DefaultErrorStrategy (ANTLR 4 Runtime 4.9.3 API)

java.lang.Object
- org.antlr.v4.runtime.DefaultErrorStrategy

All Implemented Interfaces:

ANTLRErrorStrategy

Direct Known Subclasses:

BailErrorStrategy
```
public class DefaultErrorStrategy
extends Object
implements ANTLRErrorStrategy
```
This is the default implementation of ANTLRErrorStrategy used for error reporting and recovery in ANTLR parsers.

Field Summary

Fields
Modifier and Type	Field and Description
`protected boolean`	`errorRecoveryMode` Indicates whether the error strategy is currently "recovering from an error".
`protected int`	`lastErrorIndex` The index into the input stream where the last error occurred.
`protected IntervalSet`	`lastErrorStates`
`protected ParserRuleContext`	`nextTokensContext` This field is used to propagate information about the lookahead following the previous match.
`protected int`	`nextTokensState`

Constructor Summary

Constructors
Constructor and Description

DefaultErrorStrategy()

Constructors
Constructor and Description
`DefaultErrorStrategy()`

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`protected void`	`beginErrorCondition(Parser recognizer)` This method is called to enter error recovery mode when a recognition exception is reported.
`protected void`	`consumeUntil(Parser recognizer, IntervalSet set)` Consume tokens until one matches the given token set.
`protected void`	`endErrorCondition(Parser recognizer)` This method is called to leave error recovery mode after recovering from a recognition exception.
`protected String`	`escapeWSAndQuote(String s)`
`protected IntervalSet`	`getErrorRecoverySet(Parser recognizer)`
`protected IntervalSet`	`getExpectedTokens(Parser recognizer)`
`protected Token`	`getMissingSymbol(Parser recognizer)` Conjure up a missing token during error recovery.
`protected String`	`getSymbolText(Token symbol)`
`protected int`	`getSymbolType(Token symbol)`
`protected String`	`getTokenErrorDisplay(Token t)` How should a token be displayed in an error message?
`boolean`	`inErrorRecoveryMode(Parser recognizer)` Tests whether or not `recognizer` is in the process of recovering from an error.
`void`	`recover(Parser recognizer, RecognitionException e)` This method is called to recover from exception `e`.
`Token`	`recoverInline(Parser recognizer)` This method is called when an unexpected symbol is encountered during an inline match operation, such as `Parser.match(int)`.
`void`	`reportError(Parser recognizer, RecognitionException e)` Report any kind of `RecognitionException`.
`protected void`	`reportFailedPredicate(Parser recognizer, FailedPredicateException e)` This is called by `reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)` when the exception is a `FailedPredicateException`.
`protected void`	`reportInputMismatch(Parser recognizer, InputMismatchException e)` This is called by `reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)` when the exception is an `InputMismatchException`.
`void`	`reportMatch(Parser recognizer)` This method is called by when the parser successfully matches an input symbol.
`protected void`	`reportMissingToken(Parser recognizer)` This method is called to report a syntax error which requires the insertion of a missing token into the input stream.
`protected void`	`reportNoViableAlternative(Parser recognizer, NoViableAltException e)` This is called by `reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)` when the exception is a `NoViableAltException`.
`protected void`	`reportUnwantedToken(Parser recognizer)` This method is called to report a syntax error which requires the removal of a token from the input stream.
`void`	`reset(Parser recognizer)` Reset the error handler state for the specified `recognizer`.
`protected Token`	`singleTokenDeletion(Parser recognizer)` This method implements the single-token deletion inline error recovery strategy.
`protected boolean`	`singleTokenInsertion(Parser recognizer)` This method implements the single-token insertion inline error recovery strategy.
`void`	`sync(Parser recognizer)` The default implementation of `ANTLRErrorStrategy.sync(org.antlr.v4.runtime.Parser)` makes sure that the current lookahead symbol is consistent with what were expecting at this point in the ATN.

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

- Field Detail
  - errorRecoveryMode
```
protected boolean errorRecoveryMode
```
    Indicates whether the error strategy is currently "recovering from an error". This is used to suppress reporting multiple error messages while attempting to recover from a detected syntax error.
    
