001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2025 the original author or authors.
004//
005// This library is free software; you can redistribute it and/or
006// modify it under the terms of the GNU Lesser General Public
007// License as published by the Free Software Foundation; either
008// version 2.1 of the License, or (at your option) any later version.
009//
010// This library is distributed in the hope that it will be useful,
011// but WITHOUT ANY WARRANTY; without even the implied warranty of
012// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
013// Lesser General Public License for more details.
014//
015// You should have received a copy of the GNU Lesser General Public
016// License along with this library; if not, write to the Free Software
017// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
018///////////////////////////////////////////////////////////////////////////////////////////////
019
020package com.puppycrawl.tools.checkstyle.checks.design;
021
022import com.puppycrawl.tools.checkstyle.StatelessCheck;
023import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
024import com.puppycrawl.tools.checkstyle.api.DetailAST;
025import com.puppycrawl.tools.checkstyle.api.TokenTypes;
026
027/**
028 * <div>
029 * Implements Joshua Bloch, Effective Java, Item 17 -
030 * Use Interfaces only to define types.
031 * </div>
032 *
033 * <p>
034 * According to Bloch, an interface should describe a <em>type</em>. It is therefore
035 * inappropriate to define an interface that does not contain any methods
036 * but only constants. The Standard interface
037 * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">
038 * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check.
039 * </p>
040 *
041 * <p>
042 * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable},
043 * that do not contain methods or constants at all.
044 * </p>
045 *
046 * @since 3.1
047 */
048@StatelessCheck
049public final class InterfaceIsTypeCheck
050        extends AbstractCheck {
051
052    /**
053     * A key is pointing to the warning message text in "messages.properties"
054     * file.
055     */
056    public static final String MSG_KEY = "interface.type";
057
058    /** Control whether marker interfaces like Serializable are allowed. */
059    private boolean allowMarkerInterfaces = true;
060
061    @Override
062    public int[] getDefaultTokens() {
063        return getRequiredTokens();
064    }
065
066    @Override
067    public int[] getRequiredTokens() {
068        return new int[] {TokenTypes.INTERFACE_DEF};
069    }
070
071    @Override
072    public int[] getAcceptableTokens() {
073        return getRequiredTokens();
074    }
075
076    @Override
077    public void visitToken(DetailAST ast) {
078        final DetailAST objBlock =
079                ast.findFirstToken(TokenTypes.OBJBLOCK);
080        final DetailAST methodDef =
081                objBlock.findFirstToken(TokenTypes.METHOD_DEF);
082        final DetailAST variableDef =
083                objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
084        final boolean methodRequired =
085                !allowMarkerInterfaces || variableDef != null;
086
087        if (methodDef == null && methodRequired) {
088            log(ast, MSG_KEY);
089        }
090    }
091
092    /**
093     * Setter to control whether marker interfaces like Serializable are allowed.
094     *
095     * @param flag whether to allow marker interfaces or not
096     * @since 3.1
097     */
098    public void setAllowMarkerInterfaces(boolean flag) {
099        allowMarkerInterfaces = flag;
100    }
101
102}