Type Checking - Introduction to Compiler Construction in a Java World - page 130

//position1:
if(n<=0){
return1;
}else{
returnn*factorial(n-1);

}

}

publicstaticvoidmain(String[]args){
//position2:
intx=n;
System.out.println(n+"!="+factorial(x));

}

staticintn=5;

}

The symbol table for this program, and its relationship to the AST, is illustrated in Figure
4.1.
In its entirety, the symbol table takes the form of a tree that corresponds to the shape
of the AST. A context |that is, a node in this tree|captures the region of scope corre-
sponding to the AST node that points to it. For example, in Figure 4.1,

1. The context pointer from the AST's JCompilationUnit node points to the

JCompilationUnitContext that is at the root of the symbol table.

2. The context pointer from the AST's JClassDeclaration points to a ClassContext ,

3. The context pointer from the AST's two JMethodDeclarations each point to a
MethodContext

4. The context pointer from the AST's two JBlock s each point to a LocalContext .

On the other hand, from any particular location in the program, looking back toward
the root CompilationUnitContext , the symbol table looks like a stack of contexts. Each
( surroundingContext ) link back toward the CompilationUnitContext points to the con-
text representing the surrounding lexical scope.
For example,

1. The context for Position2 pointer in Figure 4.1 points to a LocalContext , which
declares the local variable x in the body of the main() method of the Factorial
program.

2. Its surroundingContext pointer points to a MethodContext in which the formal
parameter, args is declared.

3. Its surroundingContext pointer points to a ClassContext in which nothing is de-
clared.

4. Its surroundingContext pointer points to a CompilationUnitContext at the base
of the stack, which contains the declared types for the entire program.

During analysis, when the compiler encounters a variable, it looks up that variable in the
symbol table by name, beginning at the LocalContext most recently created in the symbol
table, for example, that pointed to by the pointer labeled \context for Position1 ." Type
names are looked up in the CompilationUnitContext . To make this easier, each context
maintains three pointers to surrounding contexts, as illustrated in Figure 4.2.