Java Tutorial - Java Variables

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A variable is defined by an identifier, a type, and an optional initializer. The variables also have a scope(visibility / lifetime).

Java variable type

In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:

type identifier [ = value][, identifier [= value] ...] ;

There are three parts in variable definition:

• type could be int or float.
• identifier is the variable's name.
• Initialization includes an equal sign and a value.

To declare more than one variable of the specified type, use a comma-separated list.

int a, b, c; // declares three ints, a, b, and c.
int d = 3, e, f = 5; // declares three more ints, initializing d and f.

The following variables are defined and initialized in one expression.

public class Main {
  public static void main(String[] argv) {
    byte z = 2; // initializes z.
    double pi = 3.14; // declares an approximation of pi.
    char x = 'x'; // the variable x has the value 'x'.
  }
}

Variable cannot be used prior to its declaration.

public class Main {
  public static void main(String[] argv) {

    count = 100; // Cannot use count before it is declared! 
    int count;
  }
}

Compiling the code above generates the following error message:

Assignment Operator

The assignment operator is the single equal sign, =. It has this general form:

var = expression;

type of var must be compatible with the type of expression. The assignment operator allows you to create a chain of assignments.

 
public class Main {
  public static void main(String[] argv) {
    int x, y, z;
    x = y = z = 100; // set x, y, and z to 100
    System.out.println("x is " + x);
    System.out.println("y is " + y);
    System.out.println("z is " + z);
  }
}

The output:

Dynamic Initialization

Java allows variables to be initialized dynamically. In the following code the Math.sqrt returns the square root of 2 * 2 and assigns the result to c directly.

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

    // c is dynamically initialized
    double c = Math.sqrt(2 * 2);

    System.out.println("c is " + c);
  }
}

The output from the code above is

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