What is Math.nextAfter() method in Java?

The nextAfter() method will return the next adjacent number (floating-point value) of the first argument in the direction of the second argument.

Syntax

// numbers can be float or double
Math.nextAfter(double start, double direction);

Return value

If the start < direction, then the next adjacent number in the direction of positive infinity is returned.
If the start > direction, then the next adjacent number in the direction of negative infinity is returned.
If the start == direction, then the direction is returned.
If the start is equal to ± Float.MIN_VALUE or ± Double.MIN_VALUE and the direction has a value that is smaller than start, then 0 with the same sign as the start is returned.
If the start is equal to ± Float.MAX_VALUE or ± Double.MAX_VALUE and the direction has a value that is greater than start, then Infinity with the same sign as the start is returned.
If the start is Infinity and the direction is smaller than start then the MAX_VALUE with the sign of start will be returned.
NaN is returned if any one of the arguments is NaN.

Example

class HelloWorld {
    public static void main( String args[] ) {
        float start;
        double direction;
        // start < direction
        start = 9.999999f;
        direction = 11;
        System.out.println("Math.nextAfter(9.999999, 11) :" + Math.nextAfter(start, direction));
        // start > direction
        start = 10;
        direction = 1;
        System.out.println("Math.nextAfter(10, 1) :" + Math.nextAfter(start, direction));
        // start == direction
        start = 10;
        direction = 10;
        System.out.println("Math.nextAfter(10, 10) :" + Math.nextAfter(start, direction));
        // start = Float.MIN_VALUE  , start < direction
        start = Float.MIN_VALUE;
        direction = Double.MIN_VALUE;
        System.out.println("Math.nextAfter(Float.MIN_VALUE, Double.MIN_VALUE) :" + Math.nextAfter(start, direction));
        // start = Float.MAX_VALUE  , start < direction
        start = Float.MAX_VALUE;
        direction = Double.MAX_VALUE;
        System.out.println("Math.nextAfter(Float.MAX_VALUE, Double.MAX_VALUE) :" + Math.nextAfter(start, direction));
        // start = Infinity  , start > direction
        start = Float.POSITIVE_INFINITY;
        direction = 10;
        System.out.println("Math.nextAfter(Float.POSITIVE_INFINITY, 10) :" + Math.nextAfter(start, direction));
    }
}

In the code above, we see the following.

For the first case:

// start < direction
start = 9.999999f;
direction = 11;
Math.nextAfter(start, direction);

If the start < direction, then the next adjacent number in the direction of positive infinity is returned. In our case 10.0 is returned.

For the second case:

// start > direction
start = 10;
direction = 1;
Math.nextAfter(start, direction);

If the start > direction, then the next adjacent number in the direction of negative infinity is returned. In this case 9.999999 is returned.

For the third case:

// start == direction
start = 10;
direction = 10;
Math.nextAfter(start, direction);

If the start == direction, then the direction is returned. In this case 10 is returned.

For the fourth case:

// start = Float.MIN_VALUE  , start < direction
start = Float.MIN_VALUE;
direction = Double.MIN_VALUE;
Math.nextAfter(start, direction); // 0.0

If the start is equal to ± Float.MIN_VALUE or ± Double.MIN_VALUE and the direction has a value that is smaller than start, then 0 with the same sign as the start is returned.

For the fifth case:

// start = Float.MAX_VALUE  , start < direction
start = Float.MAX_VALUE;
direction = Double.MAX_VALUE;
Math.nextAfter(start, direction); // Infinity

If the start is equal to ± Float.MAX_VALUE or ± Double.MAX_VALUE and the direction has a value that is greater than start, then Infinity with the same sign as the start is returned.

For the sixth case:

// start = Infinity  , start > direction
start = Float.POSITIVE_INFINITY;
direction = 10;
Math.nextAfter(start, direction); // 3.4028235E38

If the start is Infinity and the direction is smaller than start, then the MAX_VALUE with the sign of start will be returned.

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