CpS 450 Notes

Symbol Table

You should create a SymbolTable class which manages the symbol table. It should manage a stack of Symbol entries (which consist of a scope number, name, and attributes), and keep track of the current scope level (initially 0). It should provide the following interface:

• Symbol push(String name,Declaration decl);
  Pushes a new symbol entry onto the stack with the given name and decl information, and the current scope level. It should return the new symbol entry. (See more about Declaration below.)
  
  If name is already defined at the current scope level, go ahead and push a new record for it onto the stack.
   

• Symbol lookup(String name);
  Searches for a symbol entry with name, starting at the top of the stack and working to the bottom. Returns the first Symbol entry found, or null if none.
   

• void beginScope();
  Increments the current scope level.
   

• void endScope();
  Pops all symbol entries in the current scope level off the stack, then decrements the current scope level. Raise an exception if the scope level would drop below 0.
   

• int getScope();
  Returns the current scope level.

Before trying to use this in your compiler, it would be a good idea to write a small JUnit test to verify that it works properly.

In Eclipse, simply right-click your SymbolTable.java in the Package Explorer, and choose New > JUnit Test from the menu, provide a name (like SymbolTableTest), and click Finish. Write a public void testSymbolTable() method that instantiates a SymbolTable, calls its methods, and asserts that they work properly, then right-click your SymbolTableTest and choose Run > Run As > JUnit Test to run your test program.

Sample SymbolTableTest.java:

import junit.framework.TestCase;

public class SymbolTableTest extends TestCase {
  public void testSymbolTable() {
    SymbolTable table = new SymbolTable();
		
    Symbol s = table.push("x", new VarDecl(Type.INT));
    assertTrue(s.getName().equals("x"));
    // ...
  }

}

Declaration

Declaration objects store information about their symbol. You will have a hierarchy of Declaration classes, because there are different kinds of identifiers. I suggest starting with the following class structure:

class Declaration {
  Type type; // all symbols have a Type
}

class VarDecl extends Declaration {
  // more will go here eventually ...
}

class MethodDecl extends Declaration {
  // more will go here eventually ...
}

class ClassDecl extends Declaration {
  // more will go here eventually ...
}