Compound Statements A Quick Overview Copyright © 2004-2017 Curt Hill

The Compound Statement The compound statement is just a pair of braces { } and any number of statements in between them It is also the executable portion of a method It allows us to wrap many statements and treat them as just one Copyright © 2004-2017 Curt Hill

Compound statements Start with a { and end with a } No semicolon follows the closing brace Any place that one statement can be a compound statement can also be Primitive variables declared in a compound statement last until end of statement Objects may survive, but not their handles Copyright © 2004-2017 Curt Hill

Review Scope That area where a variable is known The scope of a primitive starts with its declaration It ends with the enclosing brace Object handles work the same way, but the object itself may survive the compound statement Java has more complicated scope than some other languages Copyright © 2004-2017 Curt Hill

Duplicate Names Some languages like FORTRAN and many versions of BASIC only allow one instance of a name Thus code like this: int a=2; … double a = 3.4; would be illegal regardless of where in the program these lines occur Java has a rather more complicated rule Copyright © 2004-2017 Curt Hill

A Scope Block A Scope Block is the area of a compound statement Excluding any nested compound statements Blocks may nested Within one scope block all names declared must be unique However, the same name may be used in a different block, provided it does not conflict with another available Copyright © 2004-2017 Curt Hill

Scope Example Consider the following code: { int a=5; // first a int b = a * 2; { // new brace : new scope int c = a; System.out.print(c); System.out.println(“ “); } System.out.print(c); What will happen? Copyright © 2004-2017 Curt Hill

Results Syntax error The c was declared inside a compound statement There was an attempt to use it after the statement The c variable only exists inside the compound statement However, it may access things declared in surrounding compound statements Copyright © 2004-2017 Curt Hill

Finding names How does compiler connect a name with declaration? Here is what the compiler does: Look in the current block (compound statement) for the name If found stop looking If not then go the next enclosing block Keep doing this until there are no further blocks or the name is found Copyright © 2004-2017 Curt Hill

Another Scope Example How will names be found? int x,z; … { int w,y; … { int v; … v = x * y; } … } Three scope blocks Copyright © 2004-2017 Curt Hill

Duplicate Names Unlike its predecessor C++, Java only allows duplicate names where they do not overlap Duplicates may exist, however no code has access to both Consider next screen: Copyright © 2004-2017 Curt Hill

Scope Example Consider the following code: { int a=5; // first a int b = a * 2; { // new brace : new scope int c = a; // one c … } { // new brace : new scope int c = b; // second c … } Copyright © 2004-2017 Curt Hill

Discussion If you are in a scope block, you may look outside to enclosing blocks You may not look inside a scope block The variables do not exist unless execution is inside the block These two c variables did not overlap in range Copyright © 2004-2017 Curt Hill

Another Thought The above examples use compound statements in an unusual way The import statement adds new items to the global scope Perhaps a great many new items A name not found by the time of finishing the global scope is undefined Copyright © 2004-2017 Curt Hill

Classes and Scope We have already seen Math.PI This is a constant defined inside the Math object Classes act like scope blocks as well with an important difference We can look inside at public items This is done by giving first the class name, a dot, then the interior name Copyright © 2004-2017 Curt Hill

Final Thoughts It is always safe to keep your names unique within a program Upon entering a new compound statement Any name not available to you currently may be used It may be attached to any type as well Always use meaningful names One letter names are usually reserved for formulas and control variables in loops Copyright © 2004-2017 Curt Hill