Control Flow as Graph

Represent execution paths through a method or function as a graph

This notional machine represents method bodies as graphs. Statements are represented as rectangles and conditions as diamonds.

This makes the possible execution paths through the different statements of a method visible and is particularly useful for explaining compound statements (loops and conditional statements), conditional expressions, and exception handling.

30 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.

CatchAlwaysExecutes

Catch blocks always get executed

CatchProvidesOptions

Only the necessary part of a catch block executes

ConditionOneOutEdge

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

ConditionalIsSequence

If-else is equivalent to sequence of two ifs

ConditionalIsSequence

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

DeferredReturn

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

DeferredReturn

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

ElsIf

There is an elsif keyword for multi-way conditional statements

ElseAlwaysExecutes

The else branch of an if-else statement always executes

ElseAlwaysExecutes

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

ForIsConditional

The body of a for statement executes at most once

IfIsLoop

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

IfIsLoop

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

IfRequiresElse

Every if statement requires an else

InterproceduralEdge

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

LoopTerminatingCondition

For and while loops end when the condition becomes true

MissingElseTerminates

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

NoShortCircuit

&& and || always evaluate both operands

NoShortCircuit

and/or always evaluate both operands

PreIncrementBeforeLoop

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

RecursiveCallSiteNoReturn

Tail-recursive call sites have no continuation

RecursiveMethodNeedsIfElse

A recursive method needs to contain an if-else statement

RepeatDistributes

Each block in a loop is repeated individually

ResetStateEachLoopIteration

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

ReturnNoOutEdge

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

StatementMultipleOutEdges

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

SuperNotFirstStatement

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

TryFinishes

Exceptions get thrown at the end of the try block

ZeroInEdges

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

ZeroOutEdges

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