Understanding java class file

Question

I came across this question :

How can we identify whether a compilation unit is class or interface from a .class file?

The answer given was "from java source file header", with explanation

The Java source file contains a header that declares the type of class or interface, its visibility with respect to other classes, its name and any superclass it may extend, or interface it implements.

But I didn't find anything explaining how source file header dictates class or interface in any standard documentation. Wikipedia says following about the magic number (which I found to be relevant) in the class file:

CAFEBABE became the class file format, and CAFEDEAD was the persistent object format.

Can someone point me to document explaining in detail how class file tells whether it's for class or interface along with other details of class file?

Given this falls under the category of "basic syntax" questions, I have to assume that's a typo, and the question should have read "...from a .java file?". — andrewJames
– andrewJames, Commented Apr 11, 2020 at 14:31
@andrewjames The way I undertood it is that OP is given a .class file with the .java source code and has to decide based on the later if the .class file contains an interface or a class. — cegredev
– cegredev, Commented Apr 11, 2020 at 14:37
As I understand from the source webpage, the question meant given .class file tell whether it corresponds to .java file containing class or interface. — MsA
– MsA, Commented Apr 11, 2020 at 14:49

Andreas · Accepted Answer · 2020-04-11 15:09:50Z

How can we identify whether a compilation unit is class or interface from a .class file?

According to Chapter 4. The class File Format of the Java Virtual Machine Specification:

A class file consists of a single ClassFile structure:

ClassFile { u4 magic; u2 minor_version; u2 major_version; u2 constant_pool_count; cp_info constant_pool[constant_pool_count-1]; u2 access_flags; . . .

And:

The value of the access_flags item is a mask of flags used to denote access permissions to and properties of this class or interface. The interpretation of each flag, when set, is:
Flag Name Value Interpretation ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package. ACC_FINAL 0x0010 Declared final; no subclasses allowed. ACC_SUPER 0x0020 Treat superclass methods specially when invoked by the invokespecial instruction. ACC_INTERFACE 0x0200 Is an interface, not a class. . . . 

So if the 2-byte access_flags value has bit 9 set, then it is an interface, otherwise it's a class.

yikes thats precise and unnecessarily extremely subtle for java interview question...(the source webpage lists it as MCQ sort of java interview questions)

Atahan Atay · Accepted Answer · 2020-04-11 14:45:31Z

For example;

class a { ... } interface a { ... }

Also, you can extend a class, or implement an interface.

When you extend a class you can use another classes all functions, and it writes like that:

class a extends JFrame { a() { setVisible(true); //a method for jframe } }

When you use an interface you must implement it:

class b implements KeyListener { public void keyPressed(KeyEvent e) { } public void keyReleased(KeyEvent e) { } public void keyTyped(KeyEvent e) { } }

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