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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 it 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:

Code Block

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

Although the programmer's intent of the code could be have been to assign b to a and test the value of the result, 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.:

Code Block

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public void f(boolean a, boolean 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:

Code Block

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

Compliant Solution

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

Code Block

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

Risk Assessment

Errors of omission can result in unintended program flow.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
EXP51-JG	low	likely	medium	P6	L2

Related Guidelines

Noncompliant Code Example

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

Code Block

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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:

Code Block

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public void f(boolean a, boolean b, boolean flag) {
  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.

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:

Code Block

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