Initializing a variable

Initializing a variable means specifying an initial value to assign to it (i.e., before it is used at all).

Notice that a variable that is not initialized does not have a defined value, hence it cannot be used until it is assigned such a value. If the variable has been declared but not initialized, we can use an assignment statement to assign it a value.

Example: The following program contains a semantic error:

public class Java3 {
  public static void main(String[] args) {
    String line;
    System.out.println(line);
    System.out.println(line);
  }
}

The variable line is not initialized before we request to print it (the error is detected at compile time).

A variable can be initialized at the moment it is declared, through the statement:

type variableName = expression;

In Java, the above statement is equivalent to:

type variableName;
variableName = expression;

Example:

String line = "java".toUpperCase();

is equivalent to

String line;
line = "java".toUpperCase();