Java runtime object type

How to determine Type of object at runtime in Java — Runtime Type Identification Example

Runtime Type identification in Java
Determining the Type of object at runtime in Java means finding what kind of object it is. For those who are not familiar with What is a Type in Java, Type is the name of class e..g for “abc” which is a String object, Type is String. Finding the Type of any object at runtime is also known as Runtime Type Identification in Java. Determining Type becomes increasingly important for a method that accepts parameters of type java.lang.An object like compareTo method of the Comparable class. Since two objects of different types can not be equal to each other if we know how to determine the type of object from the object itself then we can, not only avoid ClassCastExcpetion but also optimized the equals method. Java provides three different ways to find the type of an object at runtime like instanceof keyword, getClass(), and isInstance() method of java.lang.Class.

Out of all three only getClass() is the one that exactly finds the Type of object while others also return true if the Type of object is the super type.

As we all know that Java throws ClassCastException if you try to cast an object into the wrong type, which makes finding the type of Object at runtime even more important for robust Java programs, Though typecasting can be minimized by using Generics in Java you still have some places where you need to cast object.

In this Java tutorial, we will see examples of three ways of determining the type of Object from the Java program itself. On the same note Java programming language does not support RTTI(Runtime type Identification) which was C++ capability to find Object type at runtime but as I said Java has its own way of facilitating this.

Java program to determine Type of object at runtime

Here is a test Java program that demonstrates Runtime type identification in Java or in simple words how to find a Type of object . It also examples that how instanceof , getClass() and isInstance() method works and how can we use them to determine Java object type at runtime.

In this Java program, we have three classes; Rule , SystemRule, and BusinessRule . Both SystemRule and BusinessRule are sub-class of Rule . We will create instances of all three classes and then try to find the correct Type for those instances.

The reason we are using both the Superclass and Subclass in this test is because of both instanceof and the isInstance() method of java.lang.Class returns true if the object is of sub-class and we are testing against superclass. Let’s see the program now :

/**
* Java program to determine type of Object at runtime in Java.
* you can identify type of any object by three ways i..e by using instanceof,
* getClass() and isInstance() method of java.lang.Class.
* Java does have capability to find out type of object but its not called
* as RTTI (Runtime type Identification) in C++.
*
* @author Javarevisited
*/

public class RuntimeTypeIdentificationTest

public static void main ( String args []) <
//creating instance of sub class and storing into type of superclass
Rule simpleRule = new BusinessRule () ;

//determining type of object in Java using instanceof keyword
System. out . println ( «Checking type of object in Java using instanceof ==>» ) ;
if ( simpleRule instanceof Rule ) <
System. out . println ( «System rule is instance of Rule» ) ;
>
if ( simpleRule instanceof SystemRule ) <
System. out . println ( «System rule is instance of SystemRule» ) ;
>
if ( simpleRule instanceof BusinessRule ) <
System. out . println ( «System rule is instance of BusinessRule» ) ;
>

//determining type of object in Java using getClass() method
System. out . println ( «Checking type of object in Java using getClass() ==>» ) ;
if ( simpleRule. getClass () == Rule. class ) <
System. out . println ( «System rule is instance of Rule» ) ;
>
if ( simpleRule. getClass () == SystemRule. class ) <
System. out . println ( «System rule is instance of SystemRule» ) ;
>
if ( simpleRule. getClass () == BusinessRule. class ) <
System. out . println ( «System rule is instance of BusinessRule» ) ;
>

//determining type of object in Java using isInstance() method
//isInstance() is similar to instanceof operator and returns true even
//if object belongs to sub class.
System. out . println ( «Checking type of object in Java using isInstance() ==>» ) ;
if ( Rule. class . isInstance ( simpleRule )) <
System. out . println ( «SystemRule is instance of Rule» ) ;
>
if ( SystemRule. class . isInstance ( simpleRule )) <
System. out . println ( «SystemRule is instance of SystemRule» ) ;
>
if ( BusinessRule. class . isInstance ( simpleRule )) <
System. out . println ( «SystemRule is instance of BusinessRule» ) ;
>
>

class Rule <
public void process () <
System. out . println ( «process method of Rule» ) ;
>
>

class SystemRule extends Rule

@Override
public void process () <
System. out . println ( «process method of SystemRule class» ) ;
>
>

class BusinessRule extends Rule

@Override
public void process () <
System. out . println ( «process method of Business Rule class» ) ;
>
>

Output:
Checking type of object in Java using instanceof == >
SystemRule is instance of Rule
SystemRule is instance of BusinessRule

Checking type of object in Java using isInstance () == >
SystemRule is instance of Rule
SystemRule is instance of BusinessRule

If you look at the output you will find that both instanceof keyword and isInstance() also consider sub type object as of Super Type. Only getClass() method returns strict type identification result and does not consider sub class object as of Super class. That’s one of the reason programmer prefer to use getClass() over instanceof while overriding equals method in Java.

