OutputStream in Java

OutputStream class in Java is an abstract class that is meant for writing the binary data from Java application to a file/console/network. It is a superclass of all output stream classes used to write binary data and contains methods for writing binary data into the file/console/network. It implements a Closeable and Flushable interface.

public abstract class OutputStream
extends Object
implements Closeable, Flushable

When we write data into the stream then data is not saved in the file directly, it is stored only in the output stream memory, and to save those data from that memory to the file, the flush() method must be called. Whenever we call the write() method then first saved inside the output stream memory and to save those data from the output stream memory to file we must call flush() method.

Up to Java 1.4 version flush() method was directly available in the OutputStream class, but in Java 1.5 version Flushable interface was introduced which contains flush() method. And OutputStream class implements the Flushable interface.

package java.io;
import java.io.IOException;
public interface Flushable {
    void flush() throws IOException;
}

package java.io;
public abstract class OutputStream
implements Closeable, Flushable {
      ………
}

Java OutputStream class methods

There are methods available in OutputStream class are,

The write() method directly can’t save the data to the file. It can store data only to the stream so to move data from the stream into the file we must call the flush() method.

Sub classes of Java OutputStream

Java OutputStream class in Java has different sub classes based on the sources and formats to the data. These sub classes are,

• FileOutputStream
• ByteArrayOutputStream
• FilterOutputStream
  • DataOutputStream
  • BufferedOutputStream
  • PrintStream
• ObjectOutputStream
• PipedOutputStream

Among these 8 classes FileOutputStream, DataOutputStream, PrintStream and ObjectOutputStream are important classes,

None of these subclasses having a 0 parameter constructor because it is a stream-based connection where one end is a Java application. Therefore second end information must be passed as input to the constructor.

Among all 8 OutputStream subclasses, only FileOutputStream can directly connect to the file. The remaining all other 7 classes need the help from FileOutputStream class to connect to the file. They connect to FileOutputStream and FileOutputStream connect to the file. So, we can say the basic source of connecting to file and writing data is FileOutputStream class and the remaining classes are depending on it.

Why SequenceInputStream is there but SequenceOutputStream is not there? SequenceInputStream reads input from multiple streams sequentially one after one with single stream. But there is no need of writing the same data in multiple stream that’s why SequenceOutputStream don’t given. Similarly, StringBufferInputStream is there but StringBufferOutputStream is not there.

DataOutputStream and ObjectOutputStream

The DataOutputStream and ObjectOutputStream class are implemented from DataOutput and ObjectOutput interface respectively. And the ObjectOutput interface is a sub-interface of the DataOutput interface.

ObjectOutput => ObjectOutputStream
DataOutput => DataOutputStream

// DataOutput.java
package java.io;
interface DataOutput {
……
}

// ObjectInput.java
package java.io;
public interface ObjectOutput extends
DataOutput, AutoCloseable {
……
}

InputStream vs OutputStream

Similarity,

• Both are subclasses of java.lang.Object class and implements a Closeable interface.
• Both belong to the java.io package.
• They are abstract classes.

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