JAVA_String、StringBuffer、StringBuilder_三者差異與記憶體內部運行機制介紹

JAVA_String、StringBuffer、StringBuilder_三者差異與記憶體內部運行機制介紹_雙等號與equals差別

- 4月 05, 2018

【結論】

效能上
StringBulder(無Thread-Safe)
快於 StringBuffer(有Thread-Safe)
快於 String(會於內部轉為StringBuffer進行較多記憶體運算)

當你在做一些純粹針對值或內容比對是否一樣時
請愛用 .equals()
盡量不要用雙等號
雙等號會多判斷是否參照到相同物件

【細部實作過程推導】
日常JAVA開發工作中最常碰到的就是我們所謂的
字串的相關處裡

這次要先來介紹關於

String 相關物件的運作差異
=============================================================
1. String
2. StringBuffer
3. StringBuilder
=============================================================

1. String 宣告寫法

在一般String 宣告時
多數JAVA程式設計師
會直接用來作為變數指派

在JAVA 官方API 文件上
可以查到其實它有許多建構子的多載

這邊先來看一段作為一般變數時指派內部運作細節

code 區塊

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package stringtest1;

/**
 *
 * @author chous
 */
public class StringTest1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        System.out.println("==========String物件作為一般變數指派=============");
        String s = "Hello";
        System.out.println("s:" + s);
        String s1 = "Hello";
        System.out.println("s1:" +s1);
        String s2 = "Hi";
        System.out.println("s2:" +s2);
        System.out.println("s == s1: " + (s == s1) );
        System.out.println("s.equals(s1): " + s.equals(s1));
    }
    
}

當你指派好一個字串內容值給String變數值時
記憶體內部會配置一個記憶體空間存放對應結果
那由於是採用一般變數指派所以會存於共享資源池的記憶體中

當你指派的值有重複時則會直接指向相同記憶體空間位址
所以目前無論是 s 或 s1 都是指向相同memory location

只有在指派不同內容時才會生出一個全新記憶體空間

並指向新位址

到底剛剛所用的雙等號與 equals 差別在哪呢??

這邊我們在嘗試使用 JAVA 作為物件實體來指派值運作結果輸出

code區塊

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package javastr1;

/**
 *
 * @author chous
 */
public class JAVAStr1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        System.out.println("==========String物件作為一般變數指派=============");
        String s = "Hello";
        System.out.println("s:" + s);
        String s1 = "Hello";
        System.out.println("s1:" +s1);
        String s2 = "Hi";
        System.out.println("s2:" +s2);
        System.out.println("s == s1: " + (s == s1) );
        System.out.println("s.equals(s1): " + s.equals(s1));
        
        
        System.out.println("==========String物件作為物件實體生成(不同記憶體位址)=============");
        System.out.println("雙等號(會看記憶體位址)_equals(只要值相同就好)");
        String strObj1 = new String("Hello");
        System.out.println("strObj1:  " + strObj1);
        
        System.out.println("s == strObj1: " +  (s == strObj1));
        System.out.println("s.equals(strObj1): " + s.equals(strObj1));
        System.out.println("=======================");
        String strObj2 = new String("Hello");
        System.out.println("strObj2:  " + strObj2);
        System.out.println("strObj1 == strObj2: " + (strObj1 == strObj2));
        System.out.println("strObj1.equals(strObj2): " + strObj1.equals(strObj2));
        
    }
    
}

當String 作為物件來使用
則會因為創出不同實體而有不同的物件記憶體位址也被配置出來
所以若在此使用雙等號即可明確看出差異

到底String 和 StringBuffer 差別在哪
何時用哪個??

事實上 String 物件是不可變動的!!!!
String Object is immutable !!!

而 StringBuffer 是可異動、同步更新的!!

StringBuffer Object is mutable !!!

code 區塊

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package javastrtest2;

/**
 *
 * @author chous
 */
public class JAVAStrTest2 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        String s = new String("Hello");
        System.out.println("s:" + s +" ,s的記憶體位址(物件編碼):" + s.hashCode());
        s+= "Hi";
        System.out.println("s:" + s+" ,s的記憶體位址(物件編碼):" + s.hashCode());
        
        
        System.out.println("===============================");
        StringBuffer s2 = new StringBuffer("Hello");
        System.out.println("s2:" + s2 + ",s2的記憶體位址(物件編碼):" + s2.hashCode());
        s2.append("Hi");
        System.out.println("s2:" + s2 + ",s2的記憶體位址(物件編碼):" + s2.hashCode());
    }
    
}

這邊我們用字串串接為示意範本!!!

