Java 源码 AbstractStringBuilder 抽象类

介绍

A mutable sequence of characters.

示例

xxxxxx

结构

源码

成员变量

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

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

/**
 * The maximum size of array to allocate.
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

构造方法

/**
 * Creates an AbstractStringBuilder of the specified capacity.
 */
AbstractStringBuilder(int capacity) {
    value = new char[capacity];
}

成员方法

/**
 * Appends the specified string to this character sequence.
 */
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;
}

/**
 * Ensures that the capacity is at least equal to the specified minimum.
 */
private void ensureCapacityInternal(int minimumCapacity) {
  if (minimumCapacity - value.length > 0) {
    value = Arrays.copyOf(value, newCapacity(minimumCapacity));
  }
}

/**
 * Returns a capacity at least as large as the given minimum capacity.
 */
private int newCapacity(int minCapacity) {
  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;
}

面试

Stringbuffer 与 Stringbuilder 的区别？
1、StringBuffer：线程安全，StringBuilder：线程不安全，因为 StringBuffer 的所有公开方法都是 synchronized 修饰的。
2、都继成了 AbstractStringBuilder 这个抽象类，实现了 CharSequence 接口。
3、初始容量都是16和扩容机制都是"旧容量*2+2"。
4、StringBuffer 比 StringBuilder 多了一个 toStringCache 字段，用来在 toString方法中进行缓存。