What is the instanceof operator used for in Java

Question

What is the instanceof operator used for  I ve seen stuff like  if  source instanceof Button                else                 But none of it made sense to me  I ve done my research  but came up only with examples without any explanations

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Very simple code example   If  object1 instanceof Class1          do something   else if  object1 instanceof Class2          do something different     Be careful here   In the example above  if Class1 is Object  the first comparison will always be true   So  just like with exceptions  hierarchical order matters

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This operator allows you to determine the type of an object  It returns a boolean value   For example  package test   import java util Date  import java util Map  import java util HashMap   public class instanceoftest       public static void main String args                  Map m new HashMap            System out println  Returns a boolean value    m instanceof Map            System out println  Returns a boolean value    m instanceof HashMap            System out println  Returns a boolean value    m instanceof Object            System out println  Returns a boolean value    m instanceof Date               the output is   Returns a boolean value true Returns a boolean value true Returns a boolean value true Returns a boolean value false

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If source is an object variable  instanceof is a way of checking to see if it is a Button or not

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The java instanceof operator is used to test whether the object is an instance of the specified type  class or subclass or interface    The instanceof in java is also known as type comparison operator as it compares the instance with type  It returns either true or false  If we apply the instanceof operator with any variable that has null value  it returns false   From JDK 14  which includes JEP 305 we can also do  Pattern Matching  for instanceof  Patterns basically test that a value has a certain type  and can extract information from the value when it has the matching type   Pattern matching allows a more clear and efficient expression of common logic in a system  namely the conditional removal of components from objects   Before Java 14  if  obj instanceof String        String str    String  obj     need to declare and cast again the object        str contains         else       str            Java 14 enhancements  if    obj instanceof String str            str contains           no need to declare str object again with casting   else          str         We can also combine the type check and other conditions together  if  obj instanceof String str  amp  amp  str length    gt  4      str contains           The use of pattern matching in instanceof should reduce the overall number of explicit casts in Java programs   PS  instanceOf will only match when the object is not null  then only it can be assigned to str

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The instanceof operator compares an object to a specified type  You can use it to test if an object is an instance of a class  an instance of a subclass  or an instance of a class that implements a particular interface     http   download oracle com javase tutorial java nutsandbolts op2 html

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public class Animal  float age     public class Lion extends Animal   int claws    public class Jungle       public static void main String args               Animal animal   new Animal             Animal animal2   new Lion             Lion lion   new Lion             Animal animal3   new Animal             Lion lion2   new Animal        won t compile  can t reference super class object with sub class reference variable            if animal instanceof Lion     false          if animal2 instanceof Lion     true          if lion insanceof Lion    true          if animal3 instanceof Animal    true

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As described on this site      The instanceof operator can be used to test if an object is of a   specific type     if  objectReference instanceof type        A quick example   String s    Hello World   return s instanceof String    result -- gt  true       However  applying instanceof on a null reference variable expression   returns false   String s   null  return s instanceof String    result -- gt  false       Since a subclass is a  type  of its superclass  you can use the   instanceof to verify this     class Parent       public Parent         class Child extends Parent       public Child             super             public class Main       public static void main String   args            Child child   new Child            System out println  child instanceof Parent              result -- gt  true    I hope this helps

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The instanceof operator is used to check whether the object is an instance of the specified type   class or subclass or interface    The instanceof is also known as type comparison operator because it compares the instance with type  It returns either true or false   class Simple1     public static void main String args        Simple1 s new Simple1      System out println s instanceof Simple1     true             If we apply the instanceof operator with any variable that has null value  it returns false

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instanceof keyword is a binary operator used to test if an object  instance  is a subtype of a given Type   Imagine   interface Domestic    class Animal    class Dog extends Animal implements Domestic    class Cat extends Animal implements Domestic      Imagine a dog object  created with Object dog   new Dog    then   dog instanceof Domestic    true - Dog implements Domestic dog instanceof Animal      true - Dog extends Animal dog instanceof Dog         true - Dog is Dog dog instanceof Object      true - Object is the parent type of all objects   However  with Object animal   new Animal      animal instanceof Dog    false   because Animal is a supertype of Dog and possibly less  refined    And   dog instanceof Cat    does not even compile    This is because Dog is neither a subtype nor a supertype of Cat  and it also does not implement it   Note that the variable used for dog above is of type Object  This is to show instanceof is a runtime operation and brings us to a the use case  to react differently based upon an objects type at runtime   Things to note  expressionThatIsNull instanceof T is false for all Types T   Happy coding

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Can be used as a shorthand in equality check  So this code if ob    null  amp  amp  this getClass      ob getClass       can be written as if ob instanceOf ClassA

