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does 'java' command compile the java program?

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14

1

Any website in Internet says: we should use 'javac; to compile a .java file and then run it by 'java' command. but today i tried to run java program without 'javac' and i got a strange result.

my file name is hello.java
and it includes below lines:

public class Myclass {

 public  static void main(String[] args){

    System.out.println("hello world");

  }

}

when i execute below command:

$ javac hello.java

it gives me below error which is reasonable:

$ hello.java:1: error: class Myclass is public, should be declared in a file named Myclass.java

public class Myclass {

       ^

1 error

but then i run it without 'javac' and it executed without any error!!!

$ java hello.java

$ hello world

Now my question is: does 'java' command compile the program too? and if the answer is yes why we need the 'javac' command?

---

Edit:

---

the version of my java is:

$ java -version

openjdk version "12.0.2" 2019-07-16

OpenJDK Runtime Environment (build 12.0.2+10)

OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)

so according to answers it is allowed in this version to run one source file using 'java' command.
thank youuuuu all:)

java javac

share|improve this question

edited 8 hours ago

milad

asked 8 hours ago

miladmilad

12388 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Which version are you using? I think they introduced Java Console in Java 9, and that might be what you experienced.
  
  – Matthieu
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You need to match the class name with its filename - that's the Java standard. Just change the file name to Myclass.java and then from the command line compile it like this javac Myclass.java and then run it like this java Myclass.
  
  – unnsse
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  yes, javac still used to compile if you don't want to deploy source code, or you have more than a single file (documentation of java for source-file option: Only used to launch a single source-file program.)
  
  – Carlos Heuberger
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Matthieu the output of "java -version" is: openjdk version "12.0.2" 2019-07-16 OpenJDK Runtime Environment (build 12.0.2+10) OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)
  
  – milad
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @unnsse thanks for your attention but I know how to fix it, please read the last part of my question
  
  – milad
  8 hours ago
  

  
  
  
  

 | 
show 2 more comments

14

1

Any website in Internet says: we should use 'javac; to compile a .java file and then run it by 'java' command. but today i tried to run java program without 'javac' and i got a strange result.

my file name is hello.java
and it includes below lines:

public class Myclass {

 public  static void main(String[] args){

    System.out.println("hello world");

  }

}

when i execute below command:

$ javac hello.java

it gives me below error which is reasonable:

$ hello.java:1: error: class Myclass is public, should be declared in a file named Myclass.java

public class Myclass {

       ^

1 error

but then i run it without 'javac' and it executed without any error!!!

$ java hello.java

$ hello world

Now my question is: does 'java' command compile the program too? and if the answer is yes why we need the 'javac' command?

---

Edit:

---

the version of my java is:

$ java -version

openjdk version "12.0.2" 2019-07-16

OpenJDK Runtime Environment (build 12.0.2+10)

OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)

so according to answers it is allowed in this version to run one source file using 'java' command.
thank youuuuu all:)

java javac

share|improve this question

edited 8 hours ago

milad

asked 8 hours ago

miladmilad

12388 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Which version are you using? I think they introduced Java Console in Java 9, and that might be what you experienced.
  
  – Matthieu
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You need to match the class name with its filename - that's the Java standard. Just change the file name to Myclass.java and then from the command line compile it like this javac Myclass.java and then run it like this java Myclass.
  
  – unnsse
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  yes, javac still used to compile if you don't want to deploy source code, or you have more than a single file (documentation of java for source-file option: Only used to launch a single source-file program.)
  
  – Carlos Heuberger
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Matthieu the output of "java -version" is: openjdk version "12.0.2" 2019-07-16 OpenJDK Runtime Environment (build 12.0.2+10) OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)
  
  – milad
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @unnsse thanks for your attention but I know how to fix it, please read the last part of my question
  
  – milad
  8 hours ago
  

  
  
  
  

 | 
show 2 more comments

Any website in Internet says: we should use 'javac; to compile a .java file and then run it by 'java' command. but today i tried to run java program without 'javac' and i got a strange result.

my file name is hello.java
and it includes below lines:

public class Myclass {

 public  static void main(String[] args){

    System.out.println("hello world");

  }

}

when i execute below command:

$ javac hello.java

it gives me below error which is reasonable:

$ hello.java:1: error: class Myclass is public, should be declared in a file named Myclass.java

public class Myclass {

       ^

1 error

but then i run it without 'javac' and it executed without any error!!!

$ java hello.java

$ hello world

Now my question is: does 'java' command compile the program too? and if the answer is yes why we need the 'javac' command?

