通过链接构造函数实例化对象

Question

My textbook has an example in the Files and Streams section that confuses me.

BufferedReader inFile = new BufferedReader (new FileReader ("data.txt"));

My thinking is we are creating an object, of type BufferedReader and constructing them with another classes constructor FileReader and then 'laying' that object into the BufferedReader constructor.

Why are we instantiating the object with two 'new' keywords and what is happening?

Does this fall under polymorphism or inheritism?

Answer 1

Perhaps this equivalent code will make more sense:

FileReader fileReader = new FileReader("data.txt");
BufferedReader inFile = new BufferedReader(fileReader);

All this does is construct a FileReader object that is used as an argument for the BufferedReader constructor. This is an example of neither polymorphism or inheritance, this is just nesting expressions inside other expressions.

Answer 2

Most stream classes can be chained together. The new operator returns an instance of the type following, using the constructor that follows. So the FileReader is initialized with a file that will be read, with the resulting object passed to a BufferedReader such that the read from the file will be buffered for efficient I/O during the actual read.