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理解String 及 String.intern() 在实际中的应用

2017-03-27 13:31 351 查看
1. 首先String不属于8种基本数据类型,String是一个对象。 

  因为对象的默认值是null,所以String的默认值也是null;但它又是一种特殊的对象,有其它对象没有的一些特性。 

  2. new String()和new String(“”)都是申明一个新的空字符串,是空串不是null; 

  3. String str=”kvill”; 

String str=new String (“kvill”);的区别: 

  在这里,我们不谈堆,也不谈栈,只先简单引入常量池这个简单的概念。 

  常量池(constant pool)指的是在编译期被确定,并被保存在已编译的.class文件中的一些数据。它包括了关于类、方法、接口等中的常量,也包括字符串常量。 

  看例1: 

String s0=”kvill”; 

String s1=”kvill”; 

String s2=”kv” + “ill”; 

System.out.println( s0==s1 ); 

System.out.println( s0==s2 );  

  结果为: 

true 

true  

  首先,我们要知道Java会确保一个字符串常量只有一个拷贝。 

  因为例子中的s0和s1中的”kvill”都是字符串常量,它们在编译期就被确定了,所以s0==s1为true;而”kv”和”ill”也都是字符串常量,当一个字符串由多个字符串常量连接而成时,它自己肯定也是字符串常量,所以s2也同样在编译期就被解析为一个字符串常量,所以s2也是常量池中”kvill”的一个引用。 

  所以我们得出s0==s1==s2; 

  用new String() 创建的字符串不是常量,不能在编译期就确定,所以new String() 创建的字符串不放入常量池中,它们有自己的地址空间。 

  看例2: 

String s0=”kvill”; 

String s1=new String(”kvill”); 

String s2=”kv” + new String(“ill”); 

System.out.println( s0==s1 ); 

System.out.println( s0==s2 ); 

System.out.println( s1==s2 );  

  结果为: 

false 

false 

false  

  例2中s0还是常量池中”kvill”的应用,s1因为无法在编译期确定,所以是运行时创建的新对象”kvill”的引用,s2因为有后半部分new String(“ill”)所以也无法在编译期确定,所以也是一个新创建对象”kvill”的应用;明白了这些也就知道为何得出此结果了。 

  4. String.intern(): 

  再补充介绍一点:存在于.class文件中的常量池,在运行期被JVM装载,并且可以扩充。String的intern()方法就是扩充常量池的一个方法;当一个String实例str调用intern()方法时,Java查找常量池中是否有相同Unicode的字符串常量,如果有,则返回其的引用,如果没有,则在常量池中增加一个Unicode等于str的字符串并返回它的引用;看例3就清楚了 

  例3: 

String s0= “kvill”; 

String s1=new String(”kvill”); 

String s2=new String(“kvill”); 

System.out.println( s0==s1 ); 

System.out.println( “**********” ); 

s1.intern(); 

s2=s2.intern(); //把常量池中“kvill”的引用赋给s2 

System.out.println( s0==s1); 

System.out.println( s0==s1.intern() ); 

System.out.println( s0==s2 );  

  结果为: 

false 

********** 

false //虽然执行了s1.intern(),但它的返回值没有赋给s1 

true //说明s1.intern()返回的是常量池中”kvill”的引用 

true  

  最后我再破除一个错误的理解: 

  有人说,“使用String.intern()方法则可以将一个String类的保存到一个全局String表中,如果具有相同值的Unicode字符串已经在这个表中,那么该方法返回表中已有字符串的地址,如果在表中没有相同值的字符串,则将自己的地址注册到表中“如果我把他说的这个全局的String表理解为常量池的话,他的最后一句话,“如果在表中没有相同值的字符串,则将自己的地址注册到表中”是错的: 

  看例4: 

String s1=new String("kvill"); 

String s2=s1.intern(); 

System.out.println( s1==s1.intern() ); 

System.out.println( s1+" "+s2 ); 

System.out.println( s2==s1.intern() );  

  结果: 

false 

kvill kvill 

true  

  在这个类中我们没有声名一个”kvill”常量,所以常量池中一开始是没有”kvill”的,当我们调用s1.intern()后就在常量池中新添加了一个”kvill”常量,原来的不在常量池中的”kvill”仍然存在,也就不是“将自己的地址注册到常量池中”了。 

  s1==s1.intern()为false说明原来的“kvill”仍然存在; 

  s2现在为常量池中“kvill”的地址,所以有s2==s1.intern()为true。 

  5. 关于equals()和==: 

  这个对于String简单来说就是比较两字符串的Unicode序列是否相当,如果相等返回true;而==是比较两字符串的地址是否相同,也就是是否是同一个字符串的引用。 

  6. 关于String是不可变的

  这一说又要说很多,大家只要知道String的实例一旦生成就不会再改变了,比如说:String str=”kv”+”ill”+” “+”ans”; 
就是有4个字符串常量,首先”kv”和”ill”生成了”kvill”存在内存中,然后”kvill”又和” “ 生成 ”kvill “存在内存中,最后又和生成了”kvill ans”;并把这个字符串的地址赋给了str,就是因为String的“不可变”产生了很多临时变量,这也就是为什么建议用StringBuffer的原因了,因为StringBuffer是可改变的

By the way,关于 String.intern() 在实际中的应用,我在tomcat的源码中找到了一个地方用到了,如下:

/*

 * Copyright 1999,2004-2005 The Apache Software Foundation.

 * 

 * Licensed 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.

 * ====================================================================

 *

 * This software consists of voluntary contributions made by many

 * individuals on behalf of the Apache Software Foundation and was

 * originally based on software copyright (c) 1999, International

 * Business Machines, Inc., http://www.apache.org.  For more

 * information on the Apache Software Foundation, please see

 * <http://www.apache.org/>.

 */

package org.apache.jasper.xmlparser;

/**

 * This class is a symbol table implementation that guarantees that

 * strings used as identifiers are unique references. Multiple calls

 * to <code>addSymbol</code> will always return the same string

 * reference.

 * <p>

 * The symbol table performs the same task as <code>String.intern()</code>

 * with the following differences:

 * <ul>

 *  <li>

 *   A new string object does not need to be created in order to

 *   retrieve a unique reference. Symbols can be added by using

 *   a series of characters in a character array.

 *  </li>

 *  <li>

 *   Users of the symbol table can provide their own symbol hashing

 *   implementation. For example, a simple string hashing algorithm

 *   may fail to produce a balanced set of hashcodes for symbols

 *   that are <em>mostly</em> unique. Strings with similar leading

 *   characters are especially prone to this poor hashing behavior.

 *  </li>

 * </ul>

 *

 * @author Andy Clark

 * @version $Id: SymbolTable.java 306179 2005-07-27 15:12:04Z yoavs $

 */

public class SymbolTable {

    //

    // Constants

    //

    /** Default table size. */

    protected static final int TABLE_SIZE = 101;

    //

    // Data

    //

    /** Buckets. */

    protected Entry[] fBuckets = null;

    // actual table size

    protected int fTableSize;

    //

    // Constructors

    //

    /** Constructs a symbol table with a default number of buckets. */

    public SymbolTable() {

        this(TABLE_SIZE);

    }

    /** Constructs a symbol table with a specified number of buckets. */

    public SymbolTable(int tableSize) {

        fTableSize = tableSize;

        fBuckets = new Entry[fTableSize];

    }

    //

    // Public methods

    //

    /**

     * Adds the specified symbol to the symbol table and returns a

     * reference to the unique symbol. If the symbol already exists,

     * the previous symbol reference is returned instead, in order

     * guarantee that symbol references remain unique.

     *

     * @param symbol The new symbol.

     */

    public String addSymbol(String symbol) {

        // search for identical symbol

        int bucket = hash(symbol) % fTableSize;

        int length = symbol.length();

        OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

            if (length == entry.characters.length) {

                for (int i = 0; i < length; i++) {

                    if (symbol.charAt(i) != entry.characters[i]) {

                        continue OUTER;

                    }

                }

                return entry.symbol;

            }

        }

        // create new entry

        Entry entry = new Entry(symbol, fBuckets[bucket]);

        fBuckets[bucket] = entry;

        return entry.symbol;

    } // addSymbol(String):String

    /**

     * Adds the specified symbol to the symbol table and returns a

     * reference to the unique symbol. If the symbol already exists,

     * the previous symbol reference is returned instead, in order

     * guarantee that symbol references remain unique.