---

Edit:

---

the version of my java is:

$ java -version

openjdk version "12.0.2" 2019-07-16

OpenJDK Runtime Environment (build 12.0.2+10)

OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)

so according to answers it is allowed in this version to run one source file using 'java' command.
thank youuuuu all:)

java javac

share|improve this question

edited 8 hours ago

milad

asked 8 hours ago

miladmilad

12388 bronze badges

public class Myclass {

 public  static void main(String[] args){

    System.out.println("hello world");

  }

}

$ javac hello.java

$ hello.java:1: error: class Myclass is public, should be declared in a file named Myclass.java

public class Myclass {

       ^

1 error

$ java hello.java

$ hello world

$ java -version

openjdk version "12.0.2" 2019-07-16

OpenJDK Runtime Environment (build 12.0.2+10)

OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)

share|improve this question

edited 8 hours ago

milad

asked 8 hours ago

miladmilad

12388 bronze badges

share|improve this question

edited 8 hours ago

milad

asked 8 hours ago

miladmilad

12388 bronze badges

asked 8 hours ago

miladmilad

12388 bronze badges

asked 8 hours ago

miladmilad

12388 bronze badges

2 Answers
2

active

oldest

votes

12

Prior to Java 11, to run your code you have to first compile it, then you can run it. Here's an example:

javac test.java

java test

Since Java 11, you can still do javac + java, or you can run java by itself to compile and auto-run your code. Note that no .class file will be generated. Here's an example:

java test.java

If you run java -help, you'll see the various allowed usages. Here's what it looks like on my machine. The last one is what you ran into: java [options] <sourcefile> [args] which will "execute a single source-file program".

$ java -help

Usage: java [options] <mainclass> [args...]

           (to execute a class)

   or  java [options] -jar <jarfile> [args...]

           (to execute a jar file)

   or  java [options] -m <module>[/<mainclass>] [args...]

       java [options] --module <module>[/<mainclass>] [args...]

           (to execute the main class in a module)

   or  java [options] <sourcefile> [args]

           (to execute a single source-file program)

share|improve this answer

edited 8 hours ago

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  introduced in Java 11: What's New or/and JEP 330: Launch Single-File Source-Code Programs
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks @CarlosHeuberger for the additional details. I made a small edit in my answer to reflect that it was introduced in Java 11.
  
  – kaan
  8 hours ago
  

  
  
  
  

add a comment
 | 

5

If you are running Java 11, there is a new feature called Java scripting that allows single source file execution. The single source compiler is more promiscuous in terms of class name versus file name, so that is how you are able to run but not succesfully compile.
https://docs.oracle.com/en/java/javase/11/scripting/java-scripting-api.html#GUID-C4A6EB7C-0AEA-45EC-8662-099BDEFC361A

If you are on a previous version of Java, then your current hello.java does not compile, because of compile errors, specifically around the class name. So there's absolutely no way that calling java hello.java compiled your code, because it does not compile.

It seems most entirely likely that you were running some previously compiled code when executing the java command.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thank you, java version is: openjdk version "12.0.2" 2019-07-16 OpenJDK Runtime Environment (build 12.0.2+10) OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)
  
  – milad
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  check Using Source-File Mode to Launch Single-File Source-Code Programs: "The compiler does not enforce the optional restriction defined at the end of JLS ??7.6, that a type in a named package should exist in a file whose name is composed from the type name followed by the .java extension."
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the link Carlos, updated my answer accordingly.
  
  – Evan
  8 hours ago
  

  
  
  
  

add a comment
 | 

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12

Prior to Java 11, to run your code you have to first compile it, then you can run it. Here's an example:

javac test.java

java test

Since Java 11, you can still do javac + java, or you can run java by itself to compile and auto-run your code. Note that no .class file will be generated. Here's an example:

java test.java

If you run java -help, you'll see the various allowed usages. Here's what it looks like on my machine. The last one is what you ran into: java [options] <sourcefile> [args] which will "execute a single source-file program".

$ java -help

Usage: java [options] <mainclass> [args...]

           (to execute a class)

   or  java [options] -jar <jarfile> [args...]

           (to execute a jar file)

   or  java [options] -m <module>[/<mainclass>] [args...]

       java [options] --module <module>[/<mainclass>] [args...]

           (to execute the main class in a module)

   or  java [options] <sourcefile> [args]

           (to execute a single source-file program)

share|improve this answer

edited 8 hours ago

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  introduced in Java 11: What's New or/and JEP 330: Launch Single-File Source-Code Programs
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks @CarlosHeuberger for the additional details. I made a small edit in my answer to reflect that it was introduced in Java 11.
  
  – kaan
  8 hours ago
  

  
  
  
  

add a comment
 | 

12

Prior to Java 11, to run your code you have to first compile it, then you can run it. Here's an example:

javac test.java

java test

Since Java 11, you can still do javac + java, or you can run java by itself to compile and auto-run your code. Note that no .class file will be generated. Here's an example:

java test.java

If you run java -help, you'll see the various allowed usages. Here's what it looks like on my machine. The last one is what you ran into: java [options] <sourcefile> [args] which will "execute a single source-file program".

$ java -help

Usage: java [options] <mainclass> [args...]

           (to execute a class)

   or  java [options] -jar <jarfile> [args...]

           (to execute a jar file)

   or  java [options] -m <module>[/<mainclass>] [args...]

       java [options] --module <module>[/<mainclass>] [args...]

