lalr_item (Java CUP Parser Generator 0.9e API)

java.lang.Object
- java_cup.lr_item_core
- - java_cup.lalr_item

```
public class lalr_item
extends lr_item_core
```
This class represents an LALR item. Each LALR item consists of a production, a "dot" at a position within that production, and a set of lookahead symbols (terminal). (The first two of these parts are provide by the super class). An item is designed to represent a configuration that the parser may be in. For example, an item of the form:
```
    [A ::= B * C d E  , {a,b,c}]
  
```
indicates that the parser is in the middle of parsing the production
```
    A ::= B C d E
  
```
that B has already been parsed, and that we will expect to see a lookahead of either a, b, or c once the complete RHS of this production has been found.
Items may initially be missing some items from their lookahead sets. Links are maintained from each item to the set of items that would need to be updated if symbols are added to its lookahead set. During "lookahead propagation", we add symbols to various lookahead sets and propagate these changes across these dependency links as needed.
Version:

last updated: 11/25/95

Author:

Scott Hudson

See Also:
lalr_item_set, lalr_state

Field Summary

Fields
Modifier and Type	Field and Description
`protected terminal_set`	`_lookahead` The lookahead symbols of the item.
`protected Stack`	`_propagate_items` Links to items that the lookahead needs to be propagated to.
`protected boolean`	`needs_propagation` Flag to indicate that this item needs to propagate its lookahead (whether it has changed or not).

Fields inherited from class java_cup.lr_item_core
_core_hash_cache, _dot_pos, _symbol_after_dot, _the_production

Constructor Summary

Constructors
Constructor and Description
`lalr_item(production prod)` Constructor with default position and empty lookahead set.
`lalr_item(production prod, int pos, terminal_set look)` Full constructor.
`lalr_item(production prod, terminal_set look)` Constructor with default position (dot at start).

Method Summary

Methods
Modifier and Type	Method and Description
`void`	`add_propagate(lalr_item prop_to)` Add a new item to the set of items we propagate to.
`terminal_set`	`calc_lookahead(terminal_set lookahead_after)` Calculate lookahead representing symbols that could appear after the symbol that the dot is currently in front of.
`boolean`	`equals(lalr_item other)` Equality comparison -- here we only require the cores to be equal since we need to do sets of items based only on core equality (ignoring lookahead sets).
`boolean`	`equals(Object other)` Generic equality comparison.
`int`	`hashCode()` Return a hash code -- here we only hash the core since we only test core matching in LALR items.
`boolean`	`lookahead_visible()` Determine if everything from the symbol one beyond the dot all the way to the end of the right hand side is nullable.
`terminal_set`	`lookahead()` The lookahead symbols of the item.
`Stack`	`propagate_items()` Links to items that the lookahead needs to be propagated to
`void`	`propagate_lookaheads(terminal_set incoming)` Propagate incoming lookaheads through this item to others need to be changed.
`lalr_item`	`shift()` Produce the new lalr_item that results from shifting the dot one position to the right.
`String`	`toString()` Convert to string.

Methods inherited from class java_cup.lr_item_core
core_equals, core_hashCode, dot_at_end, dot_before_nt, dot_pos, equals, obj_hash, shift_core, symbol_after_dot, the_production, to_simple_string

Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, wait, wait, wait

- Field Detail
  - _lookahead
```
protected terminal_set _lookahead
```
    The lookahead symbols of the item.
  - _propagate_items
```
protected Stack _propagate_items
```
    Links to items that the lookahead needs to be propagated to.
  - needs_propagation
```
protected boolean needs_propagation
```
    Flag to indicate that this item needs to propagate its lookahead (whether it has changed or not).
- Constructor Detail
  - lalr_item
```
public lalr_item(production prod,
         int pos,
         terminal_set look)
          throws internal_error
```
    Full constructor.
    
    Parameters:
    prod - the production for the item.
    pos - the position of the "dot" within the production.
    look - the set of lookahead symbols.
    
    Throws:
    
    internal_error
  - lalr_item
```
public lalr_item(production prod,
         terminal_set look)
          throws internal_error
```
    Constructor with default position (dot at start).
    
    Parameters:
    prod - the production for the item.
    look - the set of lookahead symbols.
    
    Throws:
    
    internal_error
  - lalr_item
```
public lalr_item(production prod)
          throws internal_error
```
    Constructor with default position and empty lookahead set.
    
    Parameters:
    prod - the production for the item.
    
    Throws:
    
    internal_error
- Method Detail
  - lookahead
```
public terminal_set lookahead()
```
    The lookahead symbols of the item.
  - propagate_items
```
public Stack propagate_items()
```
    Links to items that the lookahead needs to be propagated to
  - add_propagate
```
public void add_propagate(lalr_item prop_to)
```
    Add a new item to the set of items we propagate to.
  - propagate_lookaheads
```
public void propagate_lookaheads(terminal_set incoming)
                          throws internal_error
```
    Propagate incoming lookaheads through this item to others need to be changed.
    
    Throws:
    
    internal_error
  - shift
```
public lalr_item shift()
                throws internal_error
```
    Produce the new lalr_item that results from shifting the dot one position to the right.
    
    Throws:
    
    internal_error
  - calc_lookahead
```
public terminal_set calc_lookahead(terminal_set lookahead_after)
                            throws internal_error
```
    Calculate lookahead representing symbols that could appear after the symbol that the dot is currently in front of. Note: this routine must not be invoked before first sets and nullability has been calculated for all non terminals.
    
    Throws:
    
    internal_error
  - lookahead_visible
```
public boolean lookahead_visible()
                          throws internal_error
```
    Determine if everything from the symbol one beyond the dot all the way to the end of the right hand side is nullable. This would indicate that the lookahead of this item must be included in the lookaheads of all items produced as a closure of this item. Note: this routine should not be invoked until after first sets and nullability have been calculated for all non terminals.
    
    Throws:
    
    internal_error
  - equals
```
public boolean equals(lalr_item other)
```
    Equality comparison -- here we only require the cores to be equal since we need to do sets of items based only on core equality (ignoring lookahead sets).
  - equals
```
public boolean equals(Object other)
```
    Generic equality comparison.
    
    Overrides:
    
    equals in class lr_item_core
  - hashCode
```
public int hashCode()
```
    Return a hash code -- here we only hash the core since we only test core matching in LALR items.
    
    Overrides:
    
    hashCode in class lr_item_core
  - toString
```
public String toString()
```
    Convert to string.
    
    Overrides:
    
    toString in class lr_item_core

Class lalr_item

Field Summary

Fields inherited from class java_cup.lr_item_core

Constructor Summary

Method Summary

Methods inherited from class java_cup.lr_item_core

Methods inherited from class java.lang.Object

Field Detail

_lookahead

_propagate_items

needs_propagation

Constructor Detail

lalr_item

lalr_item

lalr_item

Method Detail

lookahead

propagate_items

add_propagate

propagate_lookaheads

shift

calc_lookahead

lookahead_visible

equals

equals

hashCode

toString