String 物件做串接使用 +=

StringBuffer 物件則使用 .append()

這邊事實上我們無法在JAVA中透過內定方法獲取實際記憶體位址
頂多只能使用 hashcode 回傳的物件編碼
來判斷參照到的物件是否一致

這邊會看到運行出來結果實際上
StringBuffer一直都是指向同一物件

然而
String物件
其實在內部已經又生出另一個新位址來存放串接後的內容結果
並指向另一個新位址

String 和 StringBuffer 誰比較快呢???

事實上JAVA String 做字串串接時
表面上看語法是透過簡單 "加號" 來實踐
和 StringBuffer 物件使用到的 .append()方法
看似一樣效果

實際底層運作過程
String 在使用 "+" 做字串串接時
1.會先轉為 StringBuffer 物件
2.再去call一次 .append() 方法
3.最後再將StringBuffer 資料型態轉為 String

所以你會看到它比StringBuffer於記憶體內部做了更多動作
因此較為耗時!!!!!
StringBuffer 於記憶體底層運算會比String 更精簡

StringBuffer 和 StringBuilder 差異

事實上若有看過JAVA 官方說明裡頭
你會發現這兩個物件的方法大同小異
彼此若於實體化時給予空值
預設皆設定16為大小

CODE 區塊

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package teststrapp;

/**
 *
 * @author chous
 */
public class TestStrApp {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here    
        System.out.println("=================StringBuilder預設資料空間&字串長===================");
        StringBuilder sbd0 = new StringBuilder();
        System.out.println("StringBuilder初始空內容(sbd0):"+ sbd0.toString());
        System.out.println("StringBuilder預設資料空間大小(sbd0):" + sbd0.capacity());
        System.out.println("StringBuilder預設字串長度(sbd0):" + sbd0.length());
        System.out.println("=================StringBuffer預設資料空間&字串長===================");
        StringBuffer sbf0 = new StringBuffer();
        System.out.println("StringBuffer初始空內容(sbf0):"+ sbf0.toString());
        System.out.println("StringBuffer預設資料空間大小(sbf0):" + sbf0.capacity());
        System.out.println("StringBuffer預設字串長度(sbf0):" + sbf0.length());
        System.out.println("=================StringBuilder串接新字元後之資料空間&字串長更動===================");
        sbd0.append("a");
        System.out.println("StringBuilder內容(sbd0):"+ sbd0.toString());
        System.out.println("StringBuilder資料空間大小(sbd0):" + sbd0.capacity());
        System.out.println("StringBuilder字串長度(sbd0):" + sbd0.length());
        sbd0.append("b");
        System.out.println("StringBuilder內容(sbd0):"+ sbd0.toString());
        System.out.println("StringBuilder資料空間大小(sbd0):" + sbd0.capacity());
        System.out.println("StringBuilder字串長度(sbd0):" + sbd0.length());
        sbd0.append("c");
        System.out.println("StringBuilder內容(sbd0):"+ sbd0.toString());
        System.out.println("StringBuilder資料空間大小(sbd0):" + sbd0.capacity());
        System.out.println("StringBuilder字串長度(sbd0):" + sbd0.length());
        System.out.println("=================StringBuffer串接新字元後之資料空間&字串長更動===================");
        sbf0.append("a");
        System.out.println("StringBuffer內容(sbf0):"+ sbf0.toString());
        System.out.println("StringBuffer資料空間大小(sbf0):" + sbf0.capacity());
        System.out.println("StringBuffer字串長度(sbf0):" + sbf0.length());
        sbf0.append("b");
        System.out.println("StringBuffer內容(sbf0):"+ sbf0.toString());
        System.out.println("StringBuffer資料空間大小(sbf0):" + sbf0.capacity());
        System.out.println("StringBuffer字串長度(sbf0):" + sbf0.length());
        sbf0.append("c");
        System.out.println("StringBuffer內容(sbf0):"+ sbf0.toString());
        System.out.println("StringBuffer資料空間大小(sbf0):" + sbf0.capacity());
        System.out.println("StringBuffer字串長度(sbf0):" + sbf0.length());

