer numbers have type long

In Java:
Integer numbers without L or l suffix that are too large for an int are illegal

In Scratch:
A list can have a length of 0 items

In Scratch:
A list cannot have a length of 0 items

In Scratch:
All items in a list must have the same type

In Scratch:
A list can contain items of different types

In Java:
Even an individual literal is an expression

In Java:
A literal is not an expression

In Java:
String literals always must be surrounded by quotes

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

In Java:
Local variables must be initialized explicitly

In Java:
Local variables are automatically initialized

In Java:
Lifetime of variables declared in a loop body is limited to one loop iteration

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

In Java:
For and while loops end when the condition becomes false

In Java:
For and while loops end when the condition becomes true

In JavaScript:
An expression alone, without an assignment, is already a valid statement

In JavaScript:
An expression must be assigned to have a valid statement

In JavaScript:
Map returns a new array populated with the results of the callback function

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

In Java:
Maps can store multiple values for a given key

In Java:
?

In Java:
?

In Java:
Map.put with an existing key does nothing

In Java:
To map a boolean expression to a boolean, one can just use it

In Java:
To map a boolean expression to a boolean, a conditional operator is necessary

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

In Java:
To map a boolean expression to a boolean, one can just use it

In Python:
To map a boolean expression to a bool, one can just use it

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

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

In Python:
To map a boolean expression to a bool, one can just use it

In Java:
Each object contains its own special fields for all of its methods

In Java:
Each object contains a special field referring to its class, and the class knows about its methods

In Java:
A method declaration does not need to include a return type

In Java:
?

In Java:
Methods can be defined outside a class

In Java:
?

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

In Scratch:
Blocks following an if without else will execute even if the condition is false

In Java:
A reference variable can point to multiple objects

In Java:
?

In Java:
?

In Java:
A variable can contain more than one value

In Java:
A multi-dimensional array is one thing

In Java:
A multi-dimensional is a structure of nested arrays

In Java:
A class can have multiple superclasses

In Java:
?

In Python:
Functions can return multiple values

In Python:
Functions can only return one value

In Java:
Constructors must assign values to all fields

In Java:
Constructors do not need to assign values to all fields

In Java:
To instantiate a generic type, one needs to specify type parameter names as well as types

In Java:
?

In Java:
?

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

In Java:
Packages can contain other packages

In Java:
?

In Java:
A single piece, like a literal or name, also is an expression

In Java:
Expressions must consist of more than one piece

In JavaScript:
Expressions must consist of more than one piece

In JavaScript:
A single piece, like a literal or name, also is an expression

In Python:
A single piece, like a literal or name, also is an expression

In Python:
Expressions must consist of more than one piece

In JavaScript:
Square brackets cannot be used to access properties of an object

In JavaScript:
Square brackets can be used to access properties of an object

In Java:
One cannot invoke methods on String literals

In Java:
One can invoke methods on String literals

In Java:
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

In Java:
?

In Java:
\ is a normal character in char and String literals

In Java:
\ is an escape character in char and String literals

In Java:
A constructor must do something

In Java:
A constructor's body can be empty

In Python:
The body of __init__ can be empty

In Python:
__init__ must do something

In Java:
An object contains only the fields declared in its class

In Java:
?

In Java:
Objects without instance variables can't exist

In Java:
?

In Java:
An F or f suffix can turn a number into a float literal

In Java:
There are no float literals

In JavaScript:
Multiple function calls can be chained together

In JavaScript:
It is not allowed to chain function calls

In JavaScript:
There isn't a global object

In JavaScript:
There is always a global object

In Java:
Smaller types can be automatically converted into bigger ones even without an explicit cast

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

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

In Java:
It is allow to call other methods on the same object while inside its constructor

In Java:
Multi-dimensional arrays can have a jagged shape

In Java:
Multi-dimensional arrays have a rectangular shape

In Java:
There are no local variables

In Java:
?

In Java:
There are no long literals

In Java:
An L or l suffix can turn a number into a long literal

In Java:
Subclasses inherit fields but not methods

In Java:
Subclasses inherit both fields and methods

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

In Java:
Some character sequences are reserved for use as keywords and cannot be used as identifiers

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

In Python:
Some character sequences are reserved for use as keywords and cannot be used as identifiers

In JavaScript:
Functions without return statements return no value at all

In JavaScript:
Functions without return statements return undefined

In Python:
?

