Data Types, Variables and Constants in Java

Hello students, welcome to Kwickprep. Why does Java need to know that marks is a whole number and price is a decimal? Because every value needs the right kind of box in memory. Today we learn the eight primitive types, reference types, variables, constants with final, and default values.

Let us start with two new terms. A data type tells Java what kind of value is stored, and how much memory it needs. A variable is a named box in memory that holds one value. Java has eight primitive types, which are simple types built into the language. All other types are reference types, like String, arrays and the classes we write.

Four primitive types store whole numbers, from small to big. Byte uses one byte and stores from minus one twenty eight to one twenty seven. Short uses two bytes, and goes up to about thirty two thousand. Int uses four bytes, and goes up to about two point one billion. It is the usual choice. Long uses eight bytes, for very big values like a phone number or the population of India.

The other four primitive types hold decimals, characters and truth values. Float uses four bytes and is accurate to about seven digits. Double uses eight bytes and is accurate to about fifteen digits, so it is the default for decimals. Char uses two bytes, because Java uses Unicode, which covers Hindi, Gujarati and many other scripts. Boolean holds only true or false. Its exact size is left to the J V M, though many textbooks write one bit.

Now the second family, reference types, also called non primitive types. Examples are String, arrays, classes and interfaces. A reference variable does not hold the object itself. It holds a reference, which is like the address of the object. The object is created separately in memory, usually with the word new. If a reference points to no object at all, its value is null.

Exams often ask for the differences, so compare them side by side. A primitive variable holds the value itself, and a reference variable holds an address. Int and char are primitive, and String and arrays are reference types. A primitive can never be null, but a reference can. Primitive type names are lowercase, while class names like String start with a capital.

Declaring means telling Java the type and name. Initialising means giving the first value. Here, int marks equals ninety two does both in one line. Char grade holds A in single quotes, and double fee holds fourteen ninety nine point five zero. When we print fee, Java prints fourteen ninety nine point five, because the extra zero is not shown.

Remember four rules while declaring variables. The type comes first, then the name, then a semicolon. You can declare several variables of one type, separated by commas. A big long value needs an L at the end, and a float value needs an f, or you get a compile error. A local variable, one declared inside a method, must get a value before you use it. Otherwise the error says, variable might not have been initialized.

Some values should never change, like the number of days in a week. The keyword final makes a variable a constant, which means its value is fixed after it is set. By convention, constant names are written in capitals, so DAYS. Days times twenty four is one hundred sixty eight, and that is printed.

Here are the rules for final. A final variable gets its value only once. If you write DAYS equals eight later, the compiler stops with, cannot assign a value to final variable DAYS. Constant names are written in capitals, with underscores between words. Constants also make code easier to read, because a name like DAYS explains what seven means.

Java gives default values, but only to instance variables, static variables and array elements. Whole number types start at zero. Float and double start at zero point zero. Char starts at the null character, backslash u zero zero zero zero, which prints as nothing visible. Boolean starts as false. Every reference type, like String or an array, starts as null.

Here, four static variables are declared with no values. Static means they belong to the class itself. Pause and predict the output. The int n is zero and the boolean ok is false. The String s is null and the double d is zero point zero. But if you declare int n inside main without a value and print it, you get a compile error instead. Local variables get no default.

Let us revise what we learned today. Java has eight primitive types: byte, short, int, long, float, double, char and boolean. Reference types hold addresses, and they can be null. Declare the type, then the name, and give local variables a value before use. Final makes a constant, written in capitals. Default values apply to fields and array elements, but never to local variables. Now write the size of each primitive type from memory.