java - StringBuffer的capacity()方法

Question

StringBuffer str=new StringBuffer();
StringBuffer str1=new StringBuffer(3);
System.out.println(str.capacity()); //16
System.out.println(str1.capacity()); //8
为什么str1.capacity的结果是8？

Answer 1

You may have made a mistake, the output should be:

16
3

Look at the source code:

    // minimumCapacity是当前已经存储的字符长度+要追加的字符长度
    // value.length 是当前容量
    // 所以新容量=max(2*旧容量+2，追加后的字符长度)
    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);
    }

The capacity will only be expanded when adding. During initialization, in addition to the default of 16, the number is set.

Answer 2

给你看JDK 1.8中的构造方法：
StringBuffer的两个构造方法，继承自父类的构造方法AbstractStringBuilder：
public StringBuffer() {
    super(16);
}
public StringBuffer(int capacity) {
    super(capacity);
}
AbstractStringBuilder的构造方法：
AbstractStringBuilder(int capacity) {
    value = new char[capacity];
}      

Answer 3

Run here to see Java online compilation and execution. Running environment: java v1.7.0_80