SCJP Certification

An association represents a relationship between classes. It represents a mechanism that allows objects to communicate with each other. It describes the connection between different classes.

Association can be unidirectional or bi-directional.

A unidirectional association implies that an object of the class from which the arrow is originating, i.e., the class that has the no-arrowhead side of the association may invoke methods on the class towards which the arrow is pointing.

A bi-directional association means that either object in the association may invoke methods on the other.

In Java, a narrowing conversion is also known as an explicit type conversion or casting. When the destination data type is narrower than the source data type, an explicit type conversion is required. The general form of this explicit conversion is given below:

(target-type) value

Here, target-type is the desired data type and value is the value to be converted.

For example:

int a, int b;
short c;
c= (short)(a+b);

Here, the sum of two 32-bit integer data type variables a and b is assigned to another variable c, which is of integer type short (16-bit). Therefore, an explicit conversion takes place before assigning the value to c. Otherwise, if this explicit type conversion is not performed, and the sum of variables a and b is directly assigned to variable c, a compile-time error will be generated.

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable. For example:

class Try {
void disp() {}
}

class Test extends Try {
void disp() {}
public static void main(String args[]){
Try t=new Try();
Test t1=new Test();
Try v;
v=t1;
v.disp(); // call to the disp() method from Test.
}
}

Here, the Test version of the disp() method is called because the reference variable v of the type Try points to the Test object t1.