How to Use I/o Streams in Java

This updated guide examines How to Use I/o Streams in Java and organizes the essential facts, background, and practical takeaways in clear American English.

Method 1

Using Byte and Character Streams

1. Create a text file. Give it an arbitrary name and write something in it, such asI love wikiHow!. In this article, we will name itfile.txt.
2. Create a main method. You can addthrows IOExceptiondeclaration to the method signature to avoid adding the catch block to the try-with-resources block.

   publicstaticvoidmain(String[]args){}

3. Create a try-with-resources block. Try-with-resources block will automatically close streams for us.

   publicstaticvoidmain(String[]args){try(){}}

4. CreateFileInputStreamorFileReaderin the block statement. In the constructor of the created input stream, specify the name of the text file previously created. In this article, we called itfile.txt.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt")){}}

   • The former is a byte stream, the latter is a character stream.
5. CreateFileOutputStreamorFileWriterin the block statement. In the constructor of the created output stream, specify another arbitrary text file name. In this article, we will name itfile-copy.txt.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt");FileOutputStreamout=newFileOutputStream("file-copy.txt")){}}

   • If you createdFileInputStream, you need to createFileOutputStream. The same goes for theFileReaderandFileWriter.
6. Createintvariable. This variable will be used for temporary storage of read byte or character.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt");FileOutputStreamout=newFileOutputStream("file-copy.txt")){intdata;}}

7. Read data from input stream. Create a while loop in which the data is read into the previously createdintvariable until it reads the number-1. In other words, it reads data from the file until it reaches the end of the file.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt");FileOutputStreamout=newFileOutputStream("file-copy.txt")){intdata;while((data=in.read())!=-1){}}}

8. Write data to the output stream. In the body of the while loop, write the read data to the output stream.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt");FileOutputStreamout=newFileOutputStream("file-copy.txt")){intdata;while((data=in.read())!=-1){out.write(data);}}}

9. Catch and handle IOException. Add a catch block to the try-with-resources block, catchIOExceptionand, in case of an error, print the stack trace to the console.

   publicstaticvoidmain(String[]args){try(FileInputStreamin=newFileInputStream("file.txt");FileOutputStreamout=newFileOutputStream("file-copy.txt")){intdata;while((data=in.read())!=-1){out.write(data);}}catch(IOExceptione){e.printStackTrace();}}

10. Run the program. Contents of thefile.txtare copied to thefile-copy.txtwhich, if it doesn't exist yet, is created.

Method 2

Using Buffered Streams

Buffered streams optimize input and output data. Buffer streams are more efficient than byte and character streams as they read and write from a buffer that is handled by Java platform rather than the underlying operating system. Most common usage of buffered streams is reading a textual file, line by line.

1. Create a text file. Give it an arbitrary name and write something in it, such asI love wikiHow!. In this article, we will name itfile.txt.
2. Create a main method. You can addthrows IOExceptiondeclaration to the method signature to avoid adding the catch block to the try-with-resources block.

   publicstaticvoidmain(String[]args){}

3. Create a try-with-resources block. Try-with-resources block will automatically close streams for us.

   publicstaticvoidmain(String[]args){try(){}}

4. CreateBufferedReaderin the block statement. In the constructor of the created input stream, create aFileReaderand specify the name of the text file previously created inside its constructor. In this article, we called itfile.txt.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"))){}}

5. CreateBufferedWriterin the block statement. In the constructor of the created output stream, create aFileWriterand specify another arbitrary text file name in its constructor. In this article, we will name itfile-copy.txt.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"));BufferedWriterout=newBufferedWriter(newFileWriter("file-copy.txt"))){}}

6. CreateStringvariable. This variable will be used for temporary storage of read lines.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"));BufferedWriