UnusedLocalVariable
Since Checkstyle 9.3
Description
Checks that a local variable is declared and/or assigned, but not used. Supports pattern variables. 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:
public 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:
public class Example2 {
{
int k = 12; // violation, assign and update but never use 'k'
k++;
}
int a;
Example2(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 with pattern variable:
<module name="Checker">
<module name="TreeWalker">
<module name="UnusedLocalVariable">
</module>
</module>
</module>
Example:
public class Example3 {
sealed abstract static class Shape permits Circle, Rect {}
static final class Circle extends Shape {}
static final class Rect extends Shape {}
void patternVariables(Object obj, Shape s) {
if (obj instanceof String str) {
} // violation above, unused named local variable 'str'
if (obj instanceof String t) { // ok, 't' is used
System.out.println(t);
}
if (obj instanceof String _) { // ok, '_' is unnamed variable
System.out.println("string");
}
switch (s) { // violation below, unused named local variable 'c'
case Circle c -> System.out.println("circle");
case Rect r -> System.out.println(r); // ok, 'r' is used
default -> { }
}
}
}
Example:
<module name="Checker">
<module name="TreeWalker">
<module name="UnusedLocalVariable">
<property name="allowUnnamedVariables" value="false"/>
</module>
</module>
</module>
Example:
public class Example4 {
sealed abstract static class Shape permits Circle, Rect {}
static final class Circle extends Shape {}
static final class Rect extends Shape {}
void patternVariables(Object obj, Shape s) {
if (obj instanceof String str) { // violation, unused local variable 'str'
System.out.println("string");
}
if (obj instanceof String t) { // ok, 't' is used
System.out.println(t);
}
if (obj instanceof String _) { // ok, pattern variable '_' is always skipped
System.out.println("string");
}
switch (s) { // violation below, unused local variable 'c'
case Circle c -> System.out.println("circle");
case Rect r -> System.out.println(r); // ok, 'r' is used
default -> { }
}
}
}
Example of Usage
Violation Messages
All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.
Fully Qualified Name
com.puppycrawl.tools.checkstyle.checks.coding.UnusedLocalVariableCheck
Use this fully qualified class name in configuration when an exact class reference is required.