           (to execute the main class in a module)

   or  java [options] <sourcefile> [args]

           (to execute a single source-file program)

share|improve this answer

edited 8 hours ago

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  introduced in Java 11: What's New or/and JEP 330: Launch Single-File Source-Code Programs
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thanks @CarlosHeuberger for the additional details. I made a small edit in my answer to reflect that it was introduced in Java 11.
  
  – kaan
  8 hours ago
  

  
  
  
  

add a comment
 | 

Prior to Java 11, to run your code you have to first compile it, then you can run it. Here's an example:

javac test.java

java test

Since Java 11, you can still do javac + java, or you can run java by itself to compile and auto-run your code. Note that no .class file will be generated. Here's an example:

java test.java

If you run java -help, you'll see the various allowed usages. Here's what it looks like on my machine. The last one is what you ran into: java [options] <sourcefile> [args] which will "execute a single source-file program".

$ java -help

Usage: java [options] <mainclass> [args...]

           (to execute a class)

   or  java [options] -jar <jarfile> [args...]

           (to execute a jar file)

   or  java [options] -m <module>[/<mainclass>] [args...]

       java [options] --module <module>[/<mainclass>] [args...]

           (to execute the main class in a module)

   or  java [options] <sourcefile> [args]

           (to execute a single source-file program)

share|improve this answer

edited 8 hours ago

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

javac test.java

java test

java test.java

$ java -help

Usage: java [options] <mainclass> [args...]

           (to execute a class)

   or  java [options] -jar <jarfile> [args...]

           (to execute a jar file)

   or  java [options] -m <module>[/<mainclass>] [args...]

       java [options] --module <module>[/<mainclass>] [args...]

           (to execute the main class in a module)

   or  java [options] <sourcefile> [args]

           (to execute a single source-file program)

share|improve this answer

edited 8 hours ago

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

answered 8 hours ago

kaankaan

61211 silver badge1717 bronze badges

5

If you are running Java 11, there is a new feature called Java scripting that allows single source file execution. The single source compiler is more promiscuous in terms of class name versus file name, so that is how you are able to run but not succesfully compile.
https://docs.oracle.com/en/java/javase/11/scripting/java-scripting-api.html#GUID-C4A6EB7C-0AEA-45EC-8662-099BDEFC361A

If you are on a previous version of Java, then your current hello.java does not compile, because of compile errors, specifically around the class name. So there's absolutely no way that calling java hello.java compiled your code, because it does not compile.

It seems most entirely likely that you were running some previously compiled code when executing the java command.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thank you, java version is: openjdk version "12.0.2" 2019-07-16 OpenJDK Runtime Environment (build 12.0.2+10) OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)
  
  – milad
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  check Using Source-File Mode to Launch Single-File Source-Code Programs: "The compiler does not enforce the optional restriction defined at the end of JLS ??7.6, that a type in a named package should exist in a file whose name is composed from the type name followed by the .java extension."
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the link Carlos, updated my answer accordingly.
  
  – Evan
  8 hours ago
  

  
  
  
  

add a comment
 | 

5

If you are running Java 11, there is a new feature called Java scripting that allows single source file execution. The single source compiler is more promiscuous in terms of class name versus file name, so that is how you are able to run but not succesfully compile.
https://docs.oracle.com/en/java/javase/11/scripting/java-scripting-api.html#GUID-C4A6EB7C-0AEA-45EC-8662-099BDEFC361A

If you are on a previous version of Java, then your current hello.java does not compile, because of compile errors, specifically around the class name. So there's absolutely no way that calling java hello.java compiled your code, because it does not compile.

It seems most entirely likely that you were running some previously compiled code when executing the java command.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  thank you, java version is: openjdk version "12.0.2" 2019-07-16 OpenJDK Runtime Environment (build 12.0.2+10) OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode)
  
  – milad
  8 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  check Using Source-File Mode to Launch Single-File Source-Code Programs: "The compiler does not enforce the optional restriction defined at the end of JLS ??7.6, that a type in a named package should exist in a file whose name is composed from the type name followed by the .java extension."
  
  – Carlos Heuberger
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for the link Carlos, updated my answer accordingly.
  
  – Evan
  8 hours ago
  

  
  
  
  

add a comment
 | 

If you are running Java 11, there is a new feature called Java scripting that allows single source file execution. The single source compiler is more promiscuous in terms of class name versus file name, so that is how you are able to run but not succesfully compile.
https://docs.oracle.com/en/java/javase/11/scripting/java-scripting-api.html#GUID-C4A6EB7C-0AEA-45EC-8662-099BDEFC361A

If you are on a previous version of Java, then your current hello.java does not compile, because of compile errors, specifically around the class name. So there's absolutely no way that calling java hello.java compiled your code, because it does not compile.

It seems most entirely likely that you were running some previously compiled code when executing the java command.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

share|improve this answer

edited 8 hours ago

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges

answered 8 hours ago

EvanEvan

97155 silver badges2121 bronze badges