src/main/java/com/puppycrawl/tools/checkstyle/checks/coding/SimplifyBooleanReturnCheck.java

////////////////////////////////////////////////////////////////////////////////
// checkstyle: Checks Java source code for adherence to a set of rules.
// Copyright (C) 2001-2017 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 antlr.collections.AST;
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;

/**
 * <p>
 * Checks for overly complicated boolean return statements.
 * Idea shamelessly stolen from the equivalent PMD rule (pmd.sourceforge.net).
 * </p>
 * <p>
 * An example of how to configure the check is:
 * </p>
 * <pre>
 * &lt;module name="SimplifyBooleanReturn"/&gt;
 * </pre>
 * @author Lars Kühne
 */
@StatelessCheck
public class SimplifyBooleanReturnCheck
    extends AbstractCheck {

    /**
     * A key is pointing to the warning message text in "messages.properties"
     * file.
     */
    public static final String MSG_KEY = "simplify.boolReturn";

    @Override
    public int[] getAcceptableTokens() {
        return new int[] {TokenTypes.LITERAL_IF};
    }

    @Override
    public int[] getDefaultTokens() {
        return getAcceptableTokens();
    }

    @Override
    public int[] getRequiredTokens() {
        return getAcceptableTokens();
    }

    @Override
    public void visitToken(DetailAST ast) {
        // LITERAL_IF has the following four or five children:
        // '('
        // condition
        // ')'
        // thenStatement
        // [ LITERAL_ELSE (with the elseStatement as a child) ]

        // don't bother if this is not if then else
        final AST elseLiteral =
            ast.findFirstToken(TokenTypes.LITERAL_ELSE);
        if (elseLiteral != null) {
            final AST elseStatement = elseLiteral.getFirstChild();

            // skip '(' and ')'
            final AST condition = ast.getFirstChild().getNextSibling();
            final AST thenStatement = condition.getNextSibling().getNextSibling();

            if (canReturnOnlyBooleanLiteral(thenStatement)
                && canReturnOnlyBooleanLiteral(elseStatement)) {
                log(ast.getLineNo(), ast.getColumnNo(), MSG_KEY);
            }
        }
    }

    /**
     * Returns if an AST is a return statement with a boolean literal
     * or a compound statement that contains only such a return statement.
     *
     * <p>Returns {@code true} iff ast represents
     * <br/>
     * <pre>
     * return true/false;
     * </pre>
     * or
     * <br/>
     * <pre>
     * {
     *   return true/false;
     * }
     * </pre>
     *
     * @param ast the syntax tree to check
     * @return if ast is a return statement with a boolean literal.
     */
    private static boolean canReturnOnlyBooleanLiteral(AST ast) {
        boolean result = true;
        if (!isBooleanLiteralReturnStatement(ast)) {
            final AST firstStatement = ast.getFirstChild();
            result = isBooleanLiteralReturnStatement(firstStatement);
        }
        return result;
    }

    /**
     * Returns if an AST is a return statement with a boolean literal.
     *
     * <p>Returns {@code true} iff ast represents
     * <br/>
     * <pre>
     * return true/false;
     * </pre>
     *
     * @param ast the syntax tree to check
     * @return if ast is a return statement with a boolean literal.
     */
    private static boolean isBooleanLiteralReturnStatement(AST ast) {
        boolean booleanReturnStatement = false;

        if (ast != null && ast.getType() == TokenTypes.LITERAL_RETURN) {
            final AST expr = ast.getFirstChild();

            if (expr.getType() != TokenTypes.SEMI) {
                final AST value = expr.getFirstChild();
                booleanReturnStatement = isBooleanLiteralType(value.getType());
            }
        }
        return booleanReturnStatement;
    }

    /**
     * Checks if a token type is a literal true or false.
     * @param tokenType the TokenType
     * @return true iff tokenType is LITERAL_TRUE or LITERAL_FALSE
     */
    private static boolean isBooleanLiteralType(final int tokenType) {
        final boolean isTrue = tokenType == TokenTypes.LITERAL_TRUE;
        final boolean isFalse = tokenType == TokenTypes.LITERAL_FALSE;
        return isTrue || isFalse;
    }
}