com.db4o.query

Class Predicate<ExtentType>

java.lang.Object

com.db4o.query.Predicate<ExtentType>

All Implemented Interfaces:

java.io.Serializable

public abstract class Predicate<ExtentType>
extends java.lang.Object
implements java.io.Serializable

Base class for native queries.

Native Queries provide the ability to run one or more lines of code against all instances of a class. Native query expressions should return true to mark specific instances as part of the result set. db4o will attempt to optimize native query expressions and run them against indexes and without instantiating actual objects, where this is possible.

The syntax of the enclosing object for the native query expression varies slightly, depending on the language version used. Here are some examples, how a simple native query will look like in some of the programming languages and dialects that db4o supports:

// C# .NET 2.0
IList <Cat> cats = db.Query <Cat> (delegate(Cat cat) {
return cat.Name == "Occam";
});


// Java JDK 5
List <Cat> cats = db.query(new Predicate<Cat>() {
public boolean match(Cat cat) {
return cat.getName().equals("Occam");
}
});


// Java JDK 1.2 to 1.4
List cats = db.query(new Predicate() {
public boolean match(Cat cat) {
return cat.getName().equals("Occam");
}
});


// Java JDK 1.1
ObjectSet cats = db.query(new CatOccam());

public static class CatOccam extends Predicate {
public boolean match(Cat cat) {
return cat.getName().equals("Occam");
}
});


// C# .NET 1.1
IList cats = db.Query(new CatOccam());

public class CatOccam : Predicate {
public boolean Match(Cat cat) {
return cat.Name == "Occam";
}
});

Summing up the above:
In order to run a Native Query, you can
- use the delegate notation for .NET 2.0.
- extend the Predicate class for all other language dialects

A class that extends Predicate is required to implement the #match() / #Match() method, following the native query conventions:
- The name of the method is "#match()" (Java) / "#Match()" (.NET).
- The method must be public public.
- The method returns a boolean.
- The method takes one parameter.
- The Type (.NET) / Class (Java) of the parameter specifies the extent.
- For all instances of the extent that are to be included into the resultset of the query, the match method should return true. For all instances that are not to be included, the match method should return false.

See Also:

Serialized Form

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	PREDICATEMETHOD_NAME public for implementation reasons, please ignore.

Constructor Summary

Constructors

Constructor and Description
Predicate()
Predicate(java.lang.Class<? extends ExtentType> extentType)

Method Summary

Methods

Modifier and Type	Method and Description
boolean	appliesTo(ExtentType candidate) public for implementation reasons, please ignore.
java.lang.Class<? extends ExtentType>	extentType() public for implementation reasons, please ignore.
java.lang.reflect.Method	getFilterMethod()
abstract boolean	match(ExtentType candidate) The match method that needs to be implemented by the user.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

PREDICATEMETHOD_NAME

public static final java.lang.String PREDICATEMETHOD_NAME

public for implementation reasons, please ignore.

See Also:

Constant Field Values

Constructor Detail

Predicate

public Predicate()

Predicate

public Predicate(java.lang.Class<? extends ExtentType> extentType)

Method Detail

getFilterMethod

public java.lang.reflect.Method getFilterMethod()

extentType

public java.lang.Class<? extends ExtentType> extentType()

public for implementation reasons, please ignore.

match

public abstract boolean match(ExtentType candidate)

The match method that needs to be implemented by the user.

Parameters:

candidate - the candidate object passed from db4o

Returns:

true to include an object in the resulting ObjectSet

appliesTo

public boolean appliesTo(ExtentType candidate)

public for implementation reasons, please ignore.