2.6. Constants

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2.5. Assignment Statements and Assignment Expressions

After a variable is declared, you can assign a value to it by using an assignment statement . In Java, the equal sign ( = ) is used as the assignment operator . The syntax for assignment statements is as follows :

 variable = expression; 

An expression represents a computation involving values, variables , and operators that together evaluates to a value. For example, consider the following code:

   int   x =   1   ;  // Assign 1 to variable x;    double   radius =   1.0   ;  // Assign 1.0 to variable radius;  x =   5   * (   3   /   2   ) +   3   *   2   ;  // Assign the value of the expression to x;  x = y +   1   ;  // Assign the addition of y and 1 to x;  area = radius * radius *   3.14159   ;  // Compute area  

A variable can also be used in an expression. For example,

 x = x +   1   ; 

In this assignment statement, the result of x + 1 is assigned to x . If x is 1 before the statement is executed, then it becomes 2 after the statement is executed.

To assign a value to a variable, the variable name must be on the left of the assignment operator . Thus, 1 = x would be wrong.

In Java, an assignment statement can also be treated as an expression that evaluates to the value being assigned to the variable on the left-hand side of the assignment operator. For this reason, an assignment statement is also known as an assignment expression . For example, the following statement is correct:

 System.out.println(x =   1   ); 

which is equivalent to

 x =   1   ; System.out.println(x); 

The following statement is also correct:

 i = j = k =   1   ; 

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which is equivalent to

 k =   1   ; j = k; i = j; 

Note

In an assignment statement, the data type of the variable on the left must be compatible with the data type of the value on the right. For example, int x = 1.0 would be illegal because the data type of x is int . You cannot assign a double value ( 1.0 ) to an int variable without using type casting. Type casting is introduced in §2.8, "Numeric Type Conversions."

2.5.1. Declaring and Initializing Variables in One Step

Variables often have initial values. You can declare a variable and initialize it in one step. Consider, for instance, the following code:

   int   x =   1   ; 

This is equivalent to the next two statements:

   int   x; x =   1   ; 

You can also use a shorthand form to declare and initialize variables of the same type together. For example,

   int   i =   1   , j =   2   ; 

Tip

A variable must be declared before it can be assigned a value. A variable declared in a method must be assigned a value before it can be used. Whenever possible, declare a variable and assign its initial value in one step. This will make the program easy to read and avoid programming errors.