为什么Java StringBuilder具有CharSequence的构造函数和String的构造函数？

Question

知道String实现了CharSequence接口，那么为什么StringBuilder有一个CharSequence的构造函数和另一个String的构造函数？javadoc中没有指示！

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {...}

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

...

    /**
     * Constructs a string builder initialized to the contents of the
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

    /**
     * Constructs a string builder that contains the same characters
     * as the specified {@code CharSequence}. The initial capacity of
     * the string builder is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     *
     * @param      seq   the sequence to copy.
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
...
}

Answer 1

优化。如果我没记错的话，有两种实现append的方法。append(String)比append(CharSequence)在哪里更有效CharSequence。如果我必须执行一些额外的例程来检查，以确保CharSequence它与String兼容，将其转换为String，然后运行append（String），那将比直接append（String）长。结果相同。不同的速度。