Question

根据oracle文档，请参阅此链接here

public void ensureCapacity(int minimumCapacity)

Ensures that the capacity is at least equal to the specified minimum. 
If the current capacity is less than the argument, then a new internal array 
is allocated with greater capacity. 

The new capacity is the larger of:
The minimumCapacity argument.
Twice the old capacity, plus 2.
If the minimumCapacity argument is nonpositive, this method takes no action and simply returns.

我试图用代码示例清除它：

    StringBuffer buff1 = new StringBuffer("tuts point");
    System.out.println("buffer1 = " + buff1);        // lenght = 10

    // returns the current capacity of the string buffer 1
    System.out.println("Old Capacity = " + buff1.capacity());  // capacity = 16+10=26 characters
    /*
     * increases the capacity, as needed, to the specified amount in the
     * given string buffer object
     */
    // returns twice the capacity plus 2
    buff1.ensureCapacity(30);
    System.out.println("New Capacity = " + buff1.capacity()); // 26*2 + 2 = 54

如果ensureCapacity方法中的minimumCapacity参数介于27-53之间，那么我确实得到了预期的答案（返回两倍的容量再加上2）。但是，如果参数为> = 55，则容量仅等于此参数。

    // does not return (twice the capacity plus 2)
    buff1.ensureCapacity(55);
    System.out.println("New Capacity = " + buff1.capacity());  // 55

为什么55这里是答案？

Answer 1

这是由于AbstractStringBuilder类的调用：

void expandCapacity(int minimumCapacity) {
    int newCapacity = value.length * 2 + 2;
    if (newCapacity - minimumCapacity < 0)
        newCapacity = minimumCapacity;
    if (newCapacity < 0) {
        if (minimumCapacity < 0) // overflow
            throw new OutOfMemoryError();
        newCapacity = Integer.MAX_VALUE;
    }
    value = Arrays.copyOf(value, newCapacity);
}

因此，当它增加容量时，它会将容量增加至少（2 *原始容量）+ 2，增加到所提供的值，以较大者为准。

如果容量是26，那么你通过30：

（26 * 2）+ 2 = 54，这超过30，所以新容量将为54.

如果容量是26，那么你通过55：

（26 * 2）+ 2 = 54，小于55，所以新容量将为55.

Answer 2

我确信答案将是vm的实现细节，但是

例如

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/StringBuffer.java#StringBuffer.ensureCapacity%28int%29

public synchronized void More ...ensureCapacity(int minimumCapacity) {
       if (minimumCapacity > value.length) {
            expandCapacity(minimumCapacity);
        }
    }

在AbstractStringBuilder

中调用expandCapacity

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/lang/AbstractStringBuilder.java#AbstractStringBuilder.expandCapacity%28int%29

void expandCapacity(int minimumCapacity) {
         int newCapacity = (value.length + 1) * 2;
         if (newCapacity < 0) {
             newCapacity = Integer.MAX_VALUE;
         } else if (minimumCapacity > newCapacity) {
             newCapacity = minimumCapacity;
         }
         value = Arrays.copyOf(value, newCapacity);
     }

但是我没有看到它返回55.所以更多的代码探索。是需要的（这也是openjdk的来源。不确定发生了什么变化或与你的vm不同

StringBuffer类中的ensureCapacity（int minimumCapacity）方法

2 个答案: