java - Java StringBuilder setLength 方法是否将\0 冗余分配给它的 char[] 值数组？

Question

我正在研究 Java StringBuilder 的 setLength method .

如果新的长度更大，它将新“追加”的数组索引设置为'\0':

public void setLength(int newLength) {
     if (newLength < 0)
         throw new StringIndexOutOfBoundsException(newLength);
     if (newLength > value.length)
         expandCapacity(newLength);

     if (count < newLength) {
         for (; count < newLength; count++)
             value[count] = '\0';
     } else {
         count = newLength;
     }
 }

这是不必要的吗？在 expandCapacity(newLength) 中，Arrays.copyOf 方法用于创建一个新的 char[] 数组，其大小为 newLength:

public static char[] copyOf(char[] original, int newLength) {
    char[] copy = new char[newLength];
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

Java language specification states数组中的组件被初始化为其默认值。对于 char，这是 '\u0000'，据我所知，它是 '\0' 的 unicode 等价物。

另外，StringBuilder setLength documentation状态:

If the newLength argument is greater than or equal to the current length, sufficient null characters ('\u0000') are appended so that length becomes the newLength argument.

但是可以直接访问这个数组的长度，而无需为其组件赋值:

char[] array = new char[10];
System.out.println(array.length); // prints "10"

那么，setLength 中的 for 循环是多余的吗？

Answer 1

当我们想要重用StringBuilder时，它是必要的。

假设我们在 StringBuilder

中删除这段代码

  if (count < newLength) {
         for (; count < newLength; count++)
             value[count] = '\0';
     }

我们用下面的代码进行测试:

StringBuilder builder = new StringBuilder("test");
builder.setLength(0); //the `value` still keeps "test", `count` is 0
System.out.println(builder.toString()); //print empty
builder.setLength(50); //side effect will happen here, "test" is not removed because expandCapacity still keeps the original value
System.out.println(builder.toString());  // will print test

你说的代码是jdk6的，java8的不一样