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Filter Data - Java SDK

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  • Query Engine
  • Fluent Interface
  • About the Examples In This Section
  • Comparison Operators
  • Logical Operators
  • String Operators
  • Aggregate Operators
  • Filter, Sort, Limit, Unique, and Chain Queries
  • About the Examples in This Section
  • Filters
  • Sort Results
  • Limit Results
  • Unique Results
  • Chain Queries
  • Query with Realm Query Language

Query Engine

To filter data in your realm, use the Realm query engine.

There are two ways to access the query engine with the Java SDK:

Fluent Interface

The Java SDK uses a Fluent interface to construct multi-clause queries that are passed to the query engine.

See RealmQuery API for a complete list of available methods.

There are several types of operators available to filter a Realm collection. Filters work by evaluating an operator expression for every object in the collection being filtered. If the expression resolves to true, Realm Database includes the object in the results collection.

An expression consists of one of the following:

  • The name of a property of the object currently being evaluated.

  • An operator and up to two argument expression(s).

  • A literal string, number, or date.

About the Examples In This Section

The examples in this section use a simple data set for a task list app. The two Realm object types are Project and Task. A Task has a name, assignee's name, and completed flag. There is also an arbitrary number for priority (higher is more important) and a count of minutes spent working on it. A Project has zero or more Tasks.

See the schema for these two classes, Project and Task, below:

Comparison Operators

The most straightforward operation in a search is to compare values.

Operator
Description
between
Evaluates to true if the left-hand numerical or date expression is between or equal to the right-hand range. For dates, this evaluates to true if the left-hand date is within the right-hand date range.
equalTo
Evaluates to true if the left-hand expression is equal to the right-hand expression.
greaterThan
Evaluates to true if the left-hand numerical or date expression is greater than the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is later than the right-hand date.
greaterThanOrEqualTo
Evaluates to true if the left-hand numerical or date expression is greater than or equal to the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is later than or the same as the right-hand date.
in
Evaluates to true if the left-hand expression is in the right-hand list.
lessThan
Evaluates to true if the left-hand numerical or date expression is less than the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is earlier than the right-hand date.
lessThanOrEqualTo
Evaluates to true if the left-hand numeric expression is less than or equal to the right-hand numeric expression. For dates, this evaluates to true if the left-hand date is earlier than or the same as the right-hand date.
notEqualTo
Evaluates to true if the left-hand expression is not equal to the right-hand expression.

Example

The following example uses the query engine's comparison operators to:

  • Find high priority tasks by comparing the value of the priority property value with a threshold number, above which priority can be considered high.

  • Find just-started or short-running tasks by seeing if the progressMinutes property falls within a certain range.

  • Find unassigned tasks by finding tasks where the assignee property is equal to null.

  • Find tasks assigned to specific teammates Ali or Jamie by seeing if the assignee property is in a list of names.

Logical Operators

You can make compound predicates using logical operators.

Operator
Description
and
Evaluates to true if both left-hand and right-hand expressions are true.
not
Negates the result of the given expression.
or
Evaluates to true if either expression returns true.

Example

We can use the query language's logical operators to find all of Ali's completed tasks. That is, we find all tasks where the assignee property value is equal to 'Ali' AND the isComplete property value is true:

String Operators

You can compare string values using these string operators. Regex-like wildcards allow more flexibility in search.

Operator
Description
beginsWith
Evaluates to true if the left-hand string expression begins with the right-hand string expression. This is similar to contains, but only matches if the left-hand string expression is found at the beginning of the right-hand string expression.
contains
Evaluates to true if the left-hand string expression is found anywhere in the right-hand string expression.
endsWith
Evaluates to true if the left-hand string expression ends with the right-hand string expression. This is similar to contains, but only matches if the left-hand string expression is found at the very end of the right-hand string expression.
like

Evaluates to true if the left-hand string expression matches the right-hand string wildcard string expression. A wildcard string expression is a string that uses normal characters with two special wildcard characters:

  • The * wildcard matches zero or more of any character

  • The ? wildcard matches any character.

For example, the wildcard string "d?g" matches "dog", "dig", and "dug", but not "ding", "dg", or "a dog".

equalTo
Evaluates to true if the left-hand string is lexicographically equal to the right-hand string.

Example

We use the query engine's string operators to find projects with a name starting with the letter 'e' and projects with names that contain 'ie':

Note

Case-insensitive Character Limitations

Case-insensitive string operators only support the Latin Basic, Latin Supplement, Latin Extended A, and Latin Extended B (UTF-8 range 0–591) character sets. Setting the case insensitive flag in queries when using equalTo, notEqualTo, contains, endsWith, beginsWith, or like only works on English locale characters.

