Note for Core Java - cjava By sudhansusekhar pujari
- Core Java - cjava
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Java Programming Java Programming language is developed by James Gosling at Sun Microsystems. It was developed in 1995 for developing programs for electronic devices for interactive television Java programs are platform independent On every system JVM (Java Virtual Machine) will take care of running java programs ( write once Run Anywhere) Java is developed based on the Features of C, C++. Java is Object Oriented Programming language. OOPs Concepts Class & Object Encapsulation Inheritance Polymorphism Data abstraction Data Types : Specifies the type of data stored in the system. int, char, string, Boolean, double, float, byte…etc Java Primitive Data Types and Wrapper Classes In programming languages a data type is an attribute of a piece of data that explains what kind of data is being dealt with. This involves setting constraints on the data, such as what values that data can take on, and what operations may be performed on that data. The Java programming language is strongly-typed, which means that all variables must first be defined or declared before they can be used in the actual code. This is often related to the Option Explicit statement used within Microsoft languages. Primitive Data Types byte: The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive). The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters. They can also be used in place of int where their limits help to clarify your code; the fact that a variable's range is limited can serve as a form of documentation. short: The short data type is a 16-bit signed two's complement integer. It has a minimum value of -32,768 and a maximum value of 32,767 (inclusive). As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters. int: The int data type is a 32-bit signed two's complement integer. It has a minimum value of 2,147,483,648 and a maximum value of 2,147,483,647 (inclusive). For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something
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else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead. long: The long data type is a 64-bit signed two's complement integer. It has a minimum value of -9,223,372,036,854,775,808 and a maximum value of 9,223,372,036,854,775,807 (inclusive). Use this data type when you need a range of values wider than those provided by int. float: The float data type is a single-precision 32-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math. BigDecimal class instead. Numbers and Strings covers BigDecimal and other useful classes provided by the Java platform. double: The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is beyond the scope of this discussion, but is specified in section 4.2.3 of the Java Language Specification. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency. boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined. char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive). Wrapper Classes for Primitive Data Types Primitive data types are not classes in Java. Therefore they do not come with instance variables and methods. This is good for efficiency, but seems to force us in a non-object oriented direction. To accommodate this, there are wrapper classes.
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Literals You may have noticed that the new keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation. (Sun Microsystems, 1995-2007) Variables : Variables are identifiers to store the value in the memory, the variables are available only during runtime of the program. Declare variables int x,y; char x(10); boolean y; variable declaration is compulsory
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To declare a variable in Java, all that is needed is the data type followed by the variable name: int numberOfDays; In the above example, a variable called "numberOfDays" has been declared with a data type of int. Notice how the line ends with a semi-colon. The semi-colon tells the Java compiler that the declaration is complete. Now that it has been declared, numberOfDays can only ever hold values that match the definition of the data type (i.e., for an int data type the value can only be a whole number between -2,147,483,648 to 2,147,483,647). Declaring variables for other data types is exactly the same: byte nextInStream; short hour; long totalNumberOfStars; float reactionTime; double itemPrice; Initializing Variables Before a variable can be used it must be given an initial value. This is called initializing the variable. If we try to use a variable without first giving it a value: int numberOfDays; //try and add 10 to the value of numberOfDays numberOfDays = numberOfDays + 10; The compiler will throw an error: variable numberOfDays might not have been initialized To initialize a variable we use an assignment statement. An assignment statement follows the same pattern as an equation in mathematics (e.g., 2 + 2 = 4). There is a left side of the equation, a right side and an equals sign (i.e., "=") in the middle. To give a variable a value, the left side is the name of the variable and the right side is the value: int numberOfDays; numberOfDays = 7; In the above example, numberOfDays has been declared with a data type of int and has been giving an initial value of 7. We can now add ten to the value of numberOfDays because it has been initialized: int numberOfDays; numberOfDays = 7;
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