In Python:
There is no operator that repeats sequences

In Java:
&& and || always evaluate both operands

In Java:
&& and || evaluate their right operand only if absolutely necessary

In Python:
and/or evaluate their right operand only if absolutely necessary

In Python:
and/or always evaluate both operands

In Java:
& is only a bitwise AND

In Java:
& for boolean operands is a logical AND

In Java:
One cannot invoke toString() on a String

In Java:
One can invoke toString() on a String

In JavaScript:
null and undefined are two distinct values

In JavaScript:
The values null and undefined are the same

In Java:
null is a reference pointing to no object

In Java:
null is an object

In JavaScript:
null is an object

In JavaScript:
null is a reference pointing to no object

In Java:
NullPointerExceptions are detected at compile time

In Java:
NullPointerExceptions are detected at runtime

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

In JavaScript:
Functions can be called with a different number of arguments as defined in their signature

In Java:
Numeric types cannot be coerced to boolean

In Java:
Numeric types can be coerced to boolean

In JavaScript:
Objects are passed by reference

In JavaScript:
Objects are passed by value

In Java:
?

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

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

In Java:
?

In Java:
A variable is needed to instantiate an object

In Java:
Objects have no name and can exist without a variable referring to them

In Python:
A variable is needed to instantiate an object

In Python:
Objects have no name and can exist without a variable referring to them

In Java:
Also the elements of outer arrays of a multi-dimensional array are accessible

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

In Java:
Normal developers can create packages

In Java:
Only the creators of the JDK can create packages

In Java:
Out-of-bounds array elements cannot be accessed

In Java:
Out-of-bounds array elements are null

In Python:
Nested function calls are invoked outside in

In Python:
Nested function calls are invoked inside out

In Java:
Nested method calls are invoked outside in

In Java:
Nested method calls are invoked inside out

In Java:
() are mandatory even for method calls without arguments

In Java:
() are optional for method calls without arguments

In Python:
() are optional for function calls without arguments

In Python:
() are mandatory even for function calls without arguments

In Python:
The plus operator can concatenate strings and numbers

In Python:
?

In Java:
Pre-increment in update part of for loop means same as pre-increment anywhere else

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

In Java:
?

In Java:
Primitive values are heap objects

In Java:
Type parameters of generic types can be assigned primitive types

In Java:
?

In Java:
?

In Java:
The type of a primitive variable depends on its value

In Java:
println(...) starts a new line and then prints

In Java:
println(...) prints and then starts a new line

In Java:
Private members of a superclass are not accessible from a subclass

In Java:
Private members of a superclass are accessible from a subclass

In Java:
A class where all fields are private can still be mutable

In Java:
A class where all fields are private is immutable

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

In Java:
An object can access private members of all other objects of the same class

In Java:
Static methods can access private members of instances of same class

In Java:
Static methods cannot access private members of instances of same class

In Java:
A private field cannot be changed

In Java:
Even a private field can be mutable

In JavaScript:
JavaScript is based on a class-based object model

In JavaScript:
JavaScript is based on a prototype-based object model

In Java:
Rational fractions are divisions of two integer literals

In Java:
Rational fractions are literals

In Java:
Each recursive call of a method gets its own stack frame

In Java:
Recursive calls of a method share a stack frame

In Java:
Tail-recursive call sites have no continuation

In Java:
?

In Python:
A recursive function does not necessarily need to contain an if-else statement

In Python:
A recursive function needs to contain an if-else statement

In Java:
A class having a recursive method does not imply that it is part of a recursive data structure

In Java:
A class with a recursive method represents part of a recursive data structure

In Java:
A recursive method does not necessarily need to contain an if-else statement

In Java:
A recursive method needs to contain an if-else statement

In Java:
?

In Java:
References can point into the stack

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

In Java:
?

In Java:
Every reference type can be coerced to boolean

In Java:
Not every reference type can be coerced to boolean

In Java:
One cannot cast between references and ints

In Java:
One can cast between references and ints

In Java:
References can only point to heap objects

In Java:
References can point to variables

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

In Java:
?

In Scratch:
The whole sequence of blocks in a loop is repeated

In Scratch:
Each block in a loop is repeated individually

In Scratch:
The computation of a loop iteration starts from the state that is based on everything that happened before, including prior iterations of the loop

In Scratch:
The computation of all loop iterations starts from the state before the loop

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

In Scratch:
Running a Scratch program executes the program starting with the world in its current state

In Java:
Return statements do not need () around the return value

In Java:
Return statements need () around the return value

In Python:
Return statements do not need () around the return value

In Python:
Return statements need () around the return value

In Java:
In a control-flow graph, a return statement has no outgoing edge

In Java:
?

In Java:
A return statement pops exactly one call stack frame

In Java:
A return statement can unwind multiple call stack frames

In Python:
A return statement can unwind multiple call stack frames

In Python:
A return statement pops exactly one call stack frame

In Java:
Chained accesses are invoked from left to right

In Java:
Chained accesses are invoked from right to left

In Python:
Chained accesses are invoked from right to left

In Python:
Chained accesses are invoked from left to right

In Java:
?