        System.out.println("=================StringBuilder字串反序===================");
        StringBuilder sbd1 = new StringBuilder("ABC");
        System.out.println("StringBuilder 資料大小(sbd1):" +sbd1.capacity());
        System.out.println("StringBuilder 字串長(sbd1):" +sbd1.length());
        System.out.println("StringBuilder(sbd1):" + sbd1.toString());
        sbd1.reverse();
        System.out.println("StringBuilder 反轉後(sbd1):" +sbd1.toString());
        System.out.println("=================StringBuffer字串反序===================");
        StringBuffer sbf1 = new StringBuffer("ABC");
        System.out.println("StringBuffer 資料大小(sbf1):" +sbf1.capacity());
        System.out.println("StringBuffer 字串長(sbf1):" +sbf1.length());
        System.out.println("StringBuffer(sbf1):" +sbf1.toString());
        sbf1.reverse();
        System.out.println("StringBuffer 反轉後(sbf1):" +sbf1.toString());
    }
    
}

差異在於

StringBuilder 屬於 Thread-Not-Safe 底層則沒有使用Synchronized機制
StringBuffer 屬於 Thread-Safe 底層代碼會使用Synchronized避免死結發生

StringBuilder 底層code(沒有任何Synchronized字眼)

/*
 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang;


/**
 * A mutable sequence of characters.  This class provides an API compatible
 * with {@code StringBuffer}, but with no guarantee of synchronization.
 * This class is designed for use as a drop-in replacement for
 * {@code StringBuffer} in places where the string buffer was being
 * used by a single thread (as is generally the case).   Where possible,
 * it is recommended that this class be used in preference to
 * {@code StringBuffer} as it will be faster under most implementations.
 *
 * <p>The principal operations on a {@code StringBuilder} are the
 * {@code append} and {@code insert} 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 builder. The
 * {@code append} method always adds these characters at the end
 * of the builder; the {@code insert} method adds the characters at
 * a specified point.
 * <p>
 * For example, if {@code z} refers to a string builder object
 * whose current contents are "{@code start}", then
 * the method call {@code z.append("le")} would cause the string
 * builder to contain "{@code startle}", whereas
 * {@code z.insert(4, "le")} would alter the string builder to
 * contain "{@code starlet}".
 * <p>
 * In general, if sb refers to an instance of a {@code StringBuilder},
 * then {@code sb.append(x)} has the same effect as
 * {@code sb.insert(sb.length(), x)}.
 * <p>
 * Every string builder has a capacity. As long as the length of the
 * character sequence contained in the string builder does not exceed
 * the capacity, it is not necessary to allocate a new internal
 * buffer. If the internal buffer overflows, it is automatically made larger.
 *
 * <p>Instances of {@code StringBuilder} are not safe for
 * use by multiple threads. If such synchronization is required then it is
 * recommended that {@link java.lang.StringBuffer} be used.
 *
 * <p>Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 *
 * @author      Michael McCloskey
 * @see         java.lang.StringBuffer
 * @see         java.lang.String
 * @since       1.5
 */
public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    /** use serialVersionUID for interoperability */
    static final long serialVersionUID = 4383685877147921099L;

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

    /**
     * Constructs a string builder with no characters in it and an
     * initial capacity specified by the {@code capacity} argument.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *               argument is less than {@code 0}.
     */
    public StringBuilder(int capacity) {
        super(capacity);
    }

    /**
     * Constructs a string builder initialized to the contents of the
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    /**
     * Constructs a string builder that contains the same characters
     * as the specified {@code CharSequence}. The initial capacity of
     * the string builder is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     *
     * @param      seq   the sequence to copy.
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

    @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

    /**
     * Appends the specified {@code StringBuffer} to this sequence.
     * <p>
     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to this sequence, increasing the
     * length of this sequence by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this sequence.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at index
     * <i>k</i> in the new character sequence is equal to the character at
     * index <i>k</i> in the old character sequence, if <i>k</i> is less than
     * <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
     * in the argument {@code sb}.
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     */
    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }

    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);
        return this;
    }

    /**
     * @throws     IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }

    @Override
    public StringBuilder append(char[] str) {
        super.append(str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(char[] str, int offset, int len) {
        super.append(str, offset, len);
        return this;
    }

