UnusedLocalVariable

Since Checkstyle 9.3

Description

Checks that a local variable is declared and/or assigned, but not used. Doesn't support pattern variables yet. Doesn't check array components as array components are classified as different kind of variables by JLS.

Properties

name	description	type	default value	since
allowUnnamedVariables	Allow variables named with a single underscore (known as unnamed variables in Java 21+).	boolean	`true`	10.18.0

Examples

To configure the check:


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

Example:


class Example1 {
  {
    int k = 12; // violation, assign and update but never use 'k'
    k++;
  }

  int a;
  Example1(int a) { // ok, as 'a' is a constructor parameter not a local variable
    this.a = 12;
  }

  void method(int b) {
    int[] arr = {1, 2, 3};  // violation, unused named local variable 'arr'
    int[] anotherArr = {1}; // ok, 'anotherArr' is accessed
    anotherArr[0] = 4;
  }

  String convertValue(String newValue) {
    String s = newValue.toLowerCase(); // violation, unused named local variable 's'
    String _ = newValue.toLowerCase(); // ok, '_' is unnamed variable
    return newValue.toLowerCase();
  }

  void read() throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String s; // violation, unused named local variable 's'
    while ((s = reader.readLine()) != null) {}
    try (BufferedReader reader1 = // ok, 'reader1' is a resource
                 new BufferedReader(new FileReader("abc.txt"))) {}
    try {
    } catch (Exception e) { }  // ok, 'e' is an exception parameter
  }

  void loops() {
    int j = 12;
    for (int i = 0; j < 11; i++)  // violation, unused named local variable 'i'
      for (int p = 0; j < 11; p++) p /= 2;    // ok, 'p' is used
    for (Integer _ : new  int[0]) { } // ok, '_' is unnamed variable
  }

  void lambdas() {
    Predicate<String> obj = (String str) -> true; // ok, 'str' is a lambda parameter
    obj.test("Test");
  }
}

To configure the check to violate variables named with a single underscore if they are not used:


<module name="Checker">
  <module name="TreeWalker">
    <module name="UnusedLocalVariable">
        <property name="allowUnnamedVariables" value="false"/>
    </module>
  </module>
</module>

Example:


class Example1 {
  {
    int k = 12; // violation, assign and update but never use 'k'
    k++;
  }

  int a;
  Example1(int a) { // ok, as 'a' is a constructor parameter not a local variable
    this.a = 12;
  }

  void method(int b) {
    int[] arr = {1, 2, 3};  // violation, unused local variable 'arr'
    int[] anotherArr = {1}; // ok, 'anotherArr' is accessed
    anotherArr[0] = 4;
  }

  String convertValue(String newValue) {
    String s = newValue.toLowerCase(); // violation, unused local variable 's'
    String _ = newValue.toLowerCase(); // violation, unused local variable '_'
    return newValue.toLowerCase();
  }

  void read() throws IOException {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String s; // violation, unused local variable 's'
    while ((s = reader.readLine()) != null) {}
    try (BufferedReader reader1 = // ok, 'reader1' is a resource
                 new BufferedReader(new FileReader("abc.txt"))) {}
    try {
    } catch (Exception e) { }  // ok, 'e' is an exception parameter
  }

  void loops() {
    int j = 12;
    for (int i = 0; j < 11; i++)  // violation, unused local variable 'i'
      for (int p = 0; j < 11; p++) p /= 2;   // ok, 'p' is used
    for (Integer _ : new  int[0]) { } // violation, unused local variable '_'
  }

  void lambdas() {
    Predicate<String> obj = (String str) -> true; // ok, 'str' is a lambda parameter
    obj.test("Test");
  }
}

Example of Usage

Checkstyle Style

Violation Messages

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