Constant.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 */
package org.apache.bcel.classfile;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.Serializable;

import org.apache.bcel.Constants;
import org.apache.bcel.util.BCELComparator;

/**
 * Abstract superclass for classes to represent the different constant types
 * in the constant pool of a class file. The classes keep closely to
 * the JVM specification.
 *
 * @version $Id: Constant.java 1590062 2014-04-25 15:44:42Z ebourg $
 * @author  <A HREF="mailto:m.dahm@gmx.de">M. Dahm</A>
 */
public abstract class Constant implements Cloneable, Node, Serializable {

    private static final long serialVersionUID = 5739037344085356353L;
    private static BCELComparator _cmp = new BCELComparator() {

        public boolean equals( Object o1, Object o2 ) {
            Constant THIS = (Constant) o1;
            Constant THAT = (Constant) o2;
            return THIS.toString().equals(THAT.toString());
        }


        public int hashCode( Object o ) {
            Constant THIS = (Constant) o;
            return THIS.toString().hashCode();
        }
    };
    /* In fact this tag is redundant since we can distinguish different
     * `Constant' objects by their type, i.e., via `instanceof'. In some
     * places we will use the tag for switch()es anyway.
     *
     * First, we want match the specification as closely as possible. Second we
     * need the tag as an index to select the corresponding class name from the 
     * `CONSTANT_NAMES' array.
     */
    protected byte tag;


    Constant(byte tag) {
        this.tag = tag;
    }


    /**
     * Called by objects that are traversing the nodes of the tree implicitely
     * defined by the contents of a Java class. I.e., the hierarchy of methods,
     * fields, attributes, etc. spawns a tree of objects.
     *
     * @param v Visitor object
     */
    public abstract void accept( Visitor v );


    public abstract void dump( DataOutputStream file ) throws IOException;


    /**
     * @return Tag of constant, i.e., its type. No setTag() method to avoid
     * confusion.
     */
    public final byte getTag() {
        return tag;
    }


    /**
     * @return String representation.
     */
    @Override
    public String toString() {
        return Constants.CONSTANT_NAMES[tag] + "[" + tag + "]";
    }


    /**
     * @return deep copy of this constant
     */
    public Constant copy() {
        try {
            return (Constant) super.clone();
        } catch (CloneNotSupportedException e) {
        }
        return null;
    }


    @Override
    public Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            throw new Error("Clone Not Supported"); // never happens
        }
    }


    /**
     * Read one constant from the given file, the type depends on a tag byte.
     *
     * @param file Input stream
     * @return Constant object
     */
    static Constant readConstant( DataInputStream file ) throws IOException,
            ClassFormatException {
        byte b = file.readByte(); // Read tag byte
        switch (b) {
            case Constants.CONSTANT_Class:
                return new ConstantClass(file);
            case Constants.CONSTANT_Fieldref:
                return new ConstantFieldref(file);
            case Constants.CONSTANT_Methodref:
                return new ConstantMethodref(file);
            case Constants.CONSTANT_InterfaceMethodref:
                return new ConstantInterfaceMethodref(file);
            case Constants.CONSTANT_String:
                return new ConstantString(file);
            case Constants.CONSTANT_Integer:
                return new ConstantInteger(file);
            case Constants.CONSTANT_Float:
                return new ConstantFloat(file);
            case Constants.CONSTANT_Long:
                return new ConstantLong(file);
            case Constants.CONSTANT_Double:
                return new ConstantDouble(file);
            case Constants.CONSTANT_NameAndType:
                return new ConstantNameAndType(file);
            case Constants.CONSTANT_Utf8:
                return ConstantUtf8.getInstance(file);
            case Constants.CONSTANT_MethodHandle:
                return new ConstantMethodHandle(file);
            case Constants.CONSTANT_MethodType:
                return new ConstantMethodType(file);
            case Constants.CONSTANT_InvokeDynamic:
                return new ConstantInvokeDynamic(file);
            default:
                throw new ClassFormatException("Invalid byte tag in constant pool: " + b);
        }
    }


    /**
     * @return Comparison strategy object
     */
    public static BCELComparator getComparator() {
        return _cmp;
    }


    /**
     * @param comparator Comparison strategy object
     */
    public static void setComparator( BCELComparator comparator ) {
        _cmp = comparator;
    }


    /**
     * Return value as defined by given BCELComparator strategy.
     * By default two Constant objects are said to be equal when
     * the result of toString() is equal.
     * 
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals( Object obj ) {
        return _cmp.equals(this, obj);
    }


    /**
     * Return value as defined by given BCELComparator strategy.
     * By default return the hashcode of the result of toString().
     * 
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        return _cmp.hashCode(this);
    }
}