strictfp is a keyword in java used for restricting floating-point calculations and ensuring same result on every platform while performing operations in the floating-point variable.
Floating point calculations are platform dependent i.e. different output(floating-point values) is achieved when a class file is run on different platforms(16/32/64 bit processors). To solve this types of issue, strictfp keyword was introduced in JDK 1.2 version by following IEEE 754 standards for floating-point calculations.

Important points:
strictfp modifier is used with classes, interfaces and methods only.
strictfp class Test
{
// all concrete methods here are
// implicitly strictfp.
}
strictfp interface Test
{
// all methods here becomes implicitly
// strictfp when used during inheritance.
}
class Car
{
// strictfp applied on a concrete method
strictfp void calculateSpeed(){}
}

When a class or an interface is declared with strictfp modifier, then all methods declared in the class/interface, and all nested types declared in the class, are implicitly strictfp.
strictfp cannot be used with abstract methods. However, it can be used with abstract classes/interfaces.
Since methods of an interface are implicitly abstract, strictfp cannot be used with any method inside an interface.
strictfp interface Test
{
double sum();
strictfp double mul(); // compile-time error here
}

//Java program to illustrate strictfp modifier

public class Test
{
// calculating sum using strictfp modifier
public strictfp double sum()
{
double num1 = 10e+10;

double num2 = 6e+08;

return (num1+num2);

}

public static strictfp void main(String[] args)
{
Test t = new Test();

System.out.println(t.sum());
}
}
Output:

1.006E11