源码超度：String、StringBuffer、StringBuilder

    /**     * The value is used for character storage.     */    byte[] value;

Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings.Because String objects are immutable they can be shared

 * 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.

 * 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.

    /**     * Initializes a newly created {@code String} object so that it represents     * an empty character sequence.  Note that use of this constructor is     * unnecessary since Strings are immutable.     */    public String() {        this.value = "".value;        this.coder = "".coder;    }

    /**     * Creates an AbstractStringBuilder of the specified capacity.     */    AbstractStringBuilder(int capacity) {        if (COMPACT_STRINGS) {            value = new byte[capacity];            coder = LATIN1;        } else {            value = StringUTF16.newBytesFor(capacity);            coder = UTF16;        }    }

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

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