Java基础知识强化47：StringBuffer类之StringBuffer的三个面试题

1. 面试题：String，StringBuffer，StringBuilder的区别 ？

答：String是字符串内容不可变的，而StringBuffer和StringBuilder是字符串内容长度可变的；

      StringBuffer是同步的，数据安全，效率低。

      StringBuilder是不同步的，数据不安全，效率高。

（1）. 在执行速度方面的比较：StringBuilder > StringBuffer
（2）. StringBuffer与StringBuilder，他们是字符串变量，是可改变的对象，每当我们用它们对字符串做操作时，实际上是在一个对象上操作的，不像String一样创建一些对象进行操作，所以速度就快了。
（3）. StringBuilder：线程非安全的
　       StringBuffer：线程安全的
      当我们在字符串缓冲去被多个线程使用是，JVM不能保证StringBuilder的操作是安全的，虽然他的速度最快，但是可以保证StringBuffer是可以正确操作的。当然大多数情况下就是我们是在单线程下进行的操作，所以大多数情况下是建议用StringBuilder而不用StringBuffer的，就是速度的原因。

对于三者使用的总结：• 如果要操作少量的数据用 = String
　　　　　　            • 单线程操作字符串缓冲区 下操作大量数据 = StringBuilder
　　　　　　            • 多线程操作字符串缓冲区 下操作大量数据 = StringBuffer

注意：为什么StringBuffer具有数据安全的特性?

答：观察源码，如下：

 /*
  * @(#)StringBuffer.java    1.101 05/11/17
  *
  * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  */

 package java.lang;

 /**
  * A thread-safe, mutable sequence of characters.
  * A string buffer is like a {@link String}, but can be modified. At any
  * point in time it contains some particular sequence of characters, but
  * the length and content of the sequence can be changed through certain
  * method calls.
  * <p>
  * String buffers are safe for use by multiple threads. The methods
  * are synchronized where necessary so that all the operations on any
  * particular instance behave as if they occur in some serial order
  * that is consistent with the order of the method calls made by each of
  * the individual threads involved.
  * <p>
  * The principal operations on a <code>StringBuffer</code> are the
  * <code>append</code> and <code>insert</code> methods, which are
  * overloaded so as to accept data of any type. Each effectively
  * converts a given datum to a string and then appends or inserts the
  * characters of that string to the string buffer. The
  * <code>append</code> method always adds these characters at the end
  * of the buffer; the <code>insert</code> method adds the characters at
  * a specified point.
  * <p>
  * For example, if <code>z</code> refers to a string buffer object
  * whose current contents are "<code>start</code>", then
  * the method call <code>z.append("le")</code> would cause the string
  * buffer to contain "<code>startle</code>", whereas
  * <code>z.insert(4, "le")</code> would alter the string buffer to
  * contain "<code>starlet</code>".
  * <p>
  * In general, if sb refers to an instance of a <code>StringBuffer</code>,
  * then <code>sb.append(x)</code> has the same effect as
  * <code>sb.insert(sb.length(), x)</code>.
  * <p>
  * Whenever an operation occurs involving a source sequence (such as
  * appending or inserting from a source sequence) this class synchronizes
  * only on the string buffer performing the operation, not on the source.
  * <p>
  * Every string buffer has a capacity. As long as the length of the
  * character sequence contained in the string buffer does not exceed
  * the capacity, it is not necessary to allocate a new internal
  * buffer array. If the internal buffer overflows, it is
  * automatically made larger.
  *
  * As of  release JDK 5, this class has been supplemented with an equivalent
  * class designed for use by a single thread, {@link StringBuilder}.  The
  * <tt>StringBuilder</tt> class should generally be used in preference to
  * this one, as it supports all of the same operations but it is faster, as
  * it performs no synchronization.
  *
  * @author    Arthur van Hoff
  * @version     1.101, 11/17/05
  * @see     java.lang.StringBuilder
  * @see     java.lang.String
  * @since   JDK1.0
  */
  public final class StringBuffer
     extends AbstractStringBuilder
     implements java.io.Serializable, CharSequence
 {

     /** use serialVersionUID from JDK 1.0.2 for interoperability */
     static final long serialVersionUID = 3388685877147921107L;

     /**
      * Constructs a string buffer with no characters in it and an
      * initial capacity of 16 characters.
      */
     public StringBuffer() {
     super(16);
     }

     /**
      * Constructs a string buffer with no characters in it and
      * the specified initial capacity.
      *
      * @param      capacity  the initial capacity.
      * @exception  NegativeArraySizeException  if the <code>capacity</code>
      *               argument is less than <code>0</code>.
      */
     public StringBuffer(int capacity) {
     super(capacity);
     }

     /**
      * Constructs a string buffer initialized to the contents of the
      * specified string. The initial capacity of the string buffer is
      * <code>16</code> plus the length of the string argument.
      *
      * @param   str   the initial contents of the buffer.
      * @exception NullPointerException if <code>str</code> is <code>null</code>
      */
     public StringBuffer(String str) {
     super(str.length() + 16);
     append(str);
     }

     /**
      * Constructs a string buffer that contains the same characters
      * as the specified <code>CharSequence</code>. The initial capacity of
      * the string buffer is <code>16</code> plus the length of the
      * <code>CharSequence</code> argument.
      * <p>
      * If the length of the specified <code>CharSequence</code> is
      * less than or equal to zero, then an empty buffer of capacity
      * <code>16</code> is returned.
      *
      * @param      seq   the sequence to copy.
      * @exception NullPointerException if <code>seq</code> is <code>null</code>
      * @since 1.5
      */
     public StringBuffer(CharSequence seq) {
         this(seq.length() + 16);
         append(seq);
     }

     public synchronized int length() {
     return count;
     }

     public synchronized int capacity() {
     return value.length;
     }

     ...

因为很多方法都是synchronized 

2. 面试题：StringBuffer 和数组的区别？

答：二者都可以看出是一个容器，装其他的数据。但是呢，StringBuffer的数据最终是一个字符串数据，而数组可以放置任意类型的同一种数据。

3. 面试题：形式参数问题。

String作为参数传递

StringBuffer作为参数传递

形式参数：

               基本类型：形式参数的改变不影响实际参数

               引用类型：形式参数的改变直接影响实际参数

案例演示：

package cn.itcast_08;

/*
 * 形式参数问题：
 * String作为参数传递
 * StringBuffer作为参数传递
 *
 * 形式参数：
 *         基本类型：形式参数的改变不影响实际参数
 *         引用类型：形式参数的改变直接影响实际参数
 *
 * 注意：
 *         String作为参数传递，效果和基本类型作为参数传递是一样的。
 */
public class StringBufferDemo {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "world";
        System.out.println(s1 + "---" + s2);// hello---world
        change(s1, s2);
        System.out.println(s1 + "---" + s2);// hello---world

        StringBuffer sb1 = new StringBuffer("hello");
        StringBuffer sb2 = new StringBuffer("world");
        System.out.println(sb1 + "---" + sb2);// hello---world
        change(sb1, sb2);
        System.out.println(sb1 + "---" + sb2);// hello---worldworld

    }

    public static void change(StringBuffer sb1, StringBuffer sb2) {
        sb1 = sb2;
        sb2.append(sb1);
    }

    public static void change(String s1, String s2) {
        s1 = s2;
        s2 = s1 + s2;
    }
}

运行效果如下：