Java (C++) Control Abstraction

last modified 1/4/05

Control structures in a high level language

These structured control structures for Java and C/C++

Selection	Iteration	Procedure
if-then (single way decision)	while (execute 0 or more times)	function call (pass params)
if-then-else (2 way decision)	do-while (execute 1 or more times)	return (... a value)
switch (multi-way decision)	for (execute a known number of iterations)

Control structures found in a machine language

very primitive and not abstract.
concepts covered in Computer Organization CS 220

Below are 4 basic instructions in a ML

Jump	Jump to subroutine
unconditional branch (goto target)	subroutine call (jump-to-subroutine target)
conditional branch (if cond then goto target)	return from subroutine (return)
target: [more statements]	target: [first statement of module]

Early language abstraction

FORTRAN, BASIC and other early languages often mimicked the underlying machine language in regards to control structures

Sample FORTRAN code segment	Same action in Java
160 IF (I .LT. J )GOTO 164 I = I + 3 PRINT *, I GOTO 160 164 K = 5	while (i<j) { i = i+3; System.out.println( i ); } k = 5;

With no abstraction of control structures, the code is error prone, and has no logically expressed relationship among the statements. Indentation helps the reading but is not necessary or considered by the computer system.

Advanced Control Structures

Some additional Java and C++ control statements:

goto	rarely used in C++ (should not be used)
exit	used to terminate a program
return	used to return from function/subroutine
break	used to exit a loop or switch case but has a well defined destination
continue	used to immediately start the next loop iteration

Functional (procedural) abstraction

Subroutines are an abstraction

Some basic reasons for functional abstraction (sub-themes for the course):

functional decomposition--a decomposition of a program or algorithm into smaller, manageable pieces
reusable software--the module/function can be used in several points within a program or in other programs
parameterization--the abstraction can be generalized further to allow the user to specify the different contexts in which a procedure can be used
function specification--a description of what the programmer needs to know to use the function. This is necessary for the success of reusing a function in other programs.

Parameters in Java are call-by-value (no ref or & type parameters)

Functions vs Procedures

As software designers we need to make a distinction between functions and procedures, although the Java or C++ languages do not make that distinction as clear.

Functions are designed to compute a value (or object) to be returned for use typically in a computation. They mimic mathematical functions in that they should NOT produce side effects; instead, functions compute a value or a new object based on the input parameters and simply return it.

Side effects include

input

output (error messages excepted)

changing of parameters

change or access of global variables

Example: In the statement r = sqrt(x); one doesn't expect that x is changed or that the value is printed out

Procedures (void functions in Java/C++) are designed to do a simple task and be named. They tend to have side effects such as input/output or modify a parameter or global variable.