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.

27 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

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

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