StringBuffer and StringBuilder summary

StringBuffer 和 StringBuilder

Introduction

In most cases, faster than StringBuffer String fast; faster than StringBuffer StringBuilder; StringBuffer and StringBuilder are subclasses of AbstractStringBuilder, except that most of them have synchronized StringBuffer methods modified.

Source resolve

AbstractStringBuilder

Variables and Constructor

/**
 * 用来存储字符的数组
 * The value is used for character storage.
 */
char[] value;

/**
 * 字符个数 
 * The count is the number of characters used.
 */
int count;

/**
 * This no-arg constructor is necessary for serialization of subclasses.
 */
AbstractStringBuilder() {
}

/**
 * 在构造方法中指定字符数组的长度
 * Creates an AbstractStringBuilder of the specified capacity.
 */
AbstractStringBuilder(int capacity) {
    value = new char[capacity];
}
    

Expansion

public void ensureCapacity(int minimumCapacity) {
    if (minimumCapacity > 0)
        ensureCapacityInternal(minimumCapacity);
}

private void ensureCapacityInternal(int minimumCapacity) {
    // overflow-conscious code
    if (minimumCapacity - value.length > 0) {
        value = Arrays.copyOf(value,
                newCapacity(minimumCapacity));
    }
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

private int newCapacity(int minCapacity) {
    // overflow-conscious code
    int newCapacity = (value.length << 1) + 2;
    if (newCapacity - minCapacity < 0) {
        newCapacity = minCapacity;
    }
    return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
        ? hugeCapacity(minCapacity)
        : newCapacity;
}

private int hugeCapacity(int minCapacity) {
    if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
        throw new OutOfMemoryError();
    }
    return (minCapacity > MAX_ARRAY_SIZE)
        ? minCapacity : MAX_ARRAY_SIZE;
}

The method of expansion by the ultimate newCapacity()implementation, a first capacity left (i.e., 2-fold expansion) while adding 2, if any case smaller than specified, then the capacity will be set minimumCapacity. It is then determined whether the overflow, through hugeCapacityimplementation, if the overflow (greater than length Integer.MAX_VALUE), then throw error ( OutOfMemoryError); otherwise, according to minCapacityand Integer.MAX_VALUE - 8the magnitude of comparison determination array capacity max(minCapacity, Integer.MAX_VALUE - 8). Finally, the valuevalue of the copy, this step is clearly the most time-consuming operation.

append () method

public AbstractStringBuilder append(String str) {
    if (str == null)
        return appendNull();
    int len = str.length();
    ensureCapacityInternal(count + len);
    str.getChars(0, len, value, count);
    count += len;
    return this;
}

append()Is the most commonly used method, it has many forms of overloading. The above is the most commonly used, for appending the string. If strShi null, the direct call
appendNull()method. This method is the direct additional 'n', 'u', 'l', 'l'several characters, as follows:

private AbstractStringBuilder appendNull() {
    int c = count;
    ensureCapacityInternal(c + 4);
    final char[] value = this.value;
    value[c++] = 'n';
    value[c++] = 'u';
    value[c++] = 'l';
    value[c++] = 'l';
    count = c;
    return this;
}

If not null, you first need to determine whether the array capacity is sufficient, it is not required expansion (expansion of the expansion method is to call the above analysis);
then call Stringa getChars()method to strappend to valuethe end;
finally returns the object itself, it append()can continuously invoke (a kind of similar to the chain programmed).

Think

• Why every time expansion is the expansion to twice the original?
  Personally I feel that in order to avoid the cost of expansion is often caused by consumption.
• Why do plus 2?
  Personally I thought what a good explanation, that may be because Javadevelopers think our appendtime data center often add a separator, the separator in just Javaexactly occupies two bytes. Do not know the right analysis, there are other opinions bigwigs can issue
  be discussed in.

Method comparison and StringBuffer StringBuilder

By looking at the source code analysis found both to inherit AbstractStringBuilder. And StringBufferthe reason why is thread-safe, because rewrite AbstractStringBuildertime method preceded by a synchronziedmodification of these methods; and StringBuilderrewriting, when just directly call the parent class method, do not do other operations.

In fact discovered by reading the source code StringBuilderand StringBufferthe relationship between the similar HashMapand HashTablethe relationship between.