Top-Down and Bottom-Up Approach

Top-down design

Start with the overall system and repeatedly break it into smaller sub-systems/modules until each module is simple enough to code (stepwise refinement).

• Begins with the main module and works down; lower modules are first replaced by stubs.
• Example: Banking system, then Accounts / Loans / Transactions, then Deposit, Withdraw, Transfer.
• Advantages: clear overall structure, early detection of design and interface errors, easy to manage.
• Disadvantages: low-level details come late; reuse of existing components is limited.

Bottom-up design

Start by designing and building the lowest-level, basic modules, then combine them into larger modules until the whole system is built.

• Uses drivers to test lower modules first.
• Example: Build and test Deposit, Withdraw and Transfer first, then combine them into Transactions, then the Banking system.
• Advantages: encourages reuse of existing modules, low-level modules are tested thoroughly, suits projects built on existing libraries.
• Disadvantages: overall structure and interface problems appear late; system-level design is unclear at the start.

Comparison

Basis	Top-down	Bottom-up
Starts from	Whole system / main module	Lowest-level modules
Direction	Decomposition (divide)	Composition (combine)
Testing aid	Stubs	Drivers
Strength	Clear structure, early design validation	Reuse, early detail testing
Weakness	Late low-level detail	Late integration problems

In practice a combination of both is used.