    See Also:
    
    inErrorRecoveryMode(org.antlr.v4.runtime.Parser)
  - lastErrorIndex
```
protected int lastErrorIndex
```
    The index into the input stream where the last error occurred. This is used to prevent infinite loops where an error is found but no token is consumed during recovery...another error is found, ad nauseum. This is a failsafe mechanism to guarantee that at least one token/tree node is consumed for two errors.
  - lastErrorStates
```
protected IntervalSet lastErrorStates
```
  - nextTokensContext
```
protected ParserRuleContext nextTokensContext
```
    This field is used to propagate information about the lookahead following the previous match. Since prediction prefers completing the current rule to error recovery efforts, error reporting may occur later than the original point where it was discoverable. The original context is used to compute the true expected sets as though the reporting occurred as early as possible.
  - nextTokensState
```
protected int nextTokensState
```
    See Also:
    
    nextTokensContext
- Constructor Detail
  - DefaultErrorStrategy
```
public DefaultErrorStrategy()
```
- Method Detail
  - reset
```
public void reset(Parser recognizer)
```
    Reset the error handler state for the specified recognizer.
    The default implementation simply calls endErrorCondition(org.antlr.v4.runtime.Parser) to ensure that the handler is not in error recovery mode.
    
    Specified by:
    
    reset in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
  - beginErrorCondition
```
protected void beginErrorCondition(Parser recognizer)
```
    This method is called to enter error recovery mode when a recognition exception is reported.
    
    Parameters:
    
    recognizer - the parser instance
  - inErrorRecoveryMode
```
public boolean inErrorRecoveryMode(Parser recognizer)
```
    Tests whether or not recognizer is in the process of recovering from an error. In error recovery mode, Parser.consume() adds symbols to the parse tree by calling Parser.createErrorNode(ParserRuleContext, Token) then ParserRuleContext.addErrorNode(ErrorNode) instead of Parser.createTerminalNode(ParserRuleContext, Token).
    
    Specified by:
    
    inErrorRecoveryMode in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
    
    Returns:
    
    true if the parser is currently recovering from a parse error, otherwise false
  - endErrorCondition
```
protected void endErrorCondition(Parser recognizer)
```
    This method is called to leave error recovery mode after recovering from a recognition exception.
    
    Parameters:
    
    recognizer -
  - reportMatch
```
public void reportMatch(Parser recognizer)
```
    This method is called by when the parser successfully matches an input symbol.
    The default implementation simply calls endErrorCondition(org.antlr.v4.runtime.Parser).
    
    Specified by:
    
    reportMatch in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
  - reportError
```
public void reportError(Parser recognizer,
                        RecognitionException e)
```
    Report any kind of RecognitionException. This method is called by the default exception handler generated for a rule method.
    The default implementation returns immediately if the handler is already in error recovery mode. Otherwise, it calls beginErrorCondition(org.antlr.v4.runtime.Parser) and dispatches the reporting task based on the runtime type of e according to the following table.
    - NoViableAltException: Dispatches the call to reportNoViableAlternative(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.NoViableAltException)
    - InputMismatchException: Dispatches the call to reportInputMismatch(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.InputMismatchException)
    - FailedPredicateException: Dispatches the call to reportFailedPredicate(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.FailedPredicateException)
    - All other types: calls Parser.notifyErrorListeners(java.lang.String) to report the exception
    Specified by:
    
    reportError in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
    
    e - the recognition exception to report
  - recover
```
public void recover(Parser recognizer,
                    RecognitionException e)
```
    This method is called to recover from exception e. This method is called after ANTLRErrorStrategy.reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException) by the default exception handler generated for a rule method.
    The default implementation resynchronizes the parser by consuming tokens until we find one in the resynchronization set--loosely the set of tokens that can follow the current rule.
    
    Specified by:
    
    recover in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
    
    e - the recognition exception to recover from
    
    See Also:
    
    ANTLRErrorStrategy.reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)
  - sync
```
public void sync(Parser recognizer)
          throws RecognitionException
```
    The default implementation of ANTLRErrorStrategy.sync(org.antlr.v4.runtime.Parser) makes sure that the current lookahead symbol is consistent with what were expecting at this point in the ATN. You can call this anytime but ANTLR only generates code to check before subrules/loops and each iteration.
    Implements Jim Idle's magic sync mechanism in closures and optional subrules. E.g.,
```
 a : sync ( stuff sync )* ;
 sync : {consume to what can follow sync} ;
 
```
    At the start of a sub rule upon error, sync(org.antlr.v4.runtime.Parser) performs single token deletion, if possible. If it can't do that, it bails on the current rule and uses the default error recovery, which consumes until the resynchronization set of the current rule.
    If the sub rule is optional ((...)?, (...)*, or block with an empty alternative), then the expected set includes what follows the subrule.
    
    During loop iteration, it consumes until it sees a token that can start a sub rule or what follows loop. Yes, that is pretty aggressive. We opt to stay in the loop as long as possible.
    
