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The Java Lesson 7

Bitwise operations: overview with detailed examples

1. Logical bitwise operations
2. The complement operator
3. Bitwise shift operations
4. Bitwise assignment operations andr review questions

Bitwise shift operations

• Shift the bits of the first integer operand the number of bits specified by the second integer operand. The operator used determines the direction of the shift and whether the sign of the number is to be maintained.

• The maximum number of bit positions that can be shifted is 31 bits. If the second operand specifies a shift value greater than 31, only its right-most 5 bits will be used as the shift value. In other words, a shift factor of 32 is the same as a shift factor of 0.

• Use the << (left shift), >> (signed right shift), and >>> (unsigned right shift) operators. The rules for these operations are as follows:

Operation	Rule
<<	Shifts the first operand to the left the number of bits specified by the right-most 5 bits of the second operand. The low-order bit of the first operand is replaced with binary 0.
>>	Shifts the first operand to the right the number of bits specified by the right-most 5 bits of the second operand. The sign bit of the first operand is shifted and replaced with its original value (so the sign is maintained).
>>>	Shifts the first operand to the right the number of bits specified by the right-most 5 bits of the second operand. The sign bit of the first operand is shifted and replaced with binary 0.

For example, if

byte x = 7; // Binary value: 0000 0111 Hex value: 07
byte y;

this table shows the result of executing three unrelated statements. The cast is needed in order to store the int value that results from the operation.

Statement	Binary Result	Hexadecimal	Decimal
y = (byte)(x >> 2);	0000 0001	01	01
y = (byte)(x << 2);	0001 1100	1C	28
y = (byte)(x >>> 3);	0000 0000	00	00

Example: The following program can be run to test bitwise shift operations.

public class App {
public static void main(String[] args) {

// Variables to be read from the user

int first;
int second;

// Prompt for and read the two integers

System.out.print("First integer: ");
first = Keyboard.readInt();
System.out.print("Second integer: ");
second = Keyboard.readInt();

// Display the results of bitwise shift operations

System.out.println(" " + first + " << " + second + " = " +
(first << second));
System.out.println(" " + first + " >> " + second + " = " +
(first >> second));
System.out.println(" " + first + " >>> " + second + " = " +
(first >>> second));
}
}

Notes:

1. Program results are displayed in decimal. To really understand what is happening, work out the equivalent binary and hexadecimal values.

2. Be sure to run the program several times with different integer values.

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