CODEWITHSUNDEEP
java
Sheikh Rasik
Sheikh Rasik

Reputation: 53

How to use multiple classes in java in one file?

I want to know how to use multiple classes in one file in java. I typed this code but it is showing compilation errors. 

class test {

    int a, b, c;

    void getdata(int x, int y) {
        a = x;
        b = y;
    }

    void add() {
        c = a + b;
        System.out.println("Addition = " + c);
    }
}

public class P8 {

    public static void main(String[] args) {

        test obj = new test();
        test.getdata(200, 100);
        test.add();
    }
}

Upvotes: 4

Views: 56343

Answers (7)

Tejas Chauhan
Tejas Chauhan

Reputation: 109

{ // you have to call the method by the object which you are created. then it will run without error. Test obj = new Test(); obj.getdata(20, 10);
obj.add();` }

Upvotes: -2

Stephen C
Stephen C

Reputation: 718758

The compilation errors in the classes you showed us have nothing to do with having two classes in the file. 

public static void main(String[] args) {

    test obj = new test();
    test.getdata(200, 100);  // error here
    test.add();              // error here
}

When I compile your code using javac the error messages are:

$ javac P8.java 
P8.java:21: error: non-static method getdata(int,int) cannot be referenced from a static context
        test.getdata(200, 100);
            ^
P8.java:22: error: non-static method add() cannot be referenced from a static context
        test.add();
            ^
2 errors

The problem is that test is a class name, not the name of a variable. As a result you are trying to invoke instance methods as if they were static methods.

But to my mind, this is a classic "I've shot myself in the foot Mum" moment.

You have broken one of the most widely observed rules of Java style.

Java class names should always start with an uppercase letter.

You have named your class test rather than Test. So when you wrote

test.getdata(200, 100);

test looks like a variable name, and that looks like a call of an instance method. But it isn't.

My bet is that this is part of what caused you to misconstrue the error message as being related (somehow) to having two classes in a file.

There is another stylistic howler in you code. You have called a method getdata but it actually behaves as a (sort of) setter for the Test class. If your code wasn't so small that it fits on a single page, that would be really misleading.

And finally, I agree with people who advise you not to put multiple top level classes into a single source file. It is legal code, but unnecessary. And style guides typically recommend against doing it.

Upvotes: 3

Md Alif Al Amin
Md Alif Al Amin

Reputation: 196

i hope it will help you.... i just changed test.getdata() to obj.getdata() and test.add() to obj.add() ..... check it out..

class test {
int a,b,c;
void getdata(int x, int y) {
    a=x;
    b=y;
}
void add() {
    c=a+b;
    System.out.println("Addition = "+c);
}
}

public class P8 {

public static void main(String[] args) {

    test obj = new test();
    obj.getdata(200,100);
    obj.add();
    }

}

you can not call test.getdata().. and test.add()... as its not static methods

Upvotes: 1

Anton Danilov
Anton Danilov

Reputation: 1276

You can use at most one public class per one java file (COMPILATION UNIT) and unlimited number of separate package-private classes.

Compilation unit must named as public class is.

You also can have in your public class the unlimited number of inner classes and static nested classes.

Inner classes have an intenal pointer to the enclosing class so they have access to its members as well as local vars. They can be anonymuous.

Static nested classes is just like regular pubic class but is defined within enclosing class

Upvotes: 0

JakeTheSnake
JakeTheSnake

Reputation: 402

You have to nest your classes in each other, although it is not recommended.

public class P8 {//Currently inside P8 class
    class test {//Declaring while inside P8
    private int a, b, c;//Private vars in a nested class
    void getdata(int x, int y) {
        a = x;
        b = y;
    }
    void add() {
        c = a + b;
        System.out.println("Addition = " + c);
    }
}

    public static void main(String[] args) {//Running the main for P8 class
        test obj = new test();
        test.getdata(200, 100);
        test.add();
    }
}

One of the reasons nesting classes is a bad idea is it strips the class of its privacy. The 'private' tag in java take whatever variable is tagged with it, and will only let that class access it, but if the class is inside another, both classes can freely access those private variables.

Upvotes: -2

Andy Turner
Andy Turner

Reputation: 140319

You can only have one public top-level class per file. So, remove the public from all but one (or all) of the classes.

However, there are some surprising problems that can happen if you have multiple classes in a file. Basically, you can get into trouble by (accidentally or otherwise) defining multiple classes with the same name in the same package.

If you're just a beginner, it might be hard to imagine what I'm going on about. The simple rule to avoid the problems is: one class per file, and call the file the same thing as the class it declares.

Upvotes: 9

Andres Sepulveda
Andres Sepulveda

Reputation: 17

Here's a very basic example of how to nest classes within classes. For this example, let's say that my file is named Test.java

public class Test {
    public Test() {
    }

    class Person {
         public Person() {
         }
    }
}

You should really take a look at how constructors work, because that may be one of your problems. Can't tell what else without more info, unfortunately.

Upvotes: -2

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