Inheritence

[wooden ermine]

class HelloWorld {
    public static void main(String[] args) {
        A a = new B(4);
        System.out.println(a.getClass());
        a.run();
    } 
}
class A{
    A(int speed){
        
    }
    public void run(){
        System.out.println("Runs slow");
    }
}

class B extends A{
    
    B(float speed){
        
    }
    public void run(){
        System.out.println("Runs");
    }
}```

[unreal knoll]

Detected code, here are some useful tools:

Formatted code

class HelloWorld {
  public static void main(String[] args) {
    A a = new B(4);
    System.out.println(a.getClass());
    a.run();
  }
}
class A {
  A(int speed) {
  }
  public void run() {
    System.out.println("Runs slow");
  }
}
class B extends A {
  B(float speed) {
  }
  public void run() {
    System.out.println("Runs");
  }
}

<@&987246399047479336> please have a look, thanks.

[unreal knoll]

While you are waiting for getting help, here are some tips to improve your experience:

Code is much easier to read if posted with syntax highlighting and proper formatting.

If nobody is calling back, that usually means that your question was not well asked and hence nobody feels confident enough answering. Try to use your time to elaborate, provide details, context, more code, examples and maybe some screenshots. With enough info, someone knows the answer for sure.

Don't forget to close your thread using the command </help-thread close:1027500463647621170> when your question has been answered, thanks.

[wooden ermine]

I am getting an error, due to constructors i think

[normal jacinth]

can u share the error please? 🙂

ah nvm

i see it

[wooden ermine]

B(float speed){
^
required: int
found: no arguments
reason: actual and formal argument lists differ in length

[normal jacinth]

u have to understand that when u want to create a Dog, the first thing java has to do is creating its Animal "base"

so when u write new Dog(), java first internally has to do sth like new Animal

and then it can put the dog "on top" of that

so far so good?

[wooden ermine]

I see

[normal jacinth]

now, u specified that ur A requires a parameter to be constructed

A (int speed){ ... }

so when java now wants to create ur B, its like "dude, i have to construct A first, but i have no idea what to give it for speed"

hence the error

it helps to understand that when u write a constructor, java always implicitly has to invoke the super-constructor

i.e. ur code:

B (float speed) {
}

is actually:

B (float speed) {
  super();
}

java implicitly adds this

super() means sth similar to new A()

but that call is invalid, as its missing the speed

so u have to do:

B (float speed) {
  super(123);
}

now it would work and be created with speed 123

but u probably want to use that parameter from B

so

B (float speed) {
  super(speed);
}

but now it will complain that float is not an int

not sure if u intended it to be a float though

[wooden ermine]

I am just testing out

[normal jacinth]

okay

🙂

[wooden ermine]

when I remove both constructors

it returns a as a type B

[normal jacinth]

then u get the "default constructor" automatically:

A() {}
...
B() {}

[normal jacinth]

thats always the case

u have to differentiate variables from the instances they refer/point to

A a = ... creates a variable that can refer to anything that is A

but its not an instance

the instance is what u created with new

new B()

getClass() gives u the type of the instance

[wooden ermine]

now this makes much more sense,

[normal jacinth]

which is B

[wooden ermine]

rather than creating an object called a, I create an object and assign it to a

[normal jacinth]

exactly

and when u call a method on it, such as animal.makeNoise(), the variable is resolved and the actual instance behind it is asked to please execute makeNoise()

so ur asking the dog/cat to execute the method

not animal

hence they bark or meow

[wooden ermine]

slightly different question

why B b = a; now returns an error?

[normal jacinth]

because java cant be sure that the thing behind a is actually a B

[wooden ermine]

more interestingly, when I did B b = (B)a; between println and method call, it removed the newline character

[normal jacinth]

like, the dog might as well be a cat now, who knows

[normal jacinth]

not sure what u mean by that

the cast wont mess with any prints

[wooden ermine]

but a.getClass() returns B

[normal jacinth]

yeah but at runtime

the type system is compile-time safe

java cant ensure during compile-time that the thing is a B

it is a B, but it cant know it for sure

after all, u could have messed with a in between

Animal a = new Dog();
a = new Cat(); // messing around

Dog dog = a;

now, u could say that its easy for the compiler to detect that messing-around

in this case it is

but what if

Animal a = new Dog();
a = someMethod(); // messing around

Dog dog = a;

now it would need to check what the method does

and what if

if (currentDay() == Day.MONDAY) {
  a = new Dog();
} else {
  a = new Cat();
}

now the program is safe only on mondays

but crashes on other days

its getting complex really quick

and the compiler just cant be 100% sure anymore that ur a variable still refers to a B - at compile-time

hence cast exists. which says "yo, just trust me"

[wooden ermine]

I understand

but when I do this

System.out.println(a.getClass());
B b = (B)a;
a.run();

output is:

class BRuns

[normal jacinth]

its always BRuns

cause the instance behind the variable is and will always be B

methods are executed on the instances

not the variables

[wooden ermine]

but what happened to newline character

[normal jacinth]

a.run() resolves the variable and then asks the instance to execute the method

[normal jacinth]

not sure what u mean by that. there is a newline character

[wooden ermine]

isn't output ought to be class B Runs

[normal jacinth]

yes. and it is

why do u think its not?

[wooden ermine]

it is class BRuns?

[normal jacinth]

not for me

maybe its ur console. share a screenshot please

or u edited the code and wrote print(...) instead of println(...)

[wooden ermine]

using online compiler, myb

[normal jacinth]

can u share ur code again please

since u likely did modify it from the original

[wooden ermine]

// Online Java Compiler
// Use this editor to write, compile and run your Java code online

class HelloWorld {
    public static void main(String[] args) {
        A a = new B(4);
        System.out.println(a.getClass());
        B b = (B)a;
        a.run();
    }
    
}

class A{
    
    A(int speed){
        
    }
    
    
    public void run(){
        System.out.println("Runs slow");
    }
}

class B extends A{
    
    B(float speed){
        super((int)speed);
    }
    
    public void run(){
        System.out.println("Runs");
    }
}

class C extends B{
    C(double speed){
        super((float)speed);
    }
}```

[normal jacinth]

it has a newline for me

it looks as expected

its probably ur online IDE

[wooden ermine]

I see

[normal jacinth]

maybe also a mismatch between \n and \r\n

on that online IDE

java will determine the correct line-terminator to use by checking the OS

using System.lineSeparator()

maybe that online-IDE said its a linux system, i.e. \n should be used

but then the console is maybe windows cmd

which expectes \r\n

so java maybe printed \n but the console only displays line-breaks with \r\n

🤷‍♂️

[wooden ermine]

Well since it's IDEs fault it doesn't bothers me so much hence

[normal jacinth]

i checked on jshell, cause it was the fastest thing i can have at hand for trying it out quickly

im using intellij as ide

[wooden ermine]

same here

thanks for the help