The Assignment Operator

The Assignment Operator
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Variable:
- A named space for holding data/information
- The name is often referred to as the identifier
Literal:
- A representation of a datum

Review (cont.)

Operator:
- A symbol indicating that an operation is to be performed (on one or more operands)
Expression:
- A combination of variables, operators, and literals that represent a single value
Statement:
- The smallest syntactically valid construct that conveys a complete command

Java vs. Python - Important Differences

Statements:
- In Python, statements are terminated with the end-of-line character
- In Java, statements are terminated with a ; character
White Space:
- In Python, white space is signficant
- In Java, extra white space is insignificant (e.g., the statement ends with the ;, wherever it is, even on another line)
Comments:
- In Python, everything after a # is an in-line comment
- In Java, everything after a // is an in-line comments and everything between a /* and a */ is a block comment

Assignment

The Operation:
- Placing a value into the memory identified by a variable/constant
The Operator (in Java):
- =
Operands:
- The assignment operator is a binary operator; the left-side operand is a variable/constant and the right-side operand is a literal or an expression (i.e., something that evaluates to a value)
An Important Detail (in Java):
- The assignment operation evaluates to the value of the right-side operand (so, is an expression)

Assignment of Primitive and Reference Types

Atomic/Primitive/Fundamental Types:
- The value of the right-side operand is assigned to the left-side operand
Reference Types:
- The address of the right-side operand is assigned to the left-side operand

Assignment in Java

Example Expressions (Assuming the Variables Have Been Declared):
- age = 21
- initial = 'H'
- ok = false
- course = "CS159"
Example Statements (Assuming the Variables Have Been Declared):
- age = 21;
- initial = 'H';
- ok = false;
- course = "CS159";

Assignment in Java (cont.)

Some Observations:
- A variable can only contain one value at a time
- A variable can only contain a value of the same type

A Java Example:

int   i;
i =  5;       // i contains the value 5
i = 27;       // i now contains the value 27

String s;
s = "CS159";  // s contains the address of "CS159"
s = "JMU";    // s now contains the address of "JMU"

Assignment in Java (cont.)

A Common Confusion:
- Confusing the assignment operator with the notion of "equality"
A Result of this Confusion:
- Attempting to use expressions like 21 = age (which has an inappropriate left-side operand)

A Practice to Avoid

An Example:
- ```
int   i, j;

i = j = 5;
        
```
Why it Should be Avoided:
- It can cause confusion in more complicated statements, especially those involving arithmetic operators and relational operators

An Interesting Issue

A Question:
- In the previous example, which assignment is performed first?
The General Concept:
- Associativity - determines whether (in the absence of other determinants) an expression is evaluated from left to right or right to left
Associativity of the Assignment Operator in Java:
- The assignment operator has right to left associativity, so i = j = 5 is equivalent to i = (j = 5)

Type Checking During Assignment

Strong Type Checking:
- Ensuring that the type of the right-side operand is the same as the type of the left-side operand
Conversion in Java:
- Java does not allow implicit losses of precision
- Java does allow widenings (though you should refrain from using them)

An Example:

double weight;
int    age;
	
age    = 21.0;       // Not allowed (will generate a compile-time error)
weight = 155;        // Allowed but not recommended (use 155. instead)

Type Checking During Assignment (cont.)

Typecasting:
- (type)expression

Examples:

        double  real;
        int     small;
        long    large;

        // ...
        small = (int)large;
        // ...
        small = (int)real;

Why Typecasting Should be Used Sparingly:
- Loss of precision
- Possible loss of magnitude
- Possible change of sign

The final Modifier in Java

Purpose:
- Indicate that a value can only be assigned to a variable once
Syntax:
- final type variable [, variable]... ;
Examples:
- final boolean CORRECT;
- final double BUDGET;
- final int CAPACITY;

The final Modifier in Java (cont.)

Name/Identifier Restrictions:
- Same as for other variables
Course Style Guide Requirements:
- Must be all uppercase
- Underscores between "words" improve readability
Compile-Time Errors:
- An error will be generated if an identifier that is declared to be final is the left-side operand of more than one assignment operator
A Common Convention:
- Include the assignment in the declaration