Java的Writer里面的append与write容易让人误解,且网上系统的解说的也不多, 便写了此文.
基础问题1:Writer如何追加字符串
在构造法方法中,有一个boolean append参数,这个参数表明了是否需要追加.这个参数是属于FileOutputStream的构造方法
public FileWriter(String filename, boolean append) throws IOException {
super(new FileOutputStream(filename, append));
}
/**
* Constructs a new {@code FileOutputStream} that writes to {@code path}.
* If {@code append} is true and the file already exists, it will be appended to; otherwise
* it will be truncated. The file will be created if it does not exist.
*
* @throws FileNotFoundException if the file cannot be opened for writing.
*/
public FileOutputStream(String path, boolean append) throws FileNotFoundException {
this(new File(path), append);
}
问题2: 那么在构造方法里有参数了,append方法又是干什么用的?
以下代码有什么区别?
// 程序1
FileWriter fw = new FileWriter("a.txt",true);
fw.write("a");
fw.write("b");
fw.write("c");
fw.close();
// 程序2
FileWriter fw = new FileWriter("a.txt",true);
fw.append("a");
fw.append("b");
fw.append("c");
fw.close();
老实说,没有区别, 都是在打开的文件后面追加abc而已.并且上面的重复执行10遍,都是往文件里追加了十次abc.
结果都是:
abcabcabcabcabcabcabcabcabcabc
那么append是干什么的呢, 提起append第一反应应该是追加,但是我们构造方法已经写了追加了啊.
此时我们直接点开源码好了.
/**
* Appends the character {@code c} to the target. This method works the same
* way as {@link #write(int)}.
*
* @param c
* the character to append to the target stream.
* @return this writer.
* @throws IOException
* if this writer is closed or another I/O error occurs.
*/
public Writer append(char c) throws IOException {
write(c);
return this;
}
/**
* Appends the character sequence {@code csq} to the target. This method
* works the same way as {@code Writer.write(csq.toString())}. If {@code
* csq} is {@code null}, then the string "null" is written to the target
* stream.
*
* @param csq
* the character sequence appended to the target.
* @return this writer.
* @throws IOException
* if this writer is closed or another I/O error occurs.
*/
public Writer append(CharSequence csq) throws IOException {
if (csq == null) {
csq = "null";
}
write(csq.toString());
return this;
}
/**
* Appends a subsequence of the character sequence {@code csq} to the
* target. This method works the same way as {@code
* Writer.writer(csq.subsequence(start, end).toString())}. If {@code
* csq} is {@code null}, then the specified subsequence of the string "null"
* will be written to the target.
*
* @param csq
* the character sequence appended to the target.
* @param start
* the index of the first char in the character sequence appended
* to the target.
* @param end
* the index of the character following the last character of the
* subsequence appended to the target.
* @return this writer.
* @throws IOException
* if this writer is closed or another I/O error occurs.
* @throws IndexOutOfBoundsException
* if {@code start > end}, {@code start < 0}, {@code end < 0} or
* either {@code start} or {@code end} are greater or equal than
* the length of {@code csq}.
*/
public Writer append(CharSequence csq, int start, int end) throws IOException {
if (csq == null) {
csq = "null";
}
write(csq.subSequence(start, end).toString());
return this;
}
出人意料的事情是, append里面调用了write方法, 也就是本质上append就是调用的write.也没有什么特殊的操作,要是说真的区别的话,也就在于:
append可以添加null的字符串，输出为”null” 而write会抛出空指针异常
Appendable
那么Append的意义在于, 其是Appendable接口类.
public interface Appendable {
Appendable append(char c) throws IOException;
Appendable append(CharSequence csq) throws IOException;
Appendable append(CharSequence csq, int start, int end) throws IOException;
}
而我们的Writer类,实现了这个方法而已:
public abstract class Writer implements Appendable, Closeable, Flushable {
...
}
总结
我们以后看到append, 不能直接说是追加, write是覆盖这种直觉性的反应了. 这里的append只是一种调用模式, 能够不断的返回自己append.
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