生成器模式

public class AbstractStringBuilder {
    protected char[] value;

    protected int count;

    public AbstractStringBuilder(int capacity) {
        count = 0;
        value = new char[capacity];
    }

    public AbstractStringBuilder append(char c) {
        ensureCapacityInternal(count + 1);
        value[count++] = c;
        return this;
    }

    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0)
            expandCapacity(minimumCapacity);
    }

    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);
    }
}

public class StringBuilder extends AbstractStringBuilder {
    public StringBuilder() {
        super(16);
    }

    @Override
    public String toString() {
        // Create a copy, don‘t share the array
        return new String(value, 0, count);
    }
}

public class Client {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        final int count = 26;
        for (int i = 0; i < count; i++) {
            sb.append((char) (‘a‘ + i));
        }
        System.out.println(sb.toString());
    }
}