DiamondOperatorForVariableDefinitionCheck.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.github.sevntu.checkstyle.checks.coding;

import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;

/**
 * This Check highlights variable definition statements where <a href=
 * "http://docs.oracle.com/javase/7/docs/technotes/guides/language/type-inference-generic-instance-creation.html">
 * diamond operator</a> could be used.<br>
 * <b>Rationale</b>: using diamond operator (introduced in Java 1.7) leads to shorter code<br>
 * and better code readability. It is suggested by Oracle that the diamond primarily using<br>
 * for variable declarations.<br><br>
 * E.g. of statements:
 *
 * <p><b>Without diamond operator:</b><br><code>
 * Map<String, Map<String, Integer>> someMap =
 *     new HashMap<String, Map<String, Integer>>();</code><br>
 * <b>With diamond operator:</b><br>
 * <code>
 * Map<String, Map<String, Integer>> someMap = new HashMap<>();
 * </code>
 * </p>
 * @author <a href="mailto:nesterenko-aleksey@list.ru">Aleksey Nesterenko</a>
 */
public class DiamondOperatorForVariableDefinitionCheck extends AbstractCheck {

    /** A key is pointing to the warning message text in "messages.properties" file. */
    public static final String MSG_KEY = "diamond.operator.for.variable.definition";

    @Override
    public int[] getDefaultTokens() {
        return new int[] {TokenTypes.VARIABLE_DEF};
    }

    @Override
    public void visitToken(DetailAST variableDefNode) {

        final DetailAST assignNode = variableDefNode.findFirstToken(TokenTypes.ASSIGN);

        if (assignNode != null) {

            final DetailAST newNode = assignNode.getFirstChild().getFirstChild();

            // we check only creation by NEW
            if (newNode.getType() == TokenTypes.LITERAL_NEW) {

                final DetailAST variableDefNodeType =
                        variableDefNode.findFirstToken(TokenTypes.TYPE);
                final DetailAST varDefArguments = getFirstTypeArgumentsToken(variableDefNodeType);

                // generics has to be on left side
                if (varDefArguments != null
                        && newNode.getLastChild().getType() != TokenTypes.OBJBLOCK
                        // arrays can not be generics
                        && newNode.findFirstToken(TokenTypes.ARRAY_DECLARATOR) == null) {

                    final DetailAST typeArgs = getFirstTypeArgumentsToken(newNode);

                    if (varDefArguments.equalsTree(typeArgs)) {
                        log(typeArgs, MSG_KEY);
                    }
                }
            }
        }
    }

    /**
     * Get first occurrence of TYPE_ARGUMENTS if exists.
     * @param rootToken the token to start search from.
     * @return TYPE_ARGUMENTS token if found.
     */
    private static DetailAST getFirstTypeArgumentsToken(DetailAST rootToken) {
        DetailAST resultNode = rootToken.getFirstChild();

        if (resultNode != null) {
            if (resultNode.getType() == TokenTypes.DOT) {
                resultNode = resultNode.getFirstChild().getNextSibling();
            }
            final DetailAST childNode = getFirstTypeArgumentsToken(resultNode);

            if (childNode == null) {
                resultNode = resultNode.getNextSibling();
            }
        }

        return resultNode;
    }

}