Examples of com.db4o.ObjectContainer.queryByExample()

com.db4o.ObjectContainer.queryByExample()

Query-By-Example interface to retrieve objects.

queryByExample() creates an {@link ObjectSet ObjectSet} containingall objects in the ObjectContainer that match the passed template object.

Calling queryByExample(NULL) returns all objects stored in the ObjectContainer.

Query Evaluation
All non-null members of the template object are compared against all stored objects of the same class. Primitive type members are ignored if they are 0 or false respectively.

Arrays and all supported Collection classes are evaluated for containment. Differences in length/size() are ignored.

Consult the documentation of the Configuration package to configure class-specific behaviour.

Returned Objects
The objects returned in the {@link ObjectSet ObjectSet} are instantiatedand activated to the preconfigured depth of 5. The {@link com.db4o.config.Configuration#activationDepth activation depth}may be configured {@link com.db4o.config.Configuration#activationDepth globally} or{@link com.db4o.config.ObjectClass individually for classes}.

db4o keeps track of all instantiatied objects. Queries will return references to these objects instead of instantiating them a second time.

Objects newly activated by queryByExample() can respond to the Activating callback method.

@param template object to be used as an example to find all matching objects.

@return {@link ObjectSet ObjectSet} containing all found objects.

@see com.db4o.config.Configuration#activationDepth Why activation? @see ObjectCallbacks Using callbacks @throws Db4oIOException I/O operation failed or was unexpectedly interrupted. @throws DatabaseClosedException db4o database file was closed or failed to open.


  public void toXML() {
    EmbeddedConfiguration config = Db4oEmbedded.newConfiguration();
    config.common().activationDepth(Integer.MAX_VALUE);
    ObjectContainer db4o = Db4oEmbedded.openFile(config, databaseFile);
    new XStreamFile(xstreamFile).write(db4o.queryByExample(null));
    db4o.close();
  }


  public void fromXML() {
    ObjectContainer db4o = Db4oEmbedded.openFile(databaseFile);

    }
    // DUMP DATABASE CONTENTS
    if(logDEBUG && database != null) {
    try {
    System.err.println("DUMPING DATABASE CONTENTS:");
    ObjectSet<Object> contents = database.queryByExample(new Object());
    Map<String,Integer> map = new HashMap<String, Integer>();
    for(Object o: contents) {
      String name = o.getClass().getName();
      if((map.get(name)) != null) {
        map.put(name, map.get(name)+1);

  


  private static void query() {
    ObjectContainer db = Db4o.openFile(DBFILENAME);
    try {
      ObjectSet set = db.queryByExample(TestDummy.class);
      if (ITERATIONS != set.size()) {
        System.err.println("Expected: " + ITERATIONS + ", actual: " + set.size());
      }
    } finally {
      db.close();

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