How eclipse execute java code when there are compile errors

Question

When the following java code is executed in eclipse, it gives the correct output (i.e. prints 'Class B'), but according to java specification, the code cannot be compiled (since the super class constructor requires an int argument and the constructor provided by the compiler to class B includes a call to the super class no arg constructor, which is not defined), and when try to compile the file using javac command in command line, it only compiles the super class i.e. class A and fails with the following compile error:

B.java:8: cannot find symbol
symbol  : constructor A()
location: class A
public class B extends A {
   ^
1 error

Following is the java file content:

class A {

    public A(int x) {
        //
    }
}

public class B extends A {

    public static void main(String[] args) {
        System.out.println("Class b");
    }
}

Can someone explain how eclipse executes the main method without bieng able to compile the class. Thanks in advance.

UPDATE

I deleted the .class file creatd by eclipse in the bin folder and tried to execute the class, and it could not be compiled, therefore didn't run. But when I created a source file with the public class name and saved without any content, and then added the above content, it worked. If eclipse uses the previously compiled version to execute, how could it give the output of the main method, since it did not have the main method at the time of success full compilation?

Answer 1

Eclipse uses its own Java compiler (called ecj) instead of javac. This compiler allows (partial) compilation of broken source files.

Answer 2

Actually, eclipse tries to compile the class and its not able to compile due to build errors, when you simply run those files it executed the previously compiled class, that were successfully compiled earlier (eclipse tries to compile and build the files on every save as per generic settings).

in order to resolve you can try clean the project before running, while in case of javac, it simply compiles ,

Answer 3

Eclipse shouldn't do that. It leads to ambiguous results. If a class couldn't be compiled, then its corresponding .class file should be removed.

If you check the bin folder, B.class and A.class are present. Not sure why it creates B.class even though B doesn't compile