Properties, Arrays, Lists, Maps, and Indexers

The Infra Expression Language provides support for navigating object graphs and indexing into various structures.

Numerical index values are zero-based, such as when accessing the nth element of an array in Java.
See the Safe Navigation Operator section for details on how to navigate object graphs and index into various structures using the null-safe operator.

Property Navigation

You can navigate property references within an object graph by using a period to indicate a nested property value. The instances of the Inventor class, pupin and tesla, were populated with data listed in the Classes used in the examples section. To navigate down the object graph and get Tesla’s year of birth and Pupin’s city of birth, we use the following expressions:

  • Java

// evaluates to 1856
int year = (Integer) parser.parseExpression("birthdate.year + 1900").getValue(context);

// evaluates to "Smiljan"
String city = (String) parser.parseExpression("placeOfBirth.city").getValue(context);

Case insensitivity is allowed for the first letter of property names. Thus, the expressions in the above example may be written as Birthdate.Year + 1900 and PlaceOfBirth.City, respectively. In addition, properties may optionally be accessed via method invocations — for example, getPlaceOfBirth().getCity() instead of placeOfBirth.city.

Indexing into Arrays and Collections

The nth element of an array or collection (for example, a Set or List) can be obtained by using square bracket notation, as the following example shows.

If the indexed collection is a java.util.List, the nth element will be accessed directly via list.get(n).

For any other type of Collection, the nth element will be accessed by iterating over the collection using its Iterator and returning the nth element encountered.

  • Java

ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

// Inventions Array

// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(
    context, tesla, String.class);

// Members List

// evaluates to "Nikola Tesla"
String name = parser.parseExpression("members[0].name").getValue(
    context, ieee, String.class);

// List and Array Indexing

// evaluates to "Wireless communication"
String invention = parser.parseExpression("members[0].inventions[6]").getValue(
    context, ieee, String.class);

Indexing into Strings

The nth character of a string can be obtained by specifying the index within square brackets, as demonstrated in the following example.

The nth character of a string will evaluate to a java.lang.String, not a java.lang.Character.

  • Java

// evaluates to "T" (8th letter of "Nikola Tesla")
String character = parser.parseExpression("members[0].name[7]")
    .getValue(societyContext, String.class);

Indexing into Maps

The contents of maps are obtained by specifying the key value within square brackets. In the following example, because keys for the officers map are strings, we can specify string literals such as 'president':

  • Java

// Officer's Map

// evaluates to Inventor("Pupin")
Inventor pupin = parser.parseExpression("officers['president']")
    .getValue(societyContext, Inventor.class);

// evaluates to "Idvor"
String city = parser.parseExpression("officers['president'].placeOfBirth.city")
    .getValue(societyContext, String.class);

String countryExpression = "officers['advisors'][0].placeOfBirth.country";

// setting values
parser.parseExpression(countryExpression)
    .setValue(societyContext, "Croatia");

// evaluates to "Croatia"
String country = parser.parseExpression(countryExpression)
    .getValue(societyContext, String.class);

Indexing into Objects

A property of an object can be obtained by specifying the name of the property within square brackets. This is analogous to accessing the value of a map based on its key. The following example demonstrates how to index into an object to retrieve a specific property.

  • Java

// Create an inventor to use as the root context object.
Inventor tesla = new Inventor("Nikola Tesla");

// evaluates to "Nikola Tesla"
String name = parser.parseExpression("#root['name']")
    .getValue(context, tesla, String.class);