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java.math.BigInteger.byteValueExact() was introduced in Java 8. It is an inbuilt function which converts the value of BigInteger to a byte and checks for any lost information. If the value of BigInteger is greater than 127 or less than -128, the method will throw ArithmeticException as BigInteger doesn’t fit in byte range. Syntax:

public byte byteValueExact()

Return Value: This method returns the byte value of this BigInteger. Exception: The method throws ArithmeticException if the value of BigInteger is greater than 127 or less than -128, as this range doesn’t fit in byte range. Example:

Input: 122
Output: 122
Explanation: 122 is given as input which is bigInteger
and byte value of 122 is 122

Input: -46
Output: -46
Explanation: -46 is given as input which is bigInteger 
and byte value of -46 is -46

Input: 200
Output: ArithmeticException
Explanation: When 200 is tried to convert to Byte,
since 200 > 127 (greater than a Byte's range), 
therefore it throws an arithmetic exception.

Below programs illustrates byteValueExact() method of BigInteger class: Program 1: To demonstrate byteValueExact() method for positive number <127

Java

// Java program to demonstrate byteValueExact()
// method of BigInteger Class

import java.math.*;

public class GFG {

    public static void main(String[] args)
    {
        // Creating a BigInteger object
        BigInteger big;
        big = new BigInteger("122");

        // print value of BigInteger
        System.out.println("BigInteger value : "
                           + big);

        // convert big to the byte value
        byte b1 = big.byteValueExact();

        // print byte value
        System.out.println("Byte converted value : "
                           + b1);
    }
}

Output:

BigInteger value : 122
Byte converted value : 122

Program 2: To demonstrate byteValueExact() method for negative number >-128

Java

// Java program to demonstrate byteValueExact()
// method of BigInteger Class

import java.math.*;

public class GFG {

    public static void main(String[] args)
    {

        // Creating a BigInteger object
        BigInteger big = new BigInteger("-46");

        // print value of BigInteger
        System.out.println("BigInteger value : "
                           + big);

        // convert big to the byte value
        byte b1 = big.byteValueExact();

        // print byte value
        System.out.println("Byte converted value : "
                           + b1);
    }
}

Output:

BigInteger value : -46
Byte converted value : -46

Program 3: To demonstrate byteValueExact() method for negative number <-128. It will throw Arithmetic Exception

Java

// Java program to demonstrate byteValueExact()
// method of BigInteger Class

import java.math.*;

public class GFG {

    public static void main(String[] args)
    {

        // Creating a BigInteger object
        BigInteger big = new BigInteger("-200");

        // print value of BigInteger
        System.out.println("BigInteger value : "
                           + big);

        try {
            // convert big to the byte value
            byte b1 = big.byteValueExact();

            // print byte value
            System.out.println("Byte converted value : "
                               + b1);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

Output:

BigInteger value : -200
Exception: java.lang.ArithmeticException: BigInteger out of byte range

Program 4: To demonstrate byteValueExact() method for positive number >127. It will throw Arithmetic Exception

Java

// Java program to demonstrate byteValueExact()
// method of BigInteger Class

import java.math.*;

public class GFG {

    public static void main(String[] args)
    {

        // Creating a BigInteger object
        BigInteger big = new BigInteger("200");

        // print value of BigInteger
        System.out.println("BigInteger value : "
                           + big);

        try {
            // convert big to the byte value
            byte b1 = big.byteValueExact();

            // print byte value
            System.out.println("Byte converted value : "
                               + b1);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

Output:

BigInteger value : 200
Exception: java.lang.ArithmeticException: BigInteger out of byte range

Reference: https://docs.oracle.com/javase/8/docs/api/java/math/BigInteger.html#byteValueExact--

Java 8 | BigInteger byteValueExact() method with Examples

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