Difference instanciating object inside a constructor and outside the construcctor

[Jacky Pepper]

Hi there,

What is the difference between instanciating an object inside a constructor and outside the construcctor just in the class itself?

E.g. the difference between:

public class MyClass {
	OtherClass otherClass;
	public MyClass {
		otherClass = new OtherClass();
	}

	otherMethod(){...}
}

and:

public class MyClass {
	OtherClass otherClass = new OtherClass();
	otherMethod(){...}
}

In both ways there is a OtherClass instance-object available, isn't it? Is there a difference?

[John Jai]

Where you able to create an object using this structure? Replace the otherMethod(){} with a real method that will invoke the constructor and check what happens.

public class MyClass {
	OtherClass otherClass;
	public MyClass {
		otherClass = new OtherClass();
	}

	otherMethod(){...}
}

And welcome to the Ranch, Jacky Pepper

[Winston Gutkowski]

Jacky Pepper wrote:In both ways there is a OtherClass instance-object available, isn't it? Is there a difference?

In effect: no. In flexibility: yes. The first style forces you to be able to construct the object from intrinsic data. With the second, you could pass runtime parameters to the constructor that allow you construct the object more dynamically. Note that this also applies if 'otherClass' is marked final.

Winston

[Jacky Pepper]

Thanks John and Winston!

... to be more clear, the examples should look like that:

    public class MyClass {  
        OtherClass otherClass;  
        public MyClass {  
            otherClass = new OtherClass();  
        }  
      
        myMethod(){
			otherClass.doSomething();
		}  
    }  

and

    public class MyClass {  
        OtherClass otherClass = new OtherClass();  
        myMethod(){
			otherClass.doSomething();
		} 
    } 

[Campbell Ritchie]

There is another difference. If you don’t write your own constructor, the compiler will provide a default constructor. That might not be what you want.