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# TopLink Expressions

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TopLink expressions let you specify query search criteria based on your domain object model. When you execute the query, TopLink translates these search criteria into the appropriate query language for your platform.

TopLink provides two public classes to support expressions:

- The `Expression` class represents an expression that can be anything from a simple constant to a complex clause with boolean logic. You can manipulate, group, and integrate expressions.

- The `ExpressionBuilder` class is the factory for constructing new expressions.

You can specify a selection criterion as an `Expression` with `DatabaseQuery` method `setSelectionCriteria` (see "Database Queries"), and in a finder that takes an `Expression` (see "Expression Finders").

For more information about using TopLink expressions, see "Understanding TopLink Expressions".