Read all text from a file
Here's a compact, robust idiom for Java 7, wrapped up in a utility method:
static String readFile(String path, Charset encoding)
byte encoded = Files.readAllBytes(Paths.get(path));
return new String(encoded, encoding);
Read lines of text from a file
Java 7 added a¬†convenience method to read a file as lines of text,¬†represented as a¬†List. This approach is "lossy" because the line separators are stripped from the end of each line.
List<String> lines = Files.readAllLines(Paths.get(path), encoding);
The first method, that preserves line breaks, can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), and the decoded characters (each of which is 16 bits even if encoded as 8 bits in the file) reside in memory at once. It is safest to apply to files that you know to be small relative to the available memory.
The second method, reading lines, is usually more memory efficient, because the input byte buffer for decoding doesn't need to contain the entire file. However, it's still not suitable for files that are very large relative to available memory.
For reading large files, you need a different design for your program, one that reads a chunk of text from a stream, processes it, and then moves on to the next, reusing the same fixed-sized memory block. Here, "large" depends on the computer specs. Nowadays, this threshold might be many gigabytes of RAM.
One thing that is missing from the sample in the original post is the character encoding. There are some special cases where the platform default is what you want, but they are rare, and you should be able justify your choice.
The¬†StandardCharsets¬†class define some constants for the encodings required of all Java runtimes:
String content = readFile("test.txt", StandardCharsets.UTF_8);
The platform default is available from¬†the¬†Charset¬†class¬†itself:
String content = readFile("test.txt", Charset.defaultCharset());