[Chapter 24] 24.27 java.io.FilterWriter (JDK 1.1)

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| Chapter 24 The java.io Package |

## 24.27 java.io.FilterWriter (JDK 1.1)

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This abstract class is intended to act as a superclass for character output streams that filter the data written to them before writing it to some other character output stream.

`FilterWriter` is declared `abstract`, so that it cannot be instantiated. But none of its methods are themselves abstract: they all simply invoke the corresponding method on the output stream that was passed to the `FilterWriter` constructor. If you were allowed to instantiate a `FilterWriter` object, you'd find that it acts as a "null filter"--that it simply passes the characters written to it along, without any filtration.

Because `FilterWriter` implements a "null filter," it is an ideal superclass for classes that want to implement simple filters without having to override all of the methods of `Writer`. In order to create your own filtered character output stream, you should subclass `FilterWriter` and override all of its `write()` methods to perform the desired filtering operation. Note that you can implement two of the `write()` methods in terms of the third, and thus only implement your filtering algorithm once. In some cases, you may want to override other `Writer` methods as well, and you may often want to add other methods or constructors that are specific to your subclass.

`FilterWriter` is the character-stream analog of `FilterOutputStream`.

    public abstract class FilterWriter extends Writer {
        // Protected Constructor
                protected FilterWriter(Writer out);
        // Protected Instance Variables
                protected Writer out;
        // Public Instance Methods
                public void close() throws IOException;  // Defines Writer
                public void flush() throws IOException;  // Defines Writer
                public void write(int c) throws IOException;  // Overrides Writer
                public void write(char[] cbuf, int off, int len) throws IOException;  // Defines Writer
                public void write(String str, int off, int len) throws IOException;  // Overrides Writer
    }

### Hierarchy:

Object->Writer->FilterWriter

java.io.FilterReader (JDK 1.1) |  | java.io.InputStream (JDK 1.0)