StringBuffer簡介

• StringBufferjich
• 類似於String類，但StringBuffer是執行緒安全的可變字串類；
• StringBuffer具有擴容機制，當需要儲存字元的容量不大於當前得value陣列長度，不會擴容，否則自動擴成當前需要的最小容量；
• 從jdk5開始，新增的StringBuilder基本可StringBuffer一致，但是StringBuilder是非執行緒安全的；

StringBuffer UML

StringBuffer API

StringBuffer 原始碼

package java.lang;

import java.util.Arrays;
 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    //儲存字元傳的的陣列,若修改字串時該資料就會被清空，只儲存陣列最後一次修改的值
    private transient char[] toStringCache;
    static final long serialVersionUID = 3388685877147921107L;
    //初始化繼承的陣列value大小16
    public StringBuffer() {
        super(16);
    }
    //自定義value陣列大小
    public StringBuffer(int capacity) {
        super(capacity);
    }
    //構造一個StringBuffer初始化容量為具體的字元產長度+16
    public StringBuffer(String str) {
        super(str.length() + 16);
        append(str);
    }
    //構造一個StringBuffer初始化容量為具體的字元產長度+16
    public StringBuffer(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
    //返回實際存放的字元數
    @Override
    public synchronized int length() {
        return count;
    }
    // 返回value陣列的長度
    @Override
    public synchronized int capacity() {
        return value.length;
    }
    @Override
    public synchronized void ensureCapacity(int minimumCapacity) {
        super.ensureCapacity(minimumCapacity);
    }
    //value收縮為實際含的字元數
    @Override
    public synchronized void trimToSize() {
        super.trimToSize();
    }
    // 重新設定陣列長度
    @Override
    public synchronized void setLength(int newLength) {
        toStringCache = null;
        super.setLength(newLength);
    }
    // 返回value陣列下標值
    @Override
    public synchronized char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }
    //返回指定索引字元的Unicode碼
    @Override
    public synchronized int codePointAt(int index) {
        return super.codePointAt(index);
    }

    // 返回指定索引的前一位索引位置的字元的unicode碼 = codePointAt(int index-1)
    @Override
    public synchronized int codePointBefore(int index) {
        return super.codePointBefore(index);
    }

    // 計演算法從[beginIndex,endIndex)之間共多少unicode碼
    @Override
    public synchronized int codePointCount(int beginIndex, int endIndex) {
        return super.codePointCount(beginIndex, endIndex);
    }
    // 
    @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);
    }

    // 先將toStringCache陣列清空，在更新value[index]位置的字元
    @Override
    public synchronized void setCharAt(int index, char ch) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        toStringCache = null;
        value[index] = ch;
    }
    // 先將toStringCache陣列清空，在value上追加新的字串，
    @Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }
    // 先將toStringCache陣列清空，在value上追加新的字串 
    @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
     */
    //將指定的stringbuffer加入
    public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }
  
    @Override
    synchronized StringBuffer append(AbstractStringBuilder asb) {
        toStringCache = null;
        super.append(asb);
        return this;
    }
    @Override
    public synchronized StringBuffer append(CharSequence s) {
        toStringCache = null;
        super.append(s);
        return this;
    }
    @Override
    public synchronized StringBuffer append(char[] str) {
        toStringCache = null;
        super.append(str);
        return this;
    }

    @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;
    }
    @Override
    public synchronized StringBuffer delete(int start, int end) {
        toStringCache = null;
        super.delete(start, end);
        return this;
    }

    @Override
    public synchronized StringBuffer deleteCharAt(int index) {
        toStringCache = null;
        super.deleteCharAt(index);
        return this;
    }
    @Override
    public synchronized StringBuffer replace(int start, int end, String str) {
        toStringCache = null;
        super.replace(start, end, str);
        return this;
    }
    @Override
    public synchronized String substring(int start) {
        return substring(start, count);
    }

    @Override
    public synchronized CharSequence subSequence(int start, int end) {
        return super.substring(start, end);
    }

    @Override
    public synchronized String substring(int start, int end) {
        return super.substring(start, end);
    }

    @Override
    public synchronized StringBuffer insert(int index, char[] str, int offset,
                                            int len)
    {
        toStringCache = null;
        super.insert(index, str, offset, len);
        return this;
    }

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

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

    @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;
    }

    @Override
    public synchronized StringBuffer insert(int dstOffset, CharSequence s,
            int start, int end)
    {
        toStringCache = null;
        super.insert(dstOffset, s, start, end);
        return this;
    }

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

    @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;
    }
    // toStirng 方法將value中儲存的字元拷貝到toStringCache,保障了對同一個示例的操作，都在同一塊記憶體，即保證了StringBuffer為可變字串類
    @Override
    public synchronized String toString() {
        if (toStringCache == null) {
            toStringCache = Arrays.copyOfRange(value, 0, count);
        }
        return new String(toStringCache, true);
    }

    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),
    };

    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();
    }
    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);
    }
}

 

可變字串機制的解釋

面試session

• StringBuffer StringBuilder String 的異同和使用場景？