You should be careful when comparing floating-point values (float or double) using relational operators: ==, !=, < and so on. These operators give results according to the binary representations of the floating point values. For example:

public class CompareTest {
    public static void main(String[] args) {
        double oneThird = 1.0 / 3.0;
        double one = oneThird * 3;
        System.out.println(one == 1.0);      // prints "false"
    }
}

The calculation oneThird has introduced a tiny rounding error, and when we multiply oneThird by 3 we get a result that is slightly different to 1.0.

This problem of inexact representations is more stark when we attempt to mix double and float in calculations. For example:

public class CompareTest2 {
    public static void main(String[] args) {
        float floatVal = 0.1f;
        double doubleVal = 0.1;
        double doubleValCopy = floatVal;

        System.out.println(floatVal);      // 0.1
        System.out.println(doubleVal);     // 0.1
        System.out.println(doubleValCopy); // 0.10000000149011612
        
        System.out.println(floatVal == doubleVal); // false
        System.out.println(doubleVal == doubleValCopy); // false
    }
}

The floating point representations used in Java for the float and double types have limited number of digits of precision. For the float type, the precision is 23 binary digits or about 8 decimal digits. For the double type, it is 52 bits or about 15 decimal digits. On top of that, some arithmetical operations will introduce rounding errors. Therefore, when a program compares floating point values, it standard practice to define an acceptable delta for the comparison. If the difference between the two numbers is less than the delta, they are deemed to be equal. For example

if (Math.abs(v1 - v2) < delta)

Delta compare example:

public class DeltaCompareExample {

    private static boolean deltaCompare(double v1, double v2, double delta) {
        // return true iff the difference between v1 and v2 is less than delta
        return Math.abs(v1 - v2) < delta;
    }
    
    public static void main(String[] args) {
        double[] doubles = {1.0, 1.0001, 1.0000001, 1.000000001, 1.0000000000001};
        double[] deltas = {0.01, 0.00001, 0.0000001, 0.0000000001, 0};

        // loop through all of deltas initialized above
        for (int j = 0; j < deltas.length; j++) {
            double delta = deltas[j];
            System.out.println("delta: " + delta);

            // loop through all of the doubles initialized above
            for (int i = 0; i < doubles.length - 1; i++) {
                double d1 = doubles[i];
                double d2 = doubles[i + 1];
                boolean result = deltaCompare(d1, d2, delta);

                System.out.println("" + d1 + " == " + d2 + " ? " + result);
                
            }

            System.out.println();
        }
    }
}

Result:

delta: 0.01
1.0 == 1.0001 ? true
1.0001 == 1.0000001 ? true
1.0000001 == 1.000000001 ? true
1.000000001 == 1.0000000000001 ? true

delta: 1.0E-5
1.0 == 1.0001 ? false
1.0001 == 1.0000001 ? false
1.0000001 == 1.000000001 ? true
1.000000001 == 1.0000000000001 ? true

delta: 1.0E-7
1.0 == 1.0001 ? false
1.0001 == 1.0000001 ? false
1.0000001 == 1.000000001 ? true
1.000000001 == 1.0000000000001 ? true

delta: 1.0E-10
1.0 == 1.0001 ? false
1.0001 == 1.0000001 ? false
1.0000001 == 1.000000001 ? false
1.000000001 == 1.0000000000001 ? false

delta: 0.0
1.0 == 1.0001 ? false
1.0001 == 1.0000001 ? false
1.0000001 == 1.000000001 ? false
1.000000001 == 1.0000000000001 ? false

Also for comparison of double and float primitive types static compare method of corresponding boxing type can be used. For example:

double a = 1.0;
double b = 1.0001;

System.out.println(Double.compare(a, b));//-1
System.out.println(Double.compare(b, a));//1

Finally, determining what deltas are most appropriate for a comparison can be tricky. A commonly used approach is to pick delta values that are our intuition says are about right. However, if you know scale and (true) accuracy of the input values, and the calculations performed, it may be possible to come up with mathematically sound bounds on the accuracy of the results, and hence for the deltas. (There is a formal branch of Mathematics known as Numerical Analysis that used to be taught to computational scientists that covered this kind of analysis.)