     *

     * @param buffer The buffer containing the new symbol.

     * @param offset The offset into the buffer of the new symbol.

     * @param length The length of the new symbol in the buffer.

     */

    public String addSymbol(char[] buffer, int offset, int length) {

        // search for identical symbol

        int bucket = hash(buffer, offset, length) % fTableSize;

        OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

            if (length == entry.characters.length) {

                for (int i = 0; i < length; i++) {

                    if (buffer[offset + i] != entry.characters[i]) {

                        continue OUTER;

                    }

                }

                return entry.symbol;

            }

        }

        // add new entry

        Entry entry = new Entry(buffer, offset, length, fBuckets[bucket]);

        fBuckets[bucket] = entry;

        return entry.symbol;

    } // addSymbol(char[],int,int):String

    /**

     * Returns a hashcode value for the specified symbol. The value

     * returned by this method must be identical to the value returned

     * by the <code>hash(char[],int,int)</code> method when called

     * with the character array that comprises the symbol string.

     *

     * @param symbol The symbol to hash.

     */

    public int hash(String symbol) {

        int code = 0;

        int length = symbol.length();

        for (int i = 0; i < length; i++) {

            code = code * 37 + symbol.charAt(i);

        }

        return code & 0x7FFFFFF;

    } // hash(String):int

    /**

     * Returns a hashcode value for the specified symbol information.

     * The value returned by this method must be identical to the value

     * returned by the <code>hash(String)</code> method when called

     * with the string object created from the symbol information.

     *

     * @param buffer The character buffer containing the symbol.

     * @param offset The offset into the character buffer of the start

     *               of the symbol.

     * @param length The length of the symbol.

     */

    public int hash(char[] buffer, int offset, int length) {

        int code = 0;

        for (int i = 0; i < length; i++) {

            code = code * 37 + buffer[offset + i];

        }

        return code & 0x7FFFFFF;

    } // hash(char[],int,int):int

    /**

     * Returns true if the symbol table already contains the specified

     * symbol.

     *

     * @param symbol The symbol to look for.

     */

    public boolean containsSymbol(String symbol) {

        // search for identical symbol

        int bucket = hash(symbol) % fTableSize;

        int length = symbol.length();

        OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

            if (length == entry.characters.length) {

                for (int i = 0; i < length; i++) {

                    if (symbol.charAt(i) != entry.characters[i]) {

                        continue OUTER;

                    }

                }

                return true;

            }

        }

        return false;

    } // containsSymbol(String):boolean

    /**

     * Returns true if the symbol table already contains the specified

     * symbol.

     *

     * @param buffer The buffer containing the symbol to look for.

     * @param offset The offset into the buffer.

     * @param length The length of the symbol in the buffer.

     */

    public boolean containsSymbol(char[] buffer, int offset, int length) {

        // search for identical symbol

        int bucket = hash(buffer, offset, length) % fTableSize;

        OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

            if (length == entry.characters.length) {

                for (int i = 0; i < length; i++) {

                    if (buffer[offset + i] != entry.characters[i]) {

                        continue OUTER;

                    }

                }

                return true;

            }

        }

        return false;

    } // containsSymbol(char[],int,int):boolean

    //

    // Classes

    //

    /**

     * This class is a symbol table entry. Each entry acts as a node

     * in a linked list.

     */

    protected static final class Entry {

        //

        // Data

        //

        /** Symbol. */

        public String symbol;

        /**

         * Symbol characters. This information is duplicated here for

         * comparison performance.

         */

        public char[] characters;

        /** The next entry. */

        public Entry next;

        //

        // Constructors

        //

        /**

         * Constructs a new entry from the specified symbol and next entry

         * reference.

         */

        public Entry(String symbol, Entry next) {

            this.symbol = symbol.intern();

            characters = new char[symbol.length()];

            symbol.getChars(0, characters.length, characters, 0);

            this.next = next;

        }

        /**

         * Constructs a new entry from the specified symbol information and

         * next entry reference.

         */

        public Entry(char[] ch, int offset, int length, Entry next) {

            characters = new char[length];

            System.arraycopy(ch, offset, characters, 0, length);

            symbol = new String(characters).intern();

            this.next = next;

        }

    } // class Entry

} // class SymbolTable
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