    ORIGINS
    
    Previous versions of ANTLR did a poor job of their recovery within loops. A single mismatch token or missing token would force the parser to bail out of the entire rules surrounding the loop. So, for rule
```
 classDef : 'class' ID '{' member* '}'
 
```
    input with an extra token between members would force the parser to consume until it found the next class definition rather than the next member definition of the current class.
    This functionality cost a little bit of effort because the parser has to compare token set at the start of the loop and at each iteration. If for some reason speed is suffering for you, you can turn off this functionality by simply overriding this method as a blank { }.
    Specified by:
    
    sync in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
    
    Throws:
    
    RecognitionException - if an error is detected by the error strategy but cannot be automatically recovered at the current state in the parsing process
    
    See Also:
    
    sync(org.antlr.v4.runtime.Parser)
  - reportNoViableAlternative
```
protected void reportNoViableAlternative(Parser recognizer,
                                         NoViableAltException e)
```
    This is called by reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException) when the exception is a NoViableAltException.
    
    Parameters:
    
    recognizer - the parser instance
    
    e - the recognition exception
    
    See Also:
    
    reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)
  - reportInputMismatch
```
protected void reportInputMismatch(Parser recognizer,
                                   InputMismatchException e)
```
    This is called by reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException) when the exception is an InputMismatchException.
    
    Parameters:
    
    recognizer - the parser instance
    
    e - the recognition exception
    
    See Also:
    
    reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)
  - reportFailedPredicate
```
protected void reportFailedPredicate(Parser recognizer,
                                     FailedPredicateException e)
```
    This is called by reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException) when the exception is a FailedPredicateException.
    
    Parameters:
    
    recognizer - the parser instance
    
    e - the recognition exception
    
    See Also:
    
    reportError(org.antlr.v4.runtime.Parser, org.antlr.v4.runtime.RecognitionException)
  - reportUnwantedToken
```
protected void reportUnwantedToken(Parser recognizer)
```
    This method is called to report a syntax error which requires the removal of a token from the input stream. At the time this method is called, the erroneous symbol is current LT(1) symbol and has not yet been removed from the input stream. When this method returns, recognizer is in error recovery mode.
    This method is called when singleTokenDeletion(org.antlr.v4.runtime.Parser) identifies single-token deletion as a viable recovery strategy for a mismatched input error.
    
    The default implementation simply returns if the handler is already in error recovery mode. Otherwise, it calls beginErrorCondition(org.antlr.v4.runtime.Parser) to enter error recovery mode, followed by calling Parser.notifyErrorListeners(java.lang.String).
    
    Parameters:
    
    recognizer - the parser instance
  - reportMissingToken
```
protected void reportMissingToken(Parser recognizer)
```
    This method is called to report a syntax error which requires the insertion of a missing token into the input stream. At the time this method is called, the missing token has not yet been inserted. When this method returns, recognizer is in error recovery mode.
    This method is called when singleTokenInsertion(org.antlr.v4.runtime.Parser) identifies single-token insertion as a viable recovery strategy for a mismatched input error.
    
    The default implementation simply returns if the handler is already in error recovery mode. Otherwise, it calls beginErrorCondition(org.antlr.v4.runtime.Parser) to enter error recovery mode, followed by calling Parser.notifyErrorListeners(java.lang.String).
    
    Parameters:
    
    recognizer - the parser instance
  - recoverInline
```
public Token recoverInline(Parser recognizer)
                    throws RecognitionException
```
    This method is called when an unexpected symbol is encountered during an inline match operation, such as Parser.match(int). If the error strategy successfully recovers from the match failure, this method returns the Token instance which should be treated as the successful result of the match.
    This method handles the consumption of any tokens - the caller should not call Parser.consume() after a successful recovery.
    
    Note that the calling code will not report an error if this method returns successfully. The error strategy implementation is responsible for calling Parser.notifyErrorListeners(java.lang.String) as appropriate.
    
    The default implementation attempts to recover from the mismatched input by using single token insertion and deletion as described below. If the recovery attempt fails, this method throws an InputMismatchException.
    
    EXTRA TOKEN (single token deletion)
    
    LA(1) is not what we are looking for. If LA(2) has the right token, however, then assume LA(1) is some extra spurious token and delete it. Then consume and return the next token (which was the LA(2) token) as the successful result of the match operation.
    
    This recovery strategy is implemented by singleTokenDeletion(org.antlr.v4.runtime.Parser).
    
    MISSING TOKEN (single token insertion)
    
    If current token (at LA(1)) is consistent with what could come after the expected LA(1) token, then assume the token is missing and use the parser's TokenFactory to create it on the fly. The "insertion" is performed by returning the created token as the successful result of the match operation.
    
    This recovery strategy is implemented by singleTokenInsertion(org.antlr.v4.runtime.Parser).
    
