Java 按行读取文件内容：简要方法介绍和实例演示
1、使用到的相关方法和类简介
public FileInputStream(File file) throws FileNotFoundException
Creates a FileInputStream by opening a connection to an actual file, the file named by the File object file in the file system. A new FileDescriptor object is created to represent this file connection.
If the named file does not exist, is a directory rather than a regular
file, or for some other reason cannot be opened for reading then a
FileNotFoundException is thrown.Parameters:
file - the file to be opened for reading.
Throws:
FileNotFoundException - if the file does not exist, is a directory
rather than a regular file, or for some other reason cannot be opened
for reading. SecurityException - if a security manager exists and its
checkRead method denies read access to the file.
这是查阅的java原始库中的描述，简单介绍就是传入参数file，试图通过关联一个具体的文件建立一个文件输入流，如果文件名字不存在、无法被打开或是无法被阅读，那么就抛出异常。
建立文件输入流后，进一步需要建立BufferedReader对象来对文件输入进行读取。以下给出java原文档给出的实例，其中传入BufferedReader的参数还可以是InputStreamReader等输入流。
BufferedReader in
= new BufferedReader(new FileReader(“foo.in”));
建立好字符读入后，便可以调用其中的方法进行按行读取内容。这里主要给出一个之后需要用到的方法，也就是按行读取一个字符串。
String readLine() Reads a line of text.
2、实例演示
File fileInput = new File(fileName);
ArrayList<ArrayList<Integer>> input = new ArrayList<ArrayList<Integer>>();
String str = new String();
int i = 0;
try {
FileInputStream file = new FileInputStream(fileInput);
BufferedReader br = new BufferedReader(new InputStreamReader(file, "UTF-8"));// 构造一个BufferedReader类来读取文件
while ((str = br.readLine()) != null) {
ArrayList<Integer> e = new ArrayList<Integer>();
input.add(e);
// System.out.println(str);
for (String a : str.split("\t")) {
try {
if(Integer.parseInt(a)<0)
{
System.out.println("输入中含有负数");
return null;
}
input.get(i).add(Integer.parseInt(a));
} catch (Exception w) {
System.out.println("非法输入格式或内容");
return null;
}
}
i++;
}
} catch (Exception e) {
System.out.println("读入文件错误");
return null;
}
这段代码处理的是读取文件中的不含负数和小数的矩阵，按行对矩阵进行处理，最终将矩阵保存在input中，方便后续进一步处理。代码中的fileName便是读入文件的路径。