    @Override
    public StringBuilder append(boolean b) {
        super.append(b);
        return this;
    }

    @Override
    public StringBuilder append(char c) {
        super.append(c);
        return this;
    }

    @Override
    public StringBuilder append(int i) {
        super.append(i);
        return this;
    }

    @Override
    public StringBuilder append(long lng) {
        super.append(lng);
        return this;
    }

    @Override
    public StringBuilder append(float f) {
        super.append(f);
        return this;
    }

    @Override
    public StringBuilder append(double d) {
        super.append(d);
        return this;
    }

    /**
     * @since 1.5
     */
    @Override
    public StringBuilder appendCodePoint(int codePoint) {
        super.appendCodePoint(codePoint);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder delete(int start, int end) {
        super.delete(start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder deleteCharAt(int index) {
        super.deleteCharAt(index);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder replace(int start, int end, String str) {
        super.replace(start, end, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int index, char[] str, int offset,
                                int len)
    {
        super.insert(index, str, offset, len);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, Object obj) {
            super.insert(offset, obj);
            return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, String str) {
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char[] str) {
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s) {
            super.insert(dstOffset, s);
            return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s,
                                int start, int end)
    {
        super.insert(dstOffset, s, start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, boolean b) {
        super.insert(offset, b);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char c) {
        super.insert(offset, c);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, int i) {
        super.insert(offset, i);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, long l) {
        super.insert(offset, l);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, float f) {
        super.insert(offset, f);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, double d) {
        super.insert(offset, d);
        return this;
    }

    @Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }

    @Override
    public int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }

    @Override
    public int lastIndexOf(String str) {
        return super.lastIndexOf(str);
    }

    @Override
    public int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }

    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }

    /**
     * Save the state of the {@code StringBuilder} instance to a stream
     * (that is, serialize it).
     *
     * @serialData the number of characters currently stored in the string
     *             builder ({@code int}), followed by the characters in the
     *             string builder ({@code char[]}).   The length of the
     *             {@code char} array may be greater than the number of
     *             characters currently stored in the string builder, in which
     *             case extra characters are ignored.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }

}

StringBuffer 底層code(有Synchronized字眼)

/*
 * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang;

import java.util.Arrays;

/**
 * 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} are the
 * {@code append} and {@code insert} 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} method always adds these characters at the end
 * of the buffer; the {@code insert} method adds the characters at
 * a specified point.
 * <p>
 * For example, if {@code z} refers to a string buffer object
 * whose current contents are {@code "start"}, then
 * the method call {@code z.append("le")} would cause the string
 * buffer to contain {@code "startle"}, whereas
 * {@code z.insert(4, "le")} would alter the string buffer to
 * contain {@code "starlet"}.
 * <p>
 * In general, if sb refers to an instance of a {@code StringBuffer},
 * then {@code sb.append(x)} has the same effect as
 * {@code sb.insert(sb.length(), x)}.
 * <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.
 * Note that while {@code StringBuffer} is designed to be safe to use
 * concurrently from multiple threads, if the constructor or the
 * {@code append} or {@code insert} operation is passed a source sequence
 * that is shared across threads, the calling code must ensure
 * that the operation has a consistent and unchanging view of the source
 * sequence for the duration of the operation.
 * This could be satisfied by the caller holding a lock during the
 * operation's call, by using an immutable source sequence, or by not
 * sharing the source sequence across threads.
 * <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.
 * <p>
 * Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 * <p>
 * As of  release JDK 5, this class has been supplemented with an equivalent
 * class designed for use by a single thread, {@link StringBuilder}.  The
 * {@code StringBuilder} 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
 * @see     java.lang.StringBuilder
 * @see     java.lang.String
 * @since   JDK1.0
 */
 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    /**
     * A cache of the last value returned by toString. Cleared
     * whenever the StringBuffer is modified.
     */
    private transient char[] toStringCache;

    /** 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}
     *               argument is less than {@code 0}.
     */
    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} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuffer(String str) {
        super(str.length() + 16);
        append(str);
    }

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

    @Override
    public synchronized int length() {
        return count;
    }

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


    @Override
    public synchronized void ensureCapacity(int minimumCapacity) {
        super.ensureCapacity(minimumCapacity);
    }

