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 intent of the code could be to assign b to a and test the value of the result, it 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 when a is equal to b.

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

When the assignment is intended, the following compliant solution may be used because the programmer's intent is clearer:

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

But it might 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 is in the controlling expression, as an operand of the && operator. Since && 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 when a is equal to b.

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

When the assignment is intended, one of the same solutions should be used as shown above.

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