    EXAMPLE
    
    For example, Input i=(3; is clearly missing the ')'. When the parser returns from the nested call to expr, it will have call chain:
```
 stat → expr → atom
 
```
    and it will be trying to match the ')' at this point in the derivation:
```
 => ID '=' '(' INT ')' ('+' atom)* ';'
                    ^
 
```
    The attempt to match ')' will fail when it sees ';' and call recoverInline(org.antlr.v4.runtime.Parser). To recover, it sees that LA(1)==';' is in the set of tokens that can follow the ')' token reference in rule atom. It can assume that you forgot the ')'.
    Specified by:
    
    recoverInline in interface ANTLRErrorStrategy
    
    Parameters:
    
    recognizer - the parser instance
    
    Throws:
    
    RecognitionException - if the error strategy was not able to recover from the unexpected input symbol
  - singleTokenInsertion
```
protected boolean singleTokenInsertion(Parser recognizer)
```
    This method implements the single-token insertion inline error recovery strategy. It is called by recoverInline(org.antlr.v4.runtime.Parser) if the single-token deletion strategy fails to recover from the mismatched input. If this method returns true, recognizer will be in error recovery mode.
    This method determines whether or not single-token insertion is viable by checking if the LA(1) input symbol could be successfully matched if it were instead the LA(2) symbol. If this method returns true, the caller is responsible for creating and inserting a token with the correct type to produce this behavior.
    
    Parameters:
    
    recognizer - the parser instance
    
    Returns:
    
    true if single-token insertion is a viable recovery strategy for the current mismatched input, otherwise false
  - singleTokenDeletion
```
protected Token singleTokenDeletion(Parser recognizer)
```
    This method implements the single-token deletion inline error recovery strategy. It is called by recoverInline(org.antlr.v4.runtime.Parser) to attempt to recover from mismatched input. If this method returns null, the parser and error handler state will not have changed. If this method returns non-null, recognizer will not be in error recovery mode since the returned token was a successful match.
    If the single-token deletion is successful, this method calls reportUnwantedToken(org.antlr.v4.runtime.Parser) to report the error, followed by Parser.consume() to actually "delete" the extraneous token. Then, before returning reportMatch(org.antlr.v4.runtime.Parser) is called to signal a successful match.
    
    Parameters:
    
    recognizer - the parser instance
    
    Returns:
    
    the successfully matched Token instance if single-token deletion successfully recovers from the mismatched input, otherwise null
  - getMissingSymbol
```
protected Token getMissingSymbol(Parser recognizer)
```
    Conjure up a missing token during error recovery. The recognizer attempts to recover from single missing symbols. But, actions might refer to that missing symbol. For example, x=ID {f($x);}. The action clearly assumes that there has been an identifier matched previously and that $x points at that token. If that token is missing, but the next token in the stream is what we want we assume that this token is missing and we keep going. Because we have to return some token to replace the missing token, we have to conjure one up. This method gives the user control over the tokens returned for missing tokens. Mostly, you will want to create something special for identifier tokens. For literals such as '{' and ',', the default action in the parser or tree parser works. It simply creates a CommonToken of the appropriate type. The text will be the token. If you change what tokens must be created by the lexer, override this method to create the appropriate tokens.
  - getExpectedTokens
```
protected IntervalSet getExpectedTokens(Parser recognizer)
```
  - getTokenErrorDisplay
```
protected String getTokenErrorDisplay(Token t)
```
    How should a token be displayed in an error message? The default is to display just the text, but during development you might want to have a lot of information spit out. Override in that case to use t.toString() (which, for CommonToken, dumps everything about the token). This is better than forcing you to override a method in your token objects because you don't have to go modify your lexer so that it creates a new Java type.
  - getSymbolText
```
protected String getSymbolText(Token symbol)
```
  - getSymbolType
```
protected int getSymbolType(Token symbol)
```
  - escapeWSAndQuote
```
protected String escapeWSAndQuote(String s)
```
  - getErrorRecoverySet
```
protected IntervalSet getErrorRecoverySet(Parser recognizer)
```
  - consumeUntil
```
protected void consumeUntil(Parser recognizer,
                            IntervalSet set)
```
    Consume tokens until one matches the given token set.

Class DefaultErrorStrategy

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

errorRecoveryMode

lastErrorIndex

lastErrorStates

nextTokensContext

nextTokensState

Constructor Detail

DefaultErrorStrategy

Method Detail

reset

beginErrorCondition

inErrorRecoveryMode

endErrorCondition

reportMatch

reportError

recover

sync

reportNoViableAlternative

reportInputMismatch

reportFailedPredicate

reportUnwantedToken

reportMissingToken

recoverInline

singleTokenInsertion

singleTokenDeletion

getMissingSymbol

getExpectedTokens

getTokenErrorDisplay

getSymbolText

getSymbolType

escapeWSAndQuote

getErrorRecoverySet

consumeUntil