In Java:
RuntimeExceptions are checked exceptions

In Python:
The name self is not an expression

In Python:
Even the name self on its own is an expression

In Java:
Two pieces of code with the same observable behavior have to be implemented in the same way

In Java:
Two pieces of code with the same observable behavior can be implemented in different ways

In JavaScript:
Scheduling the execution of a function after 0 milliseconds with setTimeout is equivalent to a synchronous call

In JavaScript:
Scheduling the execution of a function after 0 milliseconds with setTimeout is not equivalent to a synchronous call

In JavaScript:
SetTimeout returns an identifier of the newly scheduled timer

In JavaScript:
SetTimeout returns the value returned by the callback function

In Java:
?

In Java:
Setter methods are static

In Java:
String literals can be in single quotes

In Java:
String literals must be in double quotes

In Java:
A stack trace is the sequence of previously called methods

In Java:
?

In Java:
?

In Java:
A non-condition node in a control-flow graph can have multiple outgoing edges

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

In Java:
?

In Java:
The method to be called is determined by the static type

In Java:
The method to be called is determined by the dynamic type

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

In Java:
?

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

In Java:
One can know the length of a String object by calling its length method

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

In Java:
A String literal represents a String object and can be treated as such

In Java:
String concatenation stringifies non-String operand expressions

In Java:
String concatenation evaluates non-String operand expressions and casts value to String

In Java:
The multiplication operator can repeat a String a number of times

In Java:
The multiplication operator does not accept a String and a number

In JavaScript:
One can repeat a String by multiplying it with a number

In JavaScript:
One cannot repeat a String by multiplying it with a number

In Java:
A variable of a subtype can reference an object of a supertype

In Java:
A variable of a subtype cannot reference an object of a supertype

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

In Java:
To call a method on a superclass, it is wrong to write parentheses after the keyword super

In Java:
super() must be called in the first statement in the constructor of a subclass

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

In Java:
When instantiating an object of a subclass, just that object is allocated

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

In Java:
A variable of a supertype cannot reference an object of a subtype

In Java:
A variable of a supertype can reference an object of a subtype

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

In Java:
The type of a numerical expression is determined by the expression

In Java:
this is a special field in the object

In Java:
this is a special local variable in the stack frame

In Java:
One cannot assign to this

In Java:
One can assign to this

In JavaScript:
One can assign to this

In JavaScript:
One cannot assign to this

In Java:
this is never null

In Java:
this can be null

In Java:
Instance method or constructor call nodes in expression trees have a subtree determining the value of "this"

In Java:
Instance method or constructor call nodes in expression trees have a child labeled "this"

In Java:
this does not exist in static methods

In Java:
this is a local variable, also in static methods

In Java:
In a constructor, this is never null

In Java:
In a constructor, this is null

In Java:
The name this is not an expression

In Java:
Even the name this on its own is an expression

In Java:
Invoking toString() prints something

In Java:
Invoking toString() does not produce any output

In Java:
If code could throw an exception, you must surround that code with a try/catch block

In Java:
?

In Java:
Exceptions get thrown at the end of the try block

In Java:
?

In JavaScript:
The typeof operator applied on an array returns 'object'

In JavaScript:
The typeof operator applied on an array returns 'array'

In JavaScript:
The value null is of type 'object'

In JavaScript:
The value null is of type 'null'

In Java:
Variables don't need to be declared

In Java:
?

In Java:
The unqualified names of different classes must be different

In Java:
?

In Java:
Variable declarations don't need a type

In Java:
Variable declarations need a type

In Java:
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

In Java:
?

In Java:
= stores an expression in a variable

In Java:
= evaluates an expression and stores its value in a variable

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

In Python:
= evaluates an expression and stores a reference to its value in a variable

In Java:
A variable of a reference type contains a reference to an object

In Java:
A variable of a reference type contains a whole object

In Python:
A variable contains a whole object

In Python:
A variable contains a reference to an object

In Java:
A method with void return type can't be recursive

In Java:
A method with void return type can be recursive

In Java:
A method with void return type can return a value

In Java:
A method with void return type cannot return a value

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

In Java:
?

In Java:
Except of the entry node, all control-flow graph nodes have incoming edges

In Java:
A control-flow graph node (other than the entry node) can have zero incoming edges

In Java:
Except for the exit node, all control-flow graph nodes have outgoing edges

In Java:
A control-flow graph node (other than the exit node) can have zero outgoing edges