interface definitions and abstract Classes -

Figure 541. Interface examples

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Figure 542. Observations

Multiple standards involved:

8P8C: Mechanical dimensions and tolerances.
CAT7: Telecommunication performance of twisted-pair copper interconnects.

Note: Compatible hardware must obey both standards.

Figure 543. Writing strings to file

public class Text2File {
  private final PrintStream out;

  public Text2File(final String fileName)
     throws FileNotFoundException {
    out = new PrintStream(new File(fileName));}

  public void println(final String s) {
    out.println(s);}

  public void closeFile() {
    out.close();}
}

Figure 544. Using Text2File

final String outputFileName =
            "output.txt";
try {
  final Text2File output =
  new Text2File(outputFileName);
  output.println("Some dumb text");
  output.println("More dumb text");
  output.closeFile();
} catch (final FileNotFoundException e){
  System.err.println("Unable to open '"
   + outputFileName + "' for writing");
}

File output.txt:

Some dumb text
More dumb text

Figure 545. Possible Text2File errors:

Missing output.closeFile() call.

Some text portion may not be flushed to disk.
Calling output.println(...) after output.closeFile():
```
output.closeFile();
output.println("Too late!");
```
Last call will be silently ignored.

Figure 546. Employ “try-with-resources”

final String outputFileName =
     "output.txt";

try (final Text2File output =
     new Text2File(outputFileName)){
  output.println("Some dumb text");
  output.println("More dumb text");
} catch (FileNotFoundException e){...}

Compile time error:

Required:
   java.lang.AutoCloseable
Found:
  de.hdm_stuttgart.mi.sd1.Text2File

Figure 547. interface syntax

accessModifier interface interfaceName [throwsClause]?{
    [field]*
   [method]*
}

Figure 548. The AutoCloseable promise

package java.lang; ❶

public interface AutoCloseable {

/**
 * Closes this resource,
 * relinquishing any
 * underlying resources.
 */
void close() ❷;

}

public class Text2File
  implements AutoCloseable ❶{

  private ❷ PrintStream out;
...
  public void println(final String s){
    out.println(s); }

  public void close() ❸{
    out.close(); ❹
    out = null; ❺
  }
}

❶	Java™ standard package.
❷	Method declaration without implementing body.

❶	Promise to implement the `AutoCloseable` interface. This boils down to implement `public void close()` at ❸. Notice the `@Override` annotation resemblance to overriding base class methods in derived classes.
❷	`final` has been removed in favour of setting `out` to `null` at ❺.
❸	Renaming former `closeFile()` method to `close()` keeping the interface promise made at ❶. Caution: No `@Override` here since close() is being implemented rather tan overriding a base class method not existing anyway.
❹	Closing the `PrintStream` thereby flushing buffered strings prior to releasing allocated operating system resources.
❺	Setting `out` to `null` causes subsequent `Text2File`.`println(...)` calls throwing a `NullPointerException`. Thus silent errors become observable errors.

Figure 549. abstract class replacement

package hdm.project; ❶

abstract public
    class AutoCloseable ❷{

/**
 * Closes this resource,
 * relinquishing any
 * underlying resources.
 */
abstract void close();❸

}

public class Text2File
  extends AutoCloseable ❶{

  private  PrintStream out;
...
  public void println(final String s){
    out.println(s); }

  @Override public void close() ❷{
    out.close();
    out = null;
  }
}

❶	“Private” project package.
❷	Replacing `interface` by `abstract` class.
❸	abstract `close()` not providing an implementing body.

❶	Extending `abstract` class `AutoCloseable` replaces implementing the `AutoCloseable` interface.
❷	Overriding (and in fact implementing) the abstract base class `close()` method.

Figure 550. interface vs. abstract class

Java™ disallows multiple inheritance.
A class may implement an arbitrary number of interfaces:
```
public class X implements I1, I2, I3 {...}
```

Figure 551. interface MyAutoCloseable

/**
 * Support auto-closing of resources
 */
public interface MyAutoCloseable {
  /**
   * close resource in question. Example: Terminate
   * a database connection or a file stream.
   */
  public void close();
}

Figure 552. Extending MyAutoCloseable to flush

/**
 * Flush pending values.
 */
public interface MyFlushable extends MyAutoCloseable {

  /**
   * Save pending i.e. buffered values.
   */
  public void flush();
}

Figure 553. Using MyFlushable

public class Text2FileFlushable implements MyFlushable {
  private PrintStream out;
...
  /**
   * Flushing pending output to underlying file.
   */
  public void flush(){
    out.flush();
  }
  /**
   * Closing file thereby flushing buffer. Caution: Further calls
   * to {@link #println(String)} will fail!.
   */
  public void close() {
    out.close();
    out = null;
  }
}

Figure 554. Inheritance hierarchy

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Figure 555. Upcoming topics

Default methods.
Base classes.

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`interface` definitions and `abstract` Classes