001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2024 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 * <p>
029 * Implements Joshua Bloch, Effective Java, Item 17 -
030 * Use Interfaces only to define types.
031 * </p>
032 * <p>
033 * According to Bloch, an interface should describe a <em>type</em>. It is therefore
034 * inappropriate to define an interface that does not contain any methods
035 * but only constants. The Standard interface
036 * <a href="https://docs.oracle.com/javase/8/docs/api/javax/swing/SwingConstants.html">
037 * javax.swing.SwingConstants</a> is an example of an interface that would be flagged by this check.
038 * </p>
039 * <p>
040 * The check can be configured to also disallow marker interfaces like {@code java.io.Serializable},
041 * that do not contain methods or constants at all.
042 * </p>
043 * <ul>
044 * <li>
045 * Property {@code allowMarkerInterfaces} - Control whether marker interfaces
046 * like Serializable are allowed.
047 * Type is {@code boolean}.
048 * Default value is {@code true}.
049 * </li>
050 * </ul>
051 * <p>
052 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
053 * </p>
054 * <p>
055 * Violation Message Keys:
056 * </p>
057 * <ul>
058 * <li>
059 * {@code interface.type}
060 * </li>
061 * </ul>
062 *
063 * @since 3.1
064 */
065@StatelessCheck
066public final class InterfaceIsTypeCheck
067        extends AbstractCheck {
068
069    /**
070     * A key is pointing to the warning message text in "messages.properties"
071     * file.
072     */
073    public static final String MSG_KEY = "interface.type";
074
075    /** Control whether marker interfaces like Serializable are allowed. */
076    private boolean allowMarkerInterfaces = true;
077
078    @Override
079    public int[] getDefaultTokens() {
080        return getRequiredTokens();
081    }
082
083    @Override
084    public int[] getRequiredTokens() {
085        return new int[] {TokenTypes.INTERFACE_DEF};
086    }
087
088    @Override
089    public int[] getAcceptableTokens() {
090        return getRequiredTokens();
091    }
092
093    @Override
094    public void visitToken(DetailAST ast) {
095        final DetailAST objBlock =
096                ast.findFirstToken(TokenTypes.OBJBLOCK);
097        final DetailAST methodDef =
098                objBlock.findFirstToken(TokenTypes.METHOD_DEF);
099        final DetailAST variableDef =
100                objBlock.findFirstToken(TokenTypes.VARIABLE_DEF);
101        final boolean methodRequired =
102                !allowMarkerInterfaces || variableDef != null;
103
104        if (methodDef == null && methodRequired) {
105            log(ast, MSG_KEY);
106        }
107    }
108
109    /**
110     * Setter to control whether marker interfaces like Serializable are allowed.
111     *
112     * @param flag whether to allow marker interfaces or not
113     * @since 3.1
114     */
115    public void setAllowMarkerInterfaces(boolean flag) {
116        allowMarkerInterfaces = flag;
117    }
118
119}