Page History

Do not use Using the assignment operator in the outermost expression of an if or switch statement or a looping statement (while, do, or for) because this typically conditional expressions frequently indicates programmer error and can result in unexpected behavior. 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 outermost controlling expression in an the if statement .is an assignment expression:

public void exampleFunctionf(Objectboolean a, Objectboolean b) {

  if (a = b) {
    /* ... */
  }

}

While Although the programmer's intent of the code could be have been to assign b to a and test the value of the result for equality to zero, it is frequently a case of , this usage frequently occurs when the programmer mistakenly using used the assignment operator = instead of rather than the equals equality operator ==.

Compliant Solution

When the assignment of b to a is unintended, this conditional block is now executed The conditional block shown in this compliant solution executes only when a is equal to b.:

public void exampleFunctionf(Objectboolean a, Objectboolean b) {

  if (a == b) {
    /* ... */
  }

}

Unintended assignment of b to a cannot occur.

Compliant Solution

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

public void exampleFunctionf(Objectboolean a, Objectboolean b) {


  if ((a = b) == true) {
    /* ... */
  }

}

Compliant Solution

It may be clearer to express the Although it could be preferable to express this same logic as an explicit assignment followed by a conditionalthe if condition:

public void exampleFunctionf(Objectboolean a, Objectboolean b) {

  a = b;
  if (a) {
 == 0   /* ... */
  }
}

Noncompliant Code Example

In this noncompliant code example, an assignment expression appears as an operand of the && operator:

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

Because && is not a comparison operator, assignment is an illegal operand. 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 and flag is true:

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

...

Applicability

Errors of omission The use of the assignment operator in controlling conditional expressions frequently indicates programmer error and can result in unintended program flow.

Related Guidelines

Bibliography

unexpected behavior.

As an exception to this guideline, 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 solution:

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

Automated Detection

Bibliography

...

Image Added Image Added Image AddedImage Removed      02. Expressions (EXP)      EXP05-J. Do not write more than once to the same variable within an expression