Java Community | Help. Code. Learn.•15mo ago

Week 40 — What is instanceof patternmatching and how can it be used?

Question of the Week #40

What is instanceof patternmatching and how can it be used?

2 Replies

Java's instanceof operator allows checking whether an object can be cast to a certain type:

Object stringAsObject = "Hello World";
System.out.println(stringAsObject instanceof String);//true
System.out.println(stringAsObject instanceof LocalDate);//false

Object stringAsObject = "Hello World";
System.out.println(stringAsObject instanceof String);//true
System.out.println(stringAsObject instanceof LocalDate);//false

This is often used together with a cast. After checking the type of the object, one can cast the object to the type:

Object o = getSomeObject();
if (o instanceof String){
  //here, we know that o is a String so we can cast o to a String
  String s = (String) o;
  System.out.println("We have a String of length "+s.length());
}

Object o = getSomeObject();
if (o instanceof String){
  //here, we know that o is a String so we can cast o to a String
  String s = (String) o;
  System.out.println("We have a String of length "+s.length());
}

Since Java 16, the above code can be simplified to do the instanceof check and the cast in one step:

Object o = getSomeObject();
if (o instanceof String s){//checks whether o is a String, casts it to String and assigns it to a new String variable named s
  System.out.println("We have a String of length "+s.length());
}
//System.out.println("We have a String of length "+s.length());//compile-time error since variable is out of scope

Object o = getSomeObject();
if (o instanceof String s){//checks whether o is a String, casts it to String and assigns it to a new String variable named s
  System.out.println("We have a String of length "+s.length());
}
//System.out.println("We have a String of length "+s.length());//compile-time error since variable is out of scope

📖 Sample answer from dan1st

Eric McIntyre•15mo ago

To make a parse without need to do it in two steps, and avoid need to error handling. If (brazilian instanceof Person person) { System.out.println("Olá pessoal by "+ person.gerName()); }

Submission from fabriciogscarvalho

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Week 40 — What is instanceof patternmatching and how can it be used?