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 * <ul>
046 * <li>
047 * Property {@code allowMarkerInterfaces} - Control whether marker interfaces
048 * like Serializable are allowed.
049 * Type is {@code boolean}.
050 * Default value is {@code true}.
051 * </li>
052 * </ul>
053 *
054 * <p>
055 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
056 * </p>
057 *
058 * <p>
059 * Violation Message Keys:
060 * </p>
061 * <ul>
062 * <li>
063 * {@code interface.type}
064 * </li>
065 * </ul>
066 *
067 * @since 3.1
068 */
069@StatelessCheck
070public final class InterfaceIsTypeCheck
071        extends AbstractCheck {
072
073    /**
074     * A key is pointing to the warning message text in "messages.properties"
075     * file.
076     */
077    public static final String MSG_KEY = "interface.type";
078
079    /** Control whether marker interfaces like Serializable are allowed. */
080    private boolean allowMarkerInterfaces = true;
081
082    @Override
083    public int[] getDefaultTokens() {
084        return getRequiredTokens();
085    }
086
087    @Override
088    public int[] getRequiredTokens() {
089        return new int[] {TokenTypes.INTERFACE_DEF};
090    }
091
092    @Override
093    public int[] getAcceptableTokens() {
094        return getRequiredTokens();
095    }
096
097    @Override
098    public void visitToken(DetailAST ast) {
099        final DetailAST objBlock =
100                ast.findFirstToken(TokenTypes.OBJBLOCK);
101        final DetailAST methodDef =
102                objBlock.findFirstToken(TokenTypes.METHOD_DEF);
103        final DetailAST variableDef =
104                objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
105        final boolean methodRequired =
106                !allowMarkerInterfaces || variableDef != null;
107
108        if (methodDef == null && methodRequired) {
109            log(ast, MSG_KEY);
110        }
111    }
112
113    /**
114     * Setter to control whether marker interfaces like Serializable are allowed.
115     *
116     * @param flag whether to allow marker interfaces or not
117     * @since 3.1
118     */
119    public void setAllowMarkerInterfaces(boolean flag) {
120        allowMarkerInterfaces = flag;
121    }
122
123}