源码阅读笔记2 --- StringBuilder

• StringBuilder

public final class StringBuilder
    //StringBuilder和StringBuffer都是继承自抽象类AbstractStringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
    //调用父类的构造器
    public StringBuilder() {
        super(16);
    }

    public StringBuilder(int capacity) {
        super(capacity);
    }

    public StringBuilder(String str) {
        //调用父类的构造器创建一个str.length() + 16大小的数组
        super(str.length() + 16);
        append(str);
    }

    public StringBuilder append(String str) {
        super.append(str);
        //此处return this可以用来写方法链
        return this;
    }
}

• AbstractStringBuilder

abstract class AbstractStringBuilder implements Appendable, CharSequence {
    char[] value;

    int count;

    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

    //应该是返回obj对象的引用
    public AbstractStringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }


    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;
    }

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

• 其他

    //第二个参数是新数组的长度，这个方法通常用来增加数组的大小
    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;
    }