Assignment Statements

An assignment statement has the following basic syntax:

variable = expression;

The assignment operator (=) assigns the value of the expression on the right to the variable.

a = 5;

This is not the same as saying, “a equals five,” but is more akin to, “a receives the value five.”

The expression can be a literal constant value such as 2, 12.25, 't' or it can also be a numeric expression involving operands (variables or constants) and operators.

a = 5 + 2; b = 6 * a;

The assignment operator returns the value of the expression. Returning values in this way allows for chaining of assignment operators. Chaining is when you have more than one assignment in a statement as shown below.

a = b = 5;

The assignment operator is right-associative. This means that the above statement is really solved in this order:

a = (b = 5);// solved from right to left.

Since (b = 5) returns the integer 5, the value 5 is also assigned to variable a.

Variables contain either primitive data or object references. Notice that there is a difference between the two statements:

primitiveValue = 18234;

and

myPencil = new DrawingTool();

A variable will never actually contain an object, only a reference to an object. In the first statement, primitiveValue is a primitive type, so the assignment statement puts the data directly into it. In the second statement, myPencil is an object reference variable (the only other possibility) so a reference to the object is put into that variable. The object reference tells the program where to find an object.

Variable Type	Information It Contains	When On the Left of "="
primitive	Contains actual data	Previous data is replaced with new data
Object	Contains a reference, i.e. information on how to find the object referred to by the variable	Old reference is replaced with a new reference

The two types of variables are distinguished by how they are declared. Unless it was declared to be of a primitive type, it is an object reference variable. A variable will not change its declared type.

Be aware. Because these assignments work differently with primitive data types and with objects, you may experience unexpected behavior. Consider the following:

In this example, pencil will draw a blue line instead of red. In fact, we have only created one object here! When we say DrawingTool pen = pencil, all we are doing is assigning the variable name of ‘pen’ to the same object that pencil is already assigned to. This means pen and pencil refer to the same thing, so anything you do to either one happens to the other one!