Aggregate Operators

You can apply an aggregate operator to a collection property of a Realm object. Aggregate operators traverse a collection and reduce it to a single value.

Operator
Description
average
Evaluates to the average value of a given numerical property across a collection.
count
Evaluates to the number of objects in the given collection.
max
Evaluates to the highest value of a given numerical property across a collection.
min
Evaluates to the lowest value of a given numerical property across a collection.
sum
Evaluates to the sum of a given numerical property across a collection.

Example

We create a couple of filters to show different facets of the data:

  • Projects with average tasks priority above 5.

  • Long running projects.

Filter, Sort, Limit, Unique, and Chain Queries

About the Examples in This Section

The examples in this section use two Realm object types: Teacher and Student.

See the schema for these two classes below:

Filters

You can build filters using the operator methods of the fluent interface exposed by the RealmQuery class:

This gives you a new instance of the class RealmResults, containing teachers with the name "Ms. Langtree" or "Mrs. Jacobs".

RealmQuery includes several methods that can execute queries:

  • findAll() blocks until it finds all objects that meet the query conditions

  • findAllAsync() returns immediately and finds all objects that meet the query conditions asynchronously on a background thread

  • findFirst() blocks until it finds the first object that meets the query conditions

  • findFirstAsync() returns immediately and finds the first object that meets the query conditions asynchronously on a background thread

Queries return a list of references to the matching Realm objects using the RealmResults type.

Link Queries

When referring to an object property, you can use dot notation to refer to child properties of that object. You can refer to the properties of embedded objects and relationships with dot notation.

For example, consider a query for all teachers with a student named "Wirt" or "Greg":

You can even use dot notation to query inverse relationships:

Sort Results

Important

Realm applies the distinct(), sort() and limit() methods in the order you specify. Depending on the data set this can alter the query result. Generally, you should apply limit() last to avoid unintended result sets.

You can define the order of query results using the sort() method:

Sorts organize results in ascending order by default. To organize results in descending order, pass Sort.DESCENDING as a second argument. You can resolve sort order ties between identical property values by passing an array of properties instead of a single property: in the event of a tie, Realm sorts the tied objects by subsequent properties in order.

Note

String Sorting Limitations

Realm uses non-standard sorting for upper and lowercase letters, sorting them together rather than sorting uppercase first. As a result, '- !"#0&()*,./:;?_+<=>123aAbBcC...xXyYzZ is the actual sorting order in Realm. Additionally, sorting strings only supports the Latin Basic, Latin Supplement, Latin Extended A, and Latin Extended B (UTF-8 range 0–591) character sets.

Limit Results

You can cap the number of query results to a specific maximum number using the limit() method:

Limited result collections automatically update like any other query result. Consequently, objects might drop out of the collection as underlying data changes.

Tip

Pagination is Not Necessary for Realm Optimization

Some databases encourage paginating results with limits to avoid reading unnecessary data from disk or using too much memory.

Since Realm queries are lazy, there is no need to take such measures. Realm only loads objects from query results when they are explicitly accessed.

Tip

Deleted Notifications in Limited Results

Collection notifications report objects as deleted when they drop out of the result set. This does not necessarily mean that they have been deleted from the underlying realm, just that they are no longer part of the query result.

Unique Results

You can reduce query results to unique values for a given field or fields using the distinct() method:

You can only call distinct() on integer, long, short, and String fields; other field types will throw an exception. As with sorting, you can specify multiple fields to resolve ties.

Chain Queries

You can apply additional filters to a results collection by calling the where() method:

The where() method returns a RealmQuery that you can resolve into a RealmResults using a find method. Filtered results can only return objects of the same type as the original results set, but are otherwise able to use any filters.

Query with Realm Query Language

New in version 10.4.0.

You can also query realms using Realm Query Language, a string-based query language to constrain searches when retrieving objects from a realm.

You can use RealmQuery.rawPredicate(). For more information about syntax, usage and limitations, refer to the Realm Query Language reference.

Realm Query Language can use either the class and property names defined in your Realm Model classes or the internal names defined with @RealmField. You can combine raw predicates with other raw predicates or type-safe predicates created with RealmQuery:

Tip

See also: Realm Query Language Examples

You can also find useful Realm Query Language examples on the following pages:

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