Compile time and run time meaning

Question

To make things clear, I have elaborated my question in more detail.

I have this code here which elaborates about dynamic binding using the concept of overriding.

Here's the code:

class Test { 
         public static void main(String [] args) 
         { 

             B b = new A();   

             b.p(10);      //  10.0
             b.p(10.0);   // 10.0
         } 
  } 
  class B
  { 
    public void p(double i)
    { 
        print(i*2); 
    } 
  } 

  class A extends B 
  { 
     public void p(double i) 
    { 
          print(i); 
    } 
  }

Now, the explanation says that the compiler cannot determine which method to call during"compile time". It is during the "run time" when the compiler knows that the method that needs to be called is the subclass method because when we override, we are looking at the actual type.

Contrast with this code :

    class Test { 
         public static void main(String [] args) 
         { 

             B b = new A();   

             b.p(10);      //  10.0
             b.p(10.0);   // 10.0
         } 
       } 
  class B
 { 
   public void p(double i)() 
  { 
     print(i*2); 
  } 
} 

 class A extends B 
 { 
    public void p(int i) 
   { 
      print(i); 
   } 
}

In this example, the compiler can recognize which method to call during compile time. How can a compile recognize in this example while fail to recognize int he previous example? Question:

What exactly does the term "compile time" and "run time" mean? And how does a compiler not recognize during compile time that the function that needs to be called is the subclass function?

Check [this][1] out. You are asking the same thing. [1]: http://stackoverflow.com/questions/846103/runtime-vs-compile-time — Ruchira Gayan Ranaweera, Jun 25 '13 at 18:04
regarding your question 1. `How does your code refer to your question?` 2. `the explanation of WHAT? says that the compiler can't determine the method to call?` Are you getting some sort of error? — Sam I am says Reinstate Monica, Jun 25 '13 at 18:06
http://docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html. check this — Raghunandan, Jun 25 '13 at 18:09
The comments in your code imply that you think the output will result in `20.0`. In fact, the output will be `10.0` as `b` is an instance of `A`... — DannyMo, Jun 25 '13 at 18:13

score 5 · Answer 1 · answered Jun 25 '13 at 18:06

5

Consider the following (pseudo)code:

B b = Rand() > 0.5?new A() : new B();

b.p(10);

There is no way the compiler can know which will be called at runtime.

answered Jun 25 '13 at 18:06

AlexDev

4,049
31
36

This is the best answer here. +1. Simple and perfectly explains it. – Maroun Jun 25 '13 at 18:10

Sam I am says Reinstate Monica · Answer 2 · 2013-06-25T18:13:33.080

It's pretty self-explanitory

Compile-time and run-time refer to a period in time

Compile time is the time when your compiler is building your project.

Runtime is the time when your project is running.

And i'm presuming you're asking about why your code doesn't know that B is an A before the code runs. in the following section:

 B b = new A();   

 b.p(10);      //  20.0
 b.p(10.0);   // 20.0

The reason is because the compiler doesn't evaluate every possible path in the code to check for that stuff, This is easier to understand when you see slightly more complex implementations.

 B b = new A();   

 if(...)
 {
    ...
    b = new B();
 }


 b.p(10);      //  20.0
 b.p(10.0);   // 20.0

It only knows what b is when the code is actually executing.

score 2 · Answer 3 · answered Jun 25 '13 at 18:03

Run time means "when your program is running".

Compile time means "when your program is being compiled".

There are certain things that are only known at run time.

An example would be user input.

A compiler cannot anticipate what users will input at run time.

darijan · Answer 4 · 2013-06-25T18:09:09.523

What exactly does the term "compile time" and "run time" mean?

Compile time is when the code gets compiled to executable code (byte-code). That means, all files are linked (which you include by import expression), and byte-code, i.e. sequence of instructions is created.

Run time is when the CPU runs the compiled code - byte code.

And how does a compiler not recognize during compile time that the function that needs to be called is the subclass function?

Actually, it does. If it cannot recognize then it will not compile because compile must know unambiguously know what method to call in order to prepare the code for run time. The code you provided is valid, and the p() method from class A will be executed.

score 1 · Answer 5 · answered Jun 25 '13 at 18:04

Compile time: during compilation.
Run time: during program execution.

Consider:

List x = getsSomeList();

x could be any implementation of the List interface.

If you call x.add(foo), the correct add method won't be known until execution and there's an actual implementation on which to call add.

score 0 · Answer 6 · answered Jun 25 '13 at 18:05

0

The compiler can't know which method call when you're using polymorphism, because you can have many implementations of the same method... because of that it can only be determined at runtime.

answered Jun 25 '13 at 18:05

DGomez

1,450
9
25

score 0 · Answer 7 · answered Jun 25 '13 at 18:05

The difference between "compile time" and "runtime" is during the compiling, the JVM checks for syntax, specific Java, and other errors in the program that the programmer writes. The runtime is what is checked during the running of the application.

There is a possibility that you can be syntaxly correct, but have many errors in the runtime. Examples: mispelled words logic in the program the way it runs how things work dealing with animations etc

It depe

score 0 · Answer 8 · answered Jun 25 '13 at 18:10

The compiler cannot know which method to call because of Polymorphism, As said that the compiler cannot determine which method to call during"compile time" it meant that since multiple form of the function is available, the compiler decides which version of function it will call when the application is being compiled, so instead it decide which method to call when the program is being executed run-time).

If the extended/derived class overloads the function/method that you are trying to call it call the method from the derived class, if the method is not overloaded in derived class, then it will call the method from base class. That is how dynamic binding works in OOP oriented languages.

score -2 · Answer 9 · answered Jun 25 '13 at 18:17

-2

It looks like the example you have provided is incorrect.

The output is 10.0 in both invocations, not 20.0 as you have stated.

answered Jun 25 '13 at 18:17

JamesB

7,774
2
22
21

That's not really an answer to the question. it's moreso a tangentially-related comment – Sam I am says Reinstate Monica Jun 25 '13 at 18:19
It is relevant though as the author thinks the subclass method should be called. The fact is it is being called. – JamesB Jun 25 '13 at 18:22
1

but it's still not an answer – Sam I am says Reinstate Monica Jun 25 '13 at 18:22
Well perhaps not directly but it should confirm to the author that his/her assumptions about subclass implementations not being recognized is incorrect. – JamesB Jun 25 '13 at 18:32

Compile time and run time meaning

9 Answers9