EXP51-JG. Do not perform assignments in conditional expressions

Do not use the assignment operator in conditional expressions because it frequently indicates programmer error and can result in unexpected behavior. This means that the assignment operator should not be used in the following contexts:

• if  (controlling expression)
• while (controlling expression)
• do ... while (controlling expression)
• for (second operand)
• switch (controlling expression)
• ?:  (first operand)
• &&  (either operand)
• ||  (either operand)
• ?:  (second or third operands) where the ternary expression is used in any of these contexts

Noncompliant Code Example

In this noncompliant code example, an assignment expression is the controlling expression in an if statement:

public void f(boolean a, boolean b) {
  if (a = b) {
    /* ... */
  }
}

Although the programmer's intent could have been to assign b to a and test the value of the result, this usage frequently occurs when the programmer mistakenly used the assignment operator = rather than the equality operator ==.

Compliant Solution

When the assignment of b to a is unintended, this conditional block is now executed when a is equal to b:

public void f(boolean a, boolean b) {
  if (a == b) {
    /* ... */
  }
}

Compliant Solution

When the assignment is intended, the following compliant solution clarifies the programmer's intent:

public void f(boolean a, boolean b) {
  if ((a = b) == true) {
    /* ... */
  }
}

Compliant Solution

Nevertheless, it may be preferable to express this same logic as an assignment followed by a conditional:

public void f(boolean a, boolean b) {
  a = b;
  if (a) {
    /* ... */
  }
}

Noncompliant Code Example

In this noncompliant code example, an assignment expression appears in the controlling expression as an operand of the && operator. Because && is not a comparison operator, assignment is an illegal operand.

public void f(boolean a, boolean b, boolean flag) {
  while ( (a = b) && flag ) {
    /* ... */
  }
}

Again, this is frequently a case of the programmer mistakenly using the assignment operator = instead of the equals operator ==.

Compliant Solution

When the assignment of b to a is unintended, this conditional block is now executed only when a is equal to b:

public void f(boolean a, boolean b) {
  while ( (a == b) && flag ) {
    /* ... */
  }
}

Applicability

The use of the assignment operator in controlling conditional expressions frequently indicates programmer error and can result in unexpected behavior.

Exceptionally, it is permitted to use the assignment operator in conditional expressions when the assignment is not the controlling expression (that is, the assignment is a subexpression), as shown in the following compliant example:

public void app_ex(BufferedReader reader) throws IOException{
	String line;
    while ((line = reader.readLine()) != null) {
    	// ... work with line
    }
}

Related Guidelines

Bibliography