@XmlSchema(URL="http://schemas.opengis.net/filter/1.0.0/filter.xsd", specification=OGC_02059)

## Package org.opengis.filter.expression

An expression is a combination of one or more elements that evaluate to single `Object` value.

See:
Description

Interface Summary
Add: Encodes the operation of addition.
BinaryExpression: Abstract base class for the various filter expressions that compute some value from two input values.
Divide: Encodes the operation of division where the first argument is divided by the second argument.
Expression: Interface for all the OGC Filter elements that compute values.
ExpressionVisitor: Visitor with `visit` methods to be called by `Expression.accept(...)`.
Function: Instances of this class represent a function call into some implementation-specific function.
Literal: Instances of this interface provide a constant, literal value that can be used in expressions.
Multiply: Encodes the operation of multiplication.
PropertyName: Expression class whose value is computed by retrieving the value indicated by the provided name.
Subtract: Encodes the operation of subtraction where the second argument is subtracted from the first.

Class Summary
NilExpression: Placeholder class used to represent a NIL expression, evaultates to `null`.

## Package org.opengis.filter.expression Description

An expression is a combination of one or more elements that evaluate to single `Object` value. The following is adapted from Filter encoding specifications:

- Filter Encoding 1.0
- OpenGIS® Filter Encoding Implementation Specification Version 1.1
- OpenGIS® Catalogue Service Implementation Specification 2.0.1

An expression can be formed using the elements: `Add`, `Subtract`, `Multiply`, `Divide`, `PropertyName`, `Literal` and `Function`. They all belong to the substitution group expression which means that any of them can be used wherever an expression is called for. In addition, the combinaison of these elements are themselves expressions and can be used wherever an expression is called for.

### Arithmetic operators

The elements defined in this package are used to encode the fundamental arithmetic operations of addition, subtraction, multiplication and division. Arithmetic operators are binary operators meaning that they accept two arguments and evaluate to a single result.

### Literals

A literal value is any part of a statement or expression that is to be used exactly as it is specified, rather than as a variable or other element. The `Literal` element is used to encode literal scalar and geometric values.

### Functions

A function is a named procedure that performs a distinct computation. A function may accept zero or more arguments as input and generates a single result. A function is composed of the name of the function, encoded using the attribute name, and zero or more arguments contained within the `Function` element. The arguments themselves are expressions.

### Data Access

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Data access is acomplished by means of `PropertyName` expressions. These Expressions are independent of the data being queried and form the foundation of a, very simple, general query language offering independence specific bindings to Feature, Metadata and Record data structures.

### Filter Encodings

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At the time of this writing the Filter API has standard encodings for XML (Filter 1.0 and Filter 1.1) and Text (a BNF form is provided in the Catalog 2.0.1 specification above). You should be warned that not all content can be expressed in all encodings, please refer to the javadocs for specific limitations.

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