Description

return (x);          // parens around identifier
return (x + 1);      // parens around return value
int x = (y / 2 + 1); // parens around assignment rhs
for (int i = (0); i < 10; i++) {  // parens around literal
t -= (z + 1);                     // parens around assignment rhs
boolean a = (x > 7 && y > 5)      // parens around expression
            || z < 9;
boolean b = (~a) > -27            // parens around ~a
            && (a-- < 30);        // parens around expression
        

Notes

The check is not "type aware", that is to say, it can't tell if parentheses are unnecessary based on the types in an expression. The check is partially aware about operator precedence but unaware about operator associativity. It won't catch cases such as:

int x = (a + b) + c; // 1st Case
boolean p = true; // 2nd Case
int q = 4;
int r = 3;
if (p == (q <= r)) {}
        

In the first case, given that a, b, and c are all int variables, the parentheses around a + b are not needed. In the second case, parentheses are required as q, r are of type int and p is of type boolean and removing parentheses will give a compile-time error. Even if q and r were boolean still there will be no violation raised as check is not "type aware".

The partial support for operator precedence includes cases of the following type:

boolean a = true, b = true;
boolean c = false, d = false;
if ((a && b) || c) { // violation, unnecessary paren
}
if (a && (b || c)) { // ok
}
if ((a == b) && c) { // violation, unnecessary paren
}
String e = "e";
if ((e instanceof String) && a || b) { // violation, unnecessary paren
}
int f = 0;
int g = 0;
if (!(f >= g) // ok
        && (g > f)) { // violation, unnecessary paren
}
if ((++f) > g && a) { // violation, unnecessary paren
}
        

Properties

Examples

To configure the check:

<module name="Checker">
  <module name="TreeWalker">
    <module name="UnnecessaryParentheses"/>
  </module>
</module>

Which results in the following violations:

class Example1 {

  public int square(int a, int b) {
    // violation below, 'Unnecessary parentheses around assignment right-hand side'
    int square = (a * b);
    // violation below, 'Unnecessary parentheses around identifier 'square''
    return (square);
  }

  int sumOfSquares = 0;
  public void sumOfSquares() {
    // violation below, 'Unnecessary parentheses around literal '0''
    for (int i = (0); i < 10; i++) {
      // violation below, 'Unnecessary parentheses around assignment right-hand side'
      int x = (i + 1);
      sumOfSquares += (square(x,x));  // 2 violations
    }
  }

  List<String> myList = List.of("a1", "b1", "c1");
  public void filter() {
    myList.stream()
        // violation below, 'Unnecessary parentheses around lambda value'
        .filter((s) -> s.startsWith("c"))
            .forEach(System.out::println);
  }

  int a = 10, b = 12, c = 15;
  boolean x = true, y = false, z = true;
  public void test() {
    // 3 violations below
    if ((a >= 0 && b <= 9) || (c >= 5 && b <= 5) || (c >= 3 && a <= 7)) {
      return;
    }
    // violation below, 'Unnecessary parentheses around expression'
    if ((-a) != -27 && b > 5) {
      return;
    }
    if (x == (a <= 15)) {
      return;
    }
    if (x == (y == z)) {
      return;
    }
  }

}

To configure the check to detect unnecessary parentheses around bitwise inclusive OR '|' , bitwise AND '&' , bitwise exclusive OR '^' :

<module name="Checker">
  <module name="TreeWalker">
    <module name="UnnecessaryParentheses">
      <property name="tokens" value="BOR, BAND, BXOR" />
    </module>
  </module>
</module>

Example:

class Example2 {

  void method() {
    int x = 9, y = 8;
    // violation below, 'Unnecessary parentheses around expression'
    if (x >= 0 ^ (x <= 8 & y <= 11)
         ^ y >= 8) {
      return;
    }
    if (x >= 0 ^ x <= 8 & y <= 11 ^ y >= 8) {
      return;
    }
    // violation below, 'Unnecessary parentheses around expression'
    if (x >= 0 || (x <= 8 & y <= 11)
        && y >= 8) {
      return;
    }
    if (x >= 0 || x <= 8 & y <= 11 && y >= 8) {
      return;
    }
    if (x >= 0 & (x <= 8 ^ y <= 11) & y >= 8) {
      return;
    }
  }

}

To configure the check to detect unnecessary parentheses around conditional expressions '?' :

<module name="Checker">
  <module name="TreeWalker">
    <module name="UnnecessaryParentheses">
      <property name="tokens" value="QUESTION" />
    </module>
  </module>
</module>

Example:

class Example3 {

  void method() {
    int a = 9, b = 8;
    int c = (a > b) ? 1 : 0; // violation 'Unnecessary parentheses around expression'

    int d = c == 1 ? (b % 2 == 0) ? 1 : 0 : 5;
    // violation above 'Unnecessary parentheses around expression'
  }

}

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.paren.assign
• unnecessary.paren.expr
• unnecessary.paren.ident
• unnecessary.paren.lambda
• unnecessary.paren.literal
• unnecessary.paren.return
• unnecessary.paren.string

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker