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## org.exist.xquery
Interface Expression

All Known Implementing Classes:
  AbstractExpression

public interface Expression

Base interface implemented by all classes which are part of an XQuery/XPath expression. The main method is `eval(Sequence, Item)`. Please read the description there.

Field Summary
`static int` | `DOT_TEST`
`static int` | `EXPRESSION_ID_INVALID` Marks an invalid expression id.
`static int` | `IGNORE_CONTEXT`
`static int` | `IN_PREDICATE` Indicates that the expression is within a predicate or the where clause of a FLWOR.
`static int` | `IN_UPDATE` Indicates that the expression is used within an update statement.
`static int` | `IN_WHERE_CLAUSE` Indicates that the expression is within a where clause of a FLWOR.
`static int` | `NO_CONTEXT_ID` Indicates that no context id is supplied to an expression.
`static int` | `SINGLE_STEP_EXECUTION` Indicates that the query engine will call the expression once for every item in the context sequence.

Method Summary
`void` | `accept(ExpressionVisitor visitor)` Start traversing the expression tree using the specified `ExpressionVisitor`.
`void` | `analyze(AnalyzeContextInfo contextInfo)` Statically analyze the expression and its subexpressions.
`void` | `dump(ExpressionDumper dumper)` Write a diagnostic dump of the expression to the passed `ExpressionDumper`.
`Sequence` | `eval(Sequence contextSequence)` Evaluate the expression represented by this object.
`Sequence` | `eval(Sequence contextSequence, Item contextItem)` Evaluate the expression represented by this object.
`XQueryAST` | `getASTNode()` Returns the `XQueryAST` node from which this expression has been constructed by the parser.
`int` | `getCardinality()` The expected cardinality of the return value of the expression.
`DocumentSet` | `getContextDocSet()`
`int` | `getContextId()`
`int` | `getDependencies()` Returns a set of bit-flags, indicating some of the parameters on which this expression depends.
`int` | `getExpressionId()` Returns an id which uniquely identifies this expression within the compiled expression tree of the query.
`void` | `resetState()` Called to inform an expression that it should reset to its initial state.
`int` | `returnsType()` The static return type of the expression.
`void` | `setASTNode(XQueryAST ast)`
`void` | `setContextDocSet(DocumentSet contextSet)`
`void` | `setPrimaryAxis(int axis)`

Field Detail

### SINGLE_STEP_EXECUTION

    public static final int SINGLE_STEP_EXECUTION

  Indicates that the query engine will call the expression once for every item in the context sequence. This is what you would expect to be the normal behaviour of an XQuery processor. However, eXist tries to process some types of expressions in one single step for the whole input sequence. So if the flag is not set, the expression is only called once.

  See Also:
    Constant Field Values

### IN_PREDICATE

    public static final int IN_PREDICATE

  Indicates that the expression is within a predicate or the where clause of a FLWOR.

  See Also:
    Constant Field Values

### IN_WHERE_CLAUSE

    public static final int IN_WHERE_CLAUSE

  Indicates that the expression is within a where clause of a FLWOR. This flag will be set in addition to `IN_PREDICATE`.

  See Also:
    Constant Field Values

### IN_UPDATE

    public static final int IN_UPDATE

  Indicates that the expression is used within an update statement. Subexpressions should not cache any relevant data as it may be subject to change.

  See Also:
    Constant Field Values

### DOT_TEST

    public static final int DOT_TEST

See Also:
  Constant Field Values

### NO_CONTEXT_ID

    public static final int NO_CONTEXT_ID

  Indicates that no context id is supplied to an expression.

  See Also:
    Constant Field Values

### IGNORE_CONTEXT

    public static final int IGNORE_CONTEXT

See Also:
  Constant Field Values

### EXPRESSION_ID_INVALID

    public static final int EXPRESSION_ID_INVALID

  Marks an invalid expression id.

  See Also:
    Constant Field Values

Method Detail

### getExpressionId

    public int getExpressionId()

  Returns an id which uniquely identifies this expression within the compiled expression tree of the query.

  Returns:
    unique id or `EXPRESSION_ID_INVALID`

### analyze

    public void analyze(AnalyzeContextInfo contextInfo)
                 throws XPathException

  Statically analyze the expression and its subexpressions. During the static analysis phase, the query engine can detect unknown variables and some type errors.

  Throws:
    `XPathException`

### eval

    public Sequence eval(Sequence contextSequence,
                         Item contextItem)
                  throws XPathException

  Evaluate the expression represented by this object. Depending on the context in which this expression is executed, either the context sequence, the context item or both of them may be set. An implementing class should know how to handle this. The general contract is as follows: if the `Dependency.CONTEXT_ITEM` bit is set in the bit field returned by `getDependencies()`, the eval method will be called once for every item in the context sequence. The contextItem parameter will be set to the current item. Otherwise, the eval method will only be called once for the whole context sequence and contextItem will be null. eXist tries to process the entire context set in one, single step whenever possible. Thus, most classes only expect context to contain a list of nodes which represents the current context of the expression. The position() function in XPath is an example for an expression, which requires both, context sequence and context item to be set. The context sequence might be a node set, a sequence of atomic values or a single node or atomic value.

  Parameters:
    `contextSequence` - the current context sequence.
    `contextItem` - a single item, taken from context. This defines the item, the expression should work on.
  Throws:
    `XPathException`

### eval

    public Sequence eval(Sequence contextSequence)
                  throws XPathException

  Evaluate the expression represented by this object. An overloaded method which just passes the context sequence depending on the expression context.

  Throws:
    `XPathException`

### setPrimaryAxis

    public void setPrimaryAxis(int axis)

### returnsType

    public int returnsType()

  The static return type of the expression. This method should return one of the type constants defined in class `Type`. If the return type cannot be determined statically, return Type.ITEM.

### getCardinality

    public int getCardinality()

  The expected cardinality of the return value of the expression. Should return a bit mask with bits set as defined in class `Cardinality`.

### getDependencies

    public int getDependencies()

  Returns a set of bit-flags, indicating some of the parameters on which this expression depends. The flags are defined in `Dependency`.

  Returns:
    set of bit-flags

### resetState

    public void resetState()

  Called to inform an expression that it should reset to its initial state. All cached data in the expression object should be dropped. For example, the document() function calls this method whenever the input document set has changed.

### accept

    public void accept(ExpressionVisitor visitor)

  Start traversing the expression tree using the specified `ExpressionVisitor`.

  Parameters:
    `visitor` -

### dump

    public void dump(ExpressionDumper dumper)

  Write a diagnostic dump of the expression to the passed `ExpressionDumper`.

  Parameters:
    `dumper` - the expression dumper to write to

### setContextDocSet

    public void setContextDocSet(DocumentSet contextSet)

### getContextId

    public int getContextId()

### getContextDocSet

    public DocumentSet getContextDocSet()

### getASTNode

    public XQueryAST getASTNode()

  Returns the `XQueryAST` node from which this expression has been constructed by the parser. This node contains location information (line number and column) important for error reports.

  Returns:
    XQueryAST node

### setASTNode

    public void setASTNode(XQueryAST ast)

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Copyright (C) Wolfgang Meier. All rights reserved.