MissingSwitchDefaultCheck.java
///////////////////////////////////////////////////////////////////////////////////////////////
// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
// Copyright (C) 2001-2022 the original author or authors.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
///////////////////////////////////////////////////////////////////////////////////////////////
package com.puppycrawl.tools.checkstyle.checks.coding;
import com.puppycrawl.tools.checkstyle.StatelessCheck;
import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;
import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
/**
* <p>
* Checks that switch statement has a {@code default} clause.
* </p>
* <p>
* Rationale: It's usually a good idea to introduce a
* default case in every switch statement. Even if
* the developer is sure that all currently possible
* cases are covered, this should be expressed in the
* default branch, e.g. by using an assertion. This way
* the code is protected against later changes, e.g.
* introduction of new types in an enumeration type.
* </p>
* <p>
* This check does not validate any switch expressions. Rationale:
* The compiler requires switch expressions to be exhaustive. This means
* that all possible inputs must be covered.
* </p>
* <p>
* This check does not validate switch statements that use pattern or null
* labels. Rationale: Switch statements that use pattern or null labels are
* checked by the compiler for exhaustiveness. This means that all possible
* inputs must be covered.
* </p>
* <p>
* See the <a href="https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.28">
* Java Language Specification</a> for more information about switch statements
* and expressions.
* </p>
* <p>
* To configure the check:
* </p>
* <pre>
* <module name="MissingSwitchDefault"/>
* </pre>
* <p>Example of violation:</p>
* <pre>
* switch (i) { // violation
* case 1:
* break;
* case 2:
* break;
* }
* </pre>
* <p>Example of correct code:</p>
* <pre>
* switch (i) {
* case 1:
* break;
* case 2:
* break;
* default: // OK
* break;
* }
* switch (o) {
* case String s: // type pattern
* System.out.println(s);
* break;
* case Integer i: // type pattern
* System.out.println("Integer");
* break;
* default: // will not compile without default label, thanks to type pattern label usage
* break;
* }
* </pre>
* <p>Example of correct code which does not require default labels:</p>
* <pre>
* sealed interface S permits A, B, C {}
* final class A implements S {}
* final class B implements S {}
* record C(int i) implements S {} // Implicitly final
*
* /**
* * The completeness of a switch statement can be
* * determined by the contents of the permits clause
* * of interface S. No default label or default case
* * label is allowed by the compiler in this situation, so
* * this check does not enforce a default label in such
* * statements.
* */
* static void showSealedCompleteness(S s) {
* switch (s) {
* case A a: System.out.println("A"); break;
* case B b: System.out.println("B"); break;
* case C c: System.out.println("C"); break;
* }
* }
*
* /**
* * A total type pattern matches all possible inputs,
* * including null. A default label or
* * default case is not allowed by the compiler in this
* * situation. Accordingly, check does not enforce a
* * default label in this case.
* */
* static void showTotality(String s) {
* switch (s) {
* case Object o: // total type pattern
* System.out.println("o!");
* }
* }
*
* enum Color { RED, GREEN, BLUE }
*
* static int showSwitchExpressionExhaustiveness(Color color) {
* switch (color) {
* case RED: System.out.println("RED"); break;
* case BLUE: System.out.println("BLUE"); break;
* case GREEN: System.out.println("GREEN"); break;
* // Check will require default label below, compiler
* // does not enforce a switch statement with constants
* // to be complete.
* default: System.out.println("Something else");
* }
*
* // Check will not require default label in switch
* // expression below, because code will not compile
* // if all possible values are not handled. If the
* // 'Color' enum is extended, code will fail to compile.
* return switch (color) {
* case RED:
* yield 1;
* case GREEN:
* yield 2;
* case BLUE:
* yield 3;
* };
* }
* </pre>
* <p>
* Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
* </p>
* <p>
* Violation Message Keys:
* </p>
* <ul>
* <li>
* {@code missing.switch.default}
* </li>
* </ul>
*
* @since 3.1
*/
@StatelessCheck
public class MissingSwitchDefaultCheck extends AbstractCheck {
/**
* A key is pointing to the warning message text in "messages.properties"
* file.
*/
public static final String MSG_KEY = "missing.switch.default";
@Override
public int[] getDefaultTokens() {
return getRequiredTokens();
}
@Override
public int[] getAcceptableTokens() {
return getRequiredTokens();
}
@Override
public int[] getRequiredTokens() {
return new int[] {TokenTypes.LITERAL_SWITCH};
}
@Override
public void visitToken(DetailAST ast) {
if (!containsDefaultLabel(ast)
&& !containsPatternCaseLabelElement(ast)
&& !containsDefaultCaseLabelElement(ast)
&& !isSwitchExpression(ast)) {
log(ast, MSG_KEY);
}
}
/**
* Checks if the case group or its sibling contain the 'default' switch.
*
* @param detailAst first case group to check.
* @return true if 'default' switch found.
*/
private static boolean containsDefaultLabel(DetailAST detailAst) {
return TokenUtil.findFirstTokenByPredicate(detailAst,
ast -> ast.findFirstToken(TokenTypes.LITERAL_DEFAULT) != null
).isPresent();
}
/**
* Checks if a switch block contains a case label with a pattern variable definition.
* In this situation, the compiler enforces the given switch block to cover
* all possible inputs, and we do not need a default label.
*
* @param detailAst first case group to check.
* @return true if switch block contains a pattern case label element
*/
private static boolean containsPatternCaseLabelElement(DetailAST detailAst) {
return TokenUtil.findFirstTokenByPredicate(detailAst, ast -> {
return ast.getFirstChild() != null
&& ast.getFirstChild().findFirstToken(TokenTypes.PATTERN_VARIABLE_DEF) != null;
}).isPresent();
}
/**
* Checks if a switch block contains a default case label.
*
* @param detailAst first case group to check.
* @return true if switch block contains default case label
*/
private static boolean containsDefaultCaseLabelElement(DetailAST detailAst) {
return TokenUtil.findFirstTokenByPredicate(detailAst, ast -> {
return ast.getFirstChild() != null
&& ast.getFirstChild().findFirstToken(TokenTypes.LITERAL_DEFAULT) != null;
}).isPresent();
}
/**
* Checks if this LITERAL_SWITCH token is part of a switch expression.
*
* @param ast the switch statement we are checking
* @return true if part of a switch expression
*/
private static boolean isSwitchExpression(DetailAST ast) {
return ast.getParent().getType() == TokenTypes.EXPR
|| ast.getParent().getParent().getType() == TokenTypes.EXPR;
}
}