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You could use Map to make higher abstraction on instance of  private final Map lt Class  Consumer lt String gt  gt  actions   new HashMap lt  gt       Then having such map add some action to it   actions put String class  new Consumer lt String gt               Override         public void accept String s               System out println  action for String                              Then having an Object of not known type you could get specific action from that map   actions get someObject  accept someObject

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class Test48  public static void main  String args     Object Obj new Hello      Hello obj new Hello  System out println Obj instanceof String   System out println Obj instanceof Hello   System out println Obj instanceof Object   Hello h null  System out println h instanceof Hello   System out println h instanceof Object

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Most people have correctly explained the  What  of this question but no one explained  How  correctly   So here s a simple illustration   String s   new String  Hello    if  s instanceof String  System out println  s is instance of String       True if  s instanceof Object  System out println  s is instance of Object       True   if  s instanceof StringBuffer  System out println  s is instance of StringBuffer       Compile error Object o    Object s  if  o instanceof StringBuffer  System out println  o is instance of StringBuffer      No error  returns False else System out println  Not an instance of StringBuffer        if  o instanceof String  System out println  o is instance of String      True   Outputs   s is instance of String s is instance of Object Not an instance of StringBuffer o is instance of String   The reason for compiler error when comparing s with StringBuffer is well explained in docs      You can use it to test if an object is an instance of a class  an instance of a subclass  or an instance of a class that implements a particular interface    which implies the LHS must either be an instance of RHS or of a Class that either implements RHS or extends RHS   How to use use instanceof then  Since every Class extends Object  type-casting LHS to object will always work in your favour   String s   new String  Hello    if   Object s instanceof StringBuffer  System out println  Instance of StringBuffer      No compiler error now    else System out println  Not an instance of StringBuffer      Outputs   Not an instance of StringBuffer

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Instance of keyword is helpful when you want to know particular object s instance    Suppose you are throw exception and when you have catch then perform sum custom  operation and then again continue as per your logic  throws or log etc   Example   1  User created custom exception  InvalidExtensionsException  and throw it as per logic  2  Now in catch block  catch  Exception e       perform sum logic if exception type is  InvalidExtensionsException   InvalidExtensionsException InvalidException   InvalidExtensionsException e    3  If you are not checking instance of and exception type is Null pointer exception your code will break   So your logic should be inside of instance of      if  e instanceof InvalidExtensionsException         InvalidExtensionsException InvalidException   InvalidExtensionsException e          Above example is wrong coding practice However this example is help you to understand use of instance of it

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Best explanation is jls  Always try to check what source says  There you will get the best answer plus much more  Reproducing some parts here      The type of the RelationalExpression operand of the instanceof   operator must be a reference type or the null type  otherwise  a   compile-time error occurs       It is a compile-time error if the ReferenceType mentioned after the   instanceof operator does not denote a reference type that is reifiable      4 7        If a cast    15 16  of the RelationalExpression to the ReferenceType   would be rejected as a compile-time error  then the instanceof   relational expression likewise produces a compile-time error  In such   a situation  the result of the instanceof expression could never be   true

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It s an operator that returns true if the left side of the expression is an instance of the class name on the right side   Think about it this way  Say all the houses on your block were built from the same blueprints  Ten houses  objects   one set of blueprints  class definition    instanceof is a useful tool when you ve got a collection of objects and you re not sure what they are  Let s say you ve got a collection of controls on a form  You want to read the checked state of whatever checkboxes are there  but you can t ask a plain old object for its checked state  Instead  you d see if each object is a checkbox  and if it is  cast it to a checkbox and check its properties   if  obj instanceof Checkbox        Checkbox cb    Checkbox obj      boolean state   cb getState

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As mentioned in other answers  the canonical typical usage of instanceof is for checking if an identifier is referring to a more specific type   Example   Object someobject       some code which gets something that might be a button     if  someobject instanceof Button           then if someobject is in fact a button this block gets executed   else          otherwise execute this block     Note however  that the type of the left-hand expression must be a parent type of the right hand expression  see JLS 15 20 2 and Java Puzzlers   50  pp114    For example  the following will fail to compile   public class Test       public static void main String    args            System out println new Test   instanceof String      will fail to compile           This fails to compile with the message   Test java 6  error  inconvertible types         System out println t instanceof String                              required  String   found     Test 1 error   As Test is not a parent class of String   OTOH  this compiles perfectly and prints false as expected   public class Test       public static void main String    args            Object t   new Test               compiles fine since Object is a parent class to String         System out println t instanceof String

[java] What is the 'instanceof' operator used for in Java?

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