Important points to remember about Runtime Type Identification in Java

Few points which is worth remembering while determining Type or Class of object from Java program during runtime:

1) Always determine Type while writing methods that accept Object, which not only reduces error but also results in a robust program. You can also use Generics feature to write a parameterized method which is better than the method which accepts raw types.

2) Type identification is also useful before type casting any object into another Type to avoid ClassCastException .

3) Another use of Runtime Type identification is to implement type-specific features on methods which accepts general Type like Object or any interface.

That’s it on Runtime type identification or determining Type of object at runtime in Java program. we have seen a couple of ways to do this like instanceof operator, getClass() and finally isInstance() method of java.lang.Class. If you look at the output closely you might have figured out that except getClass() other two ways of finding Type of object also return true if the object is of Supertype and that’s why getClass() is the preferred way and I always use it while overriding equals() and hashCode() methods.

Remember Java does not support Runtime Type Identification (RTTI) as supported in C++ but does provide a few API methods to find objects at runtime.

Источник

Получение generic-типа в runtime

В Java 5 появились generic-типы, а вместе с ним и концепция type erasure, которая буквально означает стирание информации о generic-типе после компиляции. Действительно, во многих случаях это просто синтаксический сахар, помогающий писать типо-безопасный код на уровне компиляции, и в runtime с такими типами работать нельзя. Например, невозможно получить тип T внутри ArrayList , поэтому он в своей реализации создает массив Object[], а не T[] для хранения элементов.

import org.springframework.beans.factory.annotation.Autowired; import org.springframework.beans.factory.config.BeanPostProcessor; . @Autowired private Set beanPostProcessors; 

import com.fasterxml.jackson.core.type.TypeReference; import com.fasterxml.jackson.databind.ObjectMapper; . List strings = new ObjectMapper() .readValue("[1, 2, 3]", new TypeReference<>() <>); // все элементы strings - строки (не Integer и не Long) List ints = new ObjectMapper() .readValue("[1, 2, 3]", new TypeReference<>() <>); // все элементы ints - Integer (не String и не Long) 

List list = new ArrayList<>(); list.add("value"); String value = list.get(0); 

List list = new ArrayList(); list.add("value"); String value = (String) list.get(0); 

import java.lang.reflect.Field; public class ListType < private Listlist; public static void main(String[] args) throws NoSuchFieldException < Field field = ListType.class.getDeclaredField("list"); Class fieldType = field.getType(); // "interface java.util.List" System.out.println(fieldType); >> 

import java.lang.reflect.Type; import java.lang.reflect.ParameterizedType; . Type fieldGenericType = field.getGenericType(); // "class sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl" System.out.println(fieldGenericType.getClass()); // "java.util.List" System.out.println(fieldGenericType); if (fieldGenericType instanceof ParameterizedType) < Type[] typeArguments = ((ParameterizedType) fieldGenericType).getActualTypeArguments(); // "[class java.lang.Integer]" System.out.println(Arrays.toString(typeArguments)); >> > 

import java.lang.reflect.Type; import java.lang.reflect.ParameterizedType; . private List> nestedList; . Field field = ListType.class.getDeclaredField("nestedList"); Type fieldGenericType = field.getGenericType(); // "java.util.List>" System.out.println(fieldGenericType); if (fieldGenericType instanceof ParameterizedType) < Type[] typeArguments = ((ParameterizedType) fieldGenericType).getActualTypeArguments(); // "[java.util.Set]" System.out.println(Arrays.toString(typeArguments)); > > 

List ints = new ObjectMapper() .readValue("[1, 2, 3]", new TypeReference<>() <>); 

List ints = new ObjectMapper() .readValue("[1, 2, 3]", new TypeReference() <>); 

class ListType$1 extends TypeReference>

List ints = new ObjectMapper() .readValue("[1, 2, 3]", new ListType$1()); 

import com.fasterxml.jackson.databind.ObjectMapper fun main(args: Array) < val strings: List= parseJson("[1, 2, 3]") println(strings) // strings-список строк (но не чисел) > inline fun parseJson(str: String): T

import com.google.common.reflect.TypeToken; . TypeToken modelType = TypeToken.of(BeanPostProcessorComposite.class); Type actualType = modelType.resolveType( AbstractComposite.class.getDeclaredField("autowiredBeans").getGenericType() ).getType(); // "java.util.Set" System.out.println(actualType); 

import org.springframework.core.GenericTypeResolver; . Type actualType = GenericTypeResolver.resolveType( AbstractComposite.class.getDeclaredField("autowiredBeans").getGenericType(), BeanPostProcessorComposite.class ); // "java.util.Set" System.out.println(actualType);