    /**
     * @since      1.5
     */
    @Override
    public synchronized void trimToSize() {
        super.trimToSize();
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized void setLength(int newLength) {
        toStringCache = null;
        super.setLength(newLength);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }

    /**
     * @since      1.5
     */
    @Override
    public synchronized int codePointAt(int index) {
        return super.codePointAt(index);
    }

    /**
     * @since     1.5
     */
    @Override
    public synchronized int codePointBefore(int index) {
        return super.codePointBefore(index);
    }

    /**
     * @since     1.5
     */
    @Override
    public synchronized int codePointCount(int beginIndex, int endIndex) {
        return super.codePointCount(beginIndex, endIndex);
    }

    /**
     * @since     1.5
     */
    @Override
    public synchronized int offsetByCodePoints(int index, int codePointOffset) {
        return super.offsetByCodePoints(index, codePointOffset);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,
                                      int dstBegin)
    {
        super.getChars(srcBegin, srcEnd, dst, dstBegin);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized void setCharAt(int index, char ch) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        toStringCache = null;
        value[index] = ch;
    }

    @Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }

    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }

    /**
     * Appends the specified {@code StringBuffer} to this sequence.
     * <p>
     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to the contents of this {@code StringBuffer}, increasing the
     * length of this {@code StringBuffer} by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this {@code StringBuffer}.
     * <p>
     * Let <i>n</i> be the length of the old character sequence, the one
     * contained in the {@code StringBuffer} just prior to execution of the
     * {@code append} method. Then the character at index <i>k</i> in
     * the new character sequence is equal to the character at index <i>k</i>
     * in the old character sequence, if <i>k</i> is less than <i>n</i>;
     * otherwise, it is equal to the character at index <i>k-n</i> in the
     * argument {@code sb}.
     * <p>
     * This method synchronizes on {@code this}, the destination
     * object, but does not synchronize on the source ({@code sb}).
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     * @since 1.4
     */
    public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }

    /**
     * @since 1.8
     */
    @Override
    synchronized StringBuffer append(AbstractStringBuilder asb) {
        toStringCache = null;
        super.append(asb);
        return this;
    }

    /**
     * Appends the specified {@code CharSequence} to this
     * sequence.
     * <p>
     * The characters of the {@code CharSequence} argument are appended,
     * in order, increasing the length of this sequence by the length of the
     * argument.
     *
     * <p>The result of this method is exactly the same as if it were an
     * invocation of this.append(s, 0, s.length());
     *
     * <p>This method synchronizes on {@code this}, the destination
     * object, but does not synchronize on the source ({@code s}).
     *
     * <p>If {@code s} is {@code null}, then the four characters
     * {@code "null"} are appended.
     *
     * @param   s the {@code CharSequence} to append.
     * @return  a reference to this object.
     * @since 1.5
     */
    @Override
    public synchronized StringBuffer append(CharSequence s) {
        toStringCache = null;
        super.append(s);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public synchronized StringBuffer append(CharSequence s, int start, int end)
    {
        toStringCache = null;
        super.append(s, start, end);
        return this;
    }

    @Override
    public synchronized StringBuffer append(char[] str) {
        toStringCache = null;
        super.append(str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer append(char[] str, int offset, int len) {
        toStringCache = null;
        super.append(str, offset, len);
        return this;
    }

    @Override
    public synchronized StringBuffer append(boolean b) {
        toStringCache = null;
        super.append(b);
        return this;
    }

    @Override
    public synchronized StringBuffer append(char c) {
        toStringCache = null;
        super.append(c);
        return this;
    }

    @Override
    public synchronized StringBuffer append(int i) {
        toStringCache = null;
        super.append(i);
        return this;
    }

    /**
     * @since 1.5
     */
    @Override
    public synchronized StringBuffer appendCodePoint(int codePoint) {
        toStringCache = null;
        super.appendCodePoint(codePoint);
        return this;
    }

    @Override
    public synchronized StringBuffer append(long lng) {
        toStringCache = null;
        super.append(lng);
        return this;
    }

    @Override
    public synchronized StringBuffer append(float f) {
        toStringCache = null;
        super.append(f);
        return this;
    }

