关于为文本文件加行号的问题 - 黑马程序员技术交流社区

package test1;
import java.io.*;
import java.util.*;
public class MyBufferedReader {
private Reader r;
public MyBufferedReader(Reader r) {
this.r = r;
}
private String readLine(StringBuilder sb1) throws IOException {
StringBuilder sb2 = new StringBuilder();
int ch;
while ((ch = r.read()) != -1) {
if (ch == '\r') {
continue;
}
if (ch == '\n') {
return sb2.insert(0, sb1).toString();
} else {
sb2.append((char) ch);
}
}
if (sb2.length() != 0) {
return sb2.insert(0, sb1).toString();
}
return null;
}
// 读取一行，带行号功能，起始行号有用户指定
@SuppressWarnings("resource")
public String readLine(int lineNum) throws IOException {
StringBuilder sb = new StringBuilder();
Formatter formatter = new Formatter(sb);
//***********************************问题***************************************************
//主要是这里，怎么能把行号的位数不写死，比如文件有20行，这里就是%2d,如果是380行，就是%3d，依次类推
formatter.format("%3d: ", lineNum);
return readLine(sb);
}
// 读取一行
public String readLine() throws IOException {
return readLine(new StringBuilder());
}
public void myClose() throws IOException {
r.close();
}
}

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