Float data type

The float data type is a single-precision 32-bit IEEE 754 floating point.

Float overflow

Maximum possible value is 3.4028235e+38 , When it exceeds this value it produces Infinity

float f = 3.4e38f;
float result = f*2;        
System.out.println(result); //Infinity

Float UnderFlow

Minimum value is 1.4e-45f, when is goes below this value it produces 0.0

   float f = 1e-45f;
    float result = f/1000;
    System.out.println(result);

double data type

The double data type is a double-precision 64-bit IEEE 754 floating point.

Double OverFlow

Maximum possible value is 1.7976931348623157e+308 , When it exceeds this value it produces Infinity

double d = 1e308;
double result=d*2;      
System.out.println(result); //Infinity

Double UnderFlow

Minimum value is 4.9e-324, when is goes below this value it produces 0.0

   double d = 4.8e-323;
    double result = d/1000;
    System.out.println(result); //0.0

Floating point Numbers can be formatted as a decimal number using String.format with 'f' flag

   //Two digits in fracttional part are rounded
    String format1 = String.format("%.2f", 1.2399);
    System.out.println(format1); // "1.24"

    // three digits in fractional part are rounded 
    String format2 = String.format("%.3f", 1.2399);
    System.out.println(format2); // "1.240"
    
    //rounded to two digits, filled with zero 
    String format3 = String.format("%.2f", 1.2);
    System.out.println(format3); // returns "1.20"
    
    //rounder to two digits
    String format4 = String.format("%.2f", 3.19999);
    System.out.println(format4); // "3.20"

Floating point Numbers can be formatted as a decimal number using DecimalFormat

  // rounded with one digit fractional part 
    String format = new DecimalFormat("0.#").format(4.3200);
    System.out.println(format); // 4.3
    
   // rounded with two digit fractional part 
    String format = new DecimalFormat("0.##").format(1.2323000);
    System.out.println(format); //1.23

    // formatting floating numbers to decimal number
    double dv = 123456789;
    System.out.println(dv); // 1.23456789E8
    String format =  new DecimalFormat("0").format(dv);
    System.out.println(format); //123456789

By default, floating point operations on float and double do not strictly adhere to the rules of the IEEE 754 specification. An expression is allowed to use implementation-specific extensions to the range of these values; essentially allowing them to be more accurate than required.

strictfp disables this behavior. It is applied to a class, interface, or method, and applies to everything contained in it, such as classes, interfaces, methods, constructors, variable initializers, etc. With strictfp, the intermediate values of a floating-point expression must be within the float value set or the double value set. This causes the results of such expressions to be exactly those that the IEEE 754 specification predicts.

All constant expressions are implicitly strict, even if they aren't inside a strictfp scope.

Therefore, strictfp has the net effect of sometimes making certain corner case computations less accurate, and can also make floating point operations slower (as the CPU is now doing more work to ensure any native extra precision does not affect the result). However, it also causes the results to be exactly the same on all platforms. It is therefore useful in things like scientific programs, where reproducibility is more important than speed.

public class StrictFP { // No strictfp -> default lenient
    public strictfp float strict(float input) {
        return input * input / 3.4f; // Strictly adheres to the spec.
                                     // May be less accurate and may be slower.
    }

    public float lenient(float input) {
        return input * input / 3.4f; // Can sometimes be more accurate and faster,
                                     // but results may not be reproducable.
    }

    public static final strictfp class Ops { // strictfp affects all enclosed entities
        private StrictOps() {}

        public static div(double dividend, double divisor) { // implicitly strictfp
            return dividend / divisor;
        }
    }
}