    @Override
    public synchronized StringBuffer append(double d) {
        toStringCache = null;
        super.append(d);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer delete(int start, int end) {
        toStringCache = null;
        super.delete(start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer deleteCharAt(int index) {
        toStringCache = null;
        super.deleteCharAt(index);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer replace(int start, int end, String str) {
        toStringCache = null;
        super.replace(start, end, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized String substring(int start) {
        return substring(start, count);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.4
     */
    @Override
    public synchronized CharSequence subSequence(int start, int end) {
        return super.substring(start, end);
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized String substring(int start, int end) {
        return super.substring(start, end);
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer insert(int index, char[] str, int offset,
                                            int len)
    {
        toStringCache = null;
        super.insert(index, str, offset, len);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, Object obj) {
        toStringCache = null;
        super.insert(offset, String.valueOf(obj));
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, String str) {
        toStringCache = null;
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, char[] str) {
        toStringCache = null;
        super.insert(offset, str);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public StringBuffer insert(int dstOffset, CharSequence s) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        // after narrowing of s to specific type
        // Ditto for toStringCache clearing
        super.insert(dstOffset, s);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public synchronized StringBuffer insert(int dstOffset, CharSequence s,
            int start, int end)
    {
        toStringCache = null;
        super.insert(dstOffset, s, start, end);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public  StringBuffer insert(int offset, boolean b) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of b to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, b);
        return this;
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, char c) {
        toStringCache = null;
        super.insert(offset, c);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, int i) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of i to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, i);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, long l) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of l to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, l);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, float f) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of f to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, f);
        return this;
    }

    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, double d) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of d to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, d);
        return this;
    }

    /**
     * @since      1.4
     */
    @Override
    public int indexOf(String str) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        return super.indexOf(str);
    }

    /**
     * @since      1.4
     */
    @Override
    public synchronized int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }

    /**
     * @since      1.4
     */
    @Override
    public int lastIndexOf(String str) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        return lastIndexOf(str, count);
    }

    /**
     * @since      1.4
     */
    @Override
    public synchronized int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }

    /**
     * @since   JDK1.0.2
     */
    @Override
    public synchronized StringBuffer reverse() {
        toStringCache = null;
        super.reverse();
        return this;
    }

    @Override
    public synchronized String toString() {
        if (toStringCache == null) {
            toStringCache = Arrays.copyOfRange(value, 0, count);
        }
        return new String(toStringCache, true);
    }

    /**
     * Serializable fields for StringBuffer.
     *
     * @serialField value  char[]
     *              The backing character array of this StringBuffer.
     * @serialField count int
     *              The number of characters in this StringBuffer.
     * @serialField shared  boolean
     *              A flag indicating whether the backing array is shared.
     *              The value is ignored upon deserialization.
     */
    private static final java.io.ObjectStreamField[] serialPersistentFields =
    {
        new java.io.ObjectStreamField("value", char[].class),
        new java.io.ObjectStreamField("count", Integer.TYPE),
        new java.io.ObjectStreamField("shared", Boolean.TYPE),
    };

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        java.io.ObjectOutputStream.PutField fields = s.putFields();
        fields.put("value", value);
        fields.put("count", count);
        fields.put("shared", false);
        s.writeFields();
    }

    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        java.io.ObjectInputStream.GetField fields = s.readFields();
        value = (char[])fields.get("value", null);
        count = fields.get("count", 0);
    }
}

所謂的 Thread-Safe 事實上就是去避免同一時間點
會有一個以上的執行緒執行同一段任務而發生衝突

換言之，就是同一時間
只允許一個Thread針對物件進行存取處裡

所以StringBuffer會等待而導致其效能較StringBuilder慢一點

參考連結:
Java 超級新手學習筆記 - == & equals
http://sweeteason.pixnet.net/blog/post/33710656-java-超級新手學習筆記---%3D%3D-%26-equals

【程式】JAVA的Object物件
http://bbkb.pixnet.net/blog/post/10150123-【程式】java的object物件

【工作經驗知識分享】

在工作中跟在學校很大差別在於
當你在交辦事情時
盡量要簡潔並就重點論述

工作場合針對交代事項並不看重推倒過程
只看中結果、結論

盡量於交代事情或詢問時
是給予選擇題(採用哪個方案)而避免申論題
老闆並沒有這麼多美國時間!!!

過程或是相關Study文件
可以作為附件待上頭有空時做瀏覽

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第25個冬天

JAVA_String、StringBuffer、StringBuilder_三者差異與記憶體內部運行機制介紹_雙等號與equals差別

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