Pattern Matching with instanceof using strictly subtype

[Gouri Kalanidhi]

Hi there,

This is in reference to the topic Pattern Matching: Subtypes (OCP Java 17 Study guide by Scott and Jeanne: Chapter 3, page 108)

The type of the pattern variable must be a subtype of the variable on the left side of the expression. It also cannot be the same type.

Integer value = 123;
if(value instanceof Integer) {}
if(value instanceof Integer data) {} // DOES NOT COMPILE

While the second line compiles,  the last line does not compile because pattern matching requires that the pattern variable type Integer be a strict subtype of Integer.

When I try it with my own code, I don't see any error. It just works fine with the output as below.

public void checkSubtype(Integer i1){
		Integer i4 = 125;
		if(i1 instanceof Integer i2) {
			System.out.println("Integers again");
		}
		if(i4 instanceof Integer i2){
			System.out.println("both integers");
		}
	}

The output is:
Integers again
both integers

Am I missing something?

Thanks!

[Mike Simmons]

What version of Java are you using?  There were several "preview" versions of this feature, in Java 14 and Java 15.  But the final version of this feature was in Java 16, and in that version, they added the part about it being a compile error to do an instanceof check using the same type as the variable type - or a supertype of that, for that matter.  So you also can't do

if (value instanceof Object data) {}

I would note that there's really no point to using Java 14 or 15 at this point - you might as well move to Java 17 at least, since that's an LTS version (Long-Term Support).  And, any time you are using a preview version of a feature (and therefore using the --enable-preview flag while running and compiling) you should remember that those preview features you are experiencing may change their behavior in future releases.

[Stephan van Hulst]

In general, you should always use a JDK version that matches the exam you're studying for.

[Gouri Kalanidhi]

I'm using Java 17 version as I'm preparing for OCP 17 certification. Here's the exact version that eclipse IDE is using, extracted from eclipse installation details:

*** Date: Thursday, April 20, 2023 at 11:57:02 AM Central Daylight Time

*** Platform Details:

*** System properties:
java.runtime.name=Java(TM) SE Runtime Environment
java.runtime.version=17.0.6+9-LTS-190

*** System environment variables:

*** Features:

*** Plug-in Registry:

*** User Preferences:

*** Current Install Configuration:

*** Security Configuration:

[Gouri Kalanidhi]

Also, with the super type added to the original code, I'm not seeing any errors

public void checkSubtype(Integer i1) {
		Integer i4 = 125;
		if (i1 instanceof Integer i2) {
			System.out.println("Integers again");
		}
		if (i4 instanceof Integer i2) {
			System.out.println("both integers");
		}
		if (i4 instanceof Object o1) {
			System.out.println("this is an object");
		}
	}

This prints

Integers again
both integers
this is an object

[Mike Simmons]

Well, that's very interesting.  I definitely see different behavior here... either there are differences in our specific JDK versions, or one of us is not running the version we think we are.  I'm not using eclipse, rather IntelliJ - but let's take the IDE out of the equation entirely for a moment and use the command line.  I'm using this file, InstanceofPatternMatching.java:

public class InstanceofPatternMatching {
  
  public static void main(String[] args) {
    System.out.println(System.getProperty("java.version"));
    System.out.println(System.getProperty("java.vendor"));
    
    new InstanceofPatternMatching().checkSubtype(42);
  }
  public void checkSubtype(Integer i1) {
    Integer i4 = 125;
    if (i1 instanceof Integer i2) {
      System.out.println("Integers again");
    }
    if (i4 instanceof Integer i2) {
      System.out.println("both integers");
    }
    if (i4 instanceof Object o1) {
      System.out.println("this is an object");
    }
  }
}

When I compile from the command line:

% javac --version
javac 17.0.6
% javac InstanceofPatternMatching.java
InstanceofPatternMatching.java:11: error: expression type Integer is a subtype of pattern type Integer
    if (i1 instanceof Integer i2) {
           ^
InstanceofPatternMatching.java:14: error: expression type Integer is a subtype of pattern type Integer
    if (i4 instanceof Integer i2) {
           ^
InstanceofPatternMatching.java:17: error: expression type Integer is a subtype of pattern type Object
    if (i4 instanceof Object o1) {
           ^
3 errors
%

Can you compile from the command line as well?  What does "javac --version" show?

[Simon Roberts]

I concur:

simon@Ryzen:/tmp$ java -version
openjdk version "17.0.1" 2021-10-19
OpenJDK Runtime Environment Temurin-17.0.1+12 (build 17.0.1+12)
OpenJDK 64-Bit Server VM Temurin-17.0.1+12 (build 17.0.1+12, mixed mode, sharing)
simon@Ryzen:/tmp$ javac -version
javac 17.0.1
simon@Ryzen:/tmp$ cat pkg/ST.java 
package pkg;

public class ST {
  public static void main(String[] args) {
    Integer i = 1;
    if (i instanceof Integer i1) {
      System.out.println("it's integer: " + i1);
    } else {
      System.out.println("it's not");
    }
  }
}
simon@Ryzen:/tmp$ javac -d classes pkg/ST.java 
pkg/ST.java:6: error: expression type Integer is a subtype of pattern type Integer
    if (i instanceof Integer i1) {
          ^
1 error
simon@Ryzen:/tmp$

[Soner Akar]

Simon Roberts wrote:I concur:

simon@Ryzen:/tmp$ java -version
openjdk version "17.0.1" 2021-10-19
OpenJDK Runtime Environment Temurin-17.0.1+12 (build 17.0.1+12)
OpenJDK 64-Bit Server VM Temurin-17.0.1+12 (build 17.0.1+12, mixed mode, sharing)
simon@Ryzen:/tmp$ javac -version
javac 17.0.1
simon@Ryzen:/tmp$ cat pkg/ST.java 
package pkg;

public class ST {
  public static void main(String[] args) {
    Integer i = 1;
    if (i instanceof Integer i1) {
      System.out.println("it's integer: " + i1);
    } else {
      System.out.println("it's not");
    }
  }
}
simon@Ryzen:/tmp$ javac -d classes pkg/ST.java 
pkg/ST.java:6: error: expression type Integer is a subtype of pattern type Integer
    if (i instanceof Integer i1) {
          ^
1 error
simon@Ryzen:/tmp$

You are using the buggy version. Kindly consider https://github.com/openjdk/jdk/commit/739bbd03

[Mike Simmons]

It looks like they improved the error message, fixing the identification of which is the "expression type" and which is the "pattern type" - though that doesn't really come up in this example unless you include something like "instanceof Object".  Regardless, even without that bugfix the code still errors out, just with a potentially-misleading message.

[Gouri Kalanidhi]

Okay, so I'm getting compiler errors when I run the same file from command prompt.

\eclipse-workspace\oracle_certified_professional_17\src\ocp_java_17>javac PatternMatching.java
PatternMatching.java:18: error: expression type Integer is a subtype of pattern type Integer
               if (i1 instanceof Integer i2) {
                      ^
PatternMatching.java:21: error: expression type Integer is a subtype of pattern type Integer
               if (i4 instanceof Integer i2) {
                      ^
PatternMatching.java:24: error: expression type Integer is a subtype of pattern type Object
               if (i4 instanceof Object o1) {
                      ^
3 errors

System.out.println(System.getProperty("java.version"));
System.out.println(System.getProperty("java.vendor"));

These 2 getProperty() return 17.0.6 and Oracle Corporation respectively, on eclipse.

So, why is eclipse not throwing errors is still not clear. How should I figure if something is really a compiler error and eclipse does not warn?

[Paul Clapham]

Gouri Kalanidhi wrote:I'm using Java 17 version as I'm preparing for OCP 17 certification. Here's the exact version that eclipse IDE is using, extracted from eclipse installation details:

But the JRE which Eclipse uses is not necessarily the same as the JRE which you have configured into the project's build path.

[Paul Anilprem]

IDEs are meant to make development easier and they go quite beyond plain javac compiler in reporting errors, hints, and suggestions. IDEs differentiate themselves on this basis.
The certification exam, otoh, is strictly about javac. If you try match the messages from IDEs with javac, you will spend more time than is necessary to get to the point as discovered by so many other candidates: https://enthuware.com/oca-ocp-java-certification-resources/264-preparing-java-certification-ide

[Piet Souris]

Just gave it a try with java 17 and NetBeans 12.5. NetBeans reports that subtype error in my code, and when running I get a warning that the code was compiled with errors, but nevertheless it runs fine. I have the Oracle java version.

[Jeanne Boyarsky]

This gives three compiler errors with Java 17

public class MyClass {
    public static void main(String args[]) {
      Integer value = 123;
      if(value instanceof Integer) {}
      if(value instanceof Integer data) {} // DOES NOT COMPILE
    }
    
    public void checkSubtype(Integer i1){
        Integer i4 = 125;
        if(i1 instanceof Integer i2) {
            System.out.println("Integers again");
        }
        if(i4 instanceof Integer i2){
            System.out.println("both integers");
        }
    }
}

which are

/MyClass.java:5: error: expression type Integer is a subtype of pattern type Integer
      if(value instanceof Integer data) {} // DOES NOT COMPILE
               ^
/MyClass.java:10: error: expression type Integer is a subtype of pattern type Integer
        if(i1 instanceof Integer i2) {
              ^
/MyClass.java:13: error: expression type Integer is a subtype of pattern type Integer
        if(i4 instanceof Integer i2){
              ^
3 errors