1. Compound Statements A Quick Overview
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2. 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
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3. 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
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4. 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
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5. 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
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6. 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
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7. 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?
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8. 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
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9. 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
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10. Another Scope Example
How will names be found? int x,z; … { int w,y; … { int v; … v = x * y; } … }
Three scope blocks
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11. 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:
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12. 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 … }
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13. 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
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14. 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
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15. 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
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16. 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
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