& is a bitwise operator and compares each operand bitwise.It is a binary AND Operator and copies a bit to the result if it exists in both operands.Assume integer variable A holds 60 and variable B holds 13 then  (A & B) will give 12 which is 0000 1100.Whereas && is a logical AND operator and operates on boolean operands. If both the operands are true, then the condition becomes true otherwise it is false. Assume boolean variable A holds true and variable B holds false then (A && B) is false.& is to be used during bitwise operations and && is useful during logical operations.

The unary operator works on a single operand. Following are the examples of unary operators supported in java. Assume A = 60 and B = 20.OperatorDescriptionExample~ (bitwise compliment)Binary One's Complement Operator is unary and has the effect of 'flipping' bits.(~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.++ (Increment)Increases the value of operand by 1.B++ gives 21-- (Decrement)Decreases the value of operand by 1.B-- gives 19

There are following boolean operators supported by Java language.Assume variable A holds 10 and variable B holds 20, then −OperatorDescriptionExample== (equal to)Checks if the values of two operands are equal or not, if yes then condition becomes true.(A == B) is not true.!= (not equal to)Checks if the values of two operands are equal or not, if values are not equal then condition becomes true.(A != B) is true.> (greater than)Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.(A > B) is not true.< (less than)Checks if the ... Read More

Widening refers to passing a lower size data type like int to a higher size data type like long. No casting is required in such a case.public class MyFirstJavaProgram {    public static void main(String []args) {       int a = 300;       long b = a;       System.out.println(b);    } }

Narrowing refers to passing a higher size data type like int to a lower size data type like short. It may lead to data loss. Casting is required for narrowing conversion. Following program output will be 44. public class MyFirstJavaProgram { public static void main(String []args) { int a = 300; byte b = (byte)a; // narrowing System.out.println(b); } }

We can convert one data types into another data type using casting. Narrowing ConversionNarrowing refers to passing a higher size data type like int to a lower size data type like short. It may lead to data loss. Following program output will be 44.public class MyFirstJavaProgram {    public static void main(String []args) {       int a = 300;       byte b = (byte)a; // narrowing       System.out.println(b);    } }Widening/Promotion ConversionWidening refers to passing a lower size data type like int to a higher size data type like long. public class MyFirstJavaProgram {    public ... Read More

OverflowOverflow occurs when we assign such a value to a variable which is more than the maximum permissible value.UnderflowUnderflow occurs when we assign such a value to a variable which is less than the minimum permissible value.JVM does not throw any exception in case Overflow or underflow occurs, it simply changes the value. Its programmer responsibility to check the possibility of an overflow/underflow condition and act accordingly. Example (Overflow)Consider the case of int variable, it is of 32 bit and any value which is more than Integer.MAX_VALUE (2147483647) is rolled over. For example, Integer.MAX_VALUE + 1 returns -2147483648 (Integer.MIN_VALUE).As int data ... Read More

Following are the best practices while declaring a variable.Variables names should be short or long enough as per the scope. For example, loop counter variable, i is fine whereas employee as a loop variable.Specific words should not be used as equals, compare, data.Use meaningful names which can explain the purpose of the variable. For example cnt Vs counter.Don't use _ to declare a variable name, Use camel casing. For example, employeeName is better than employee_name.Each organization has its own syntax specific standards. Follow those rules to maintain consistency and readability.

Yes, from Java 8 onwards, java.util.concurrent.atomic package contains classes which support atomic operations on single variables preventing race conditions or do not face synchronization issues. All classes in the atomic package have get/set methods. Each set method has a happens-before relationship with any subsequent get() method call on the same variable. import java.util.concurrent.atomic.AtomicInteger; class AtomicCounter { private AtomicInteger counter = new AtomicInteger(0); public void increment() { counter.incrementAndGet(); } public void decrement() { counter.decrementAndGet(); } public int value() { return counter.get(); } }

There is no global variables support in Java. Static variables can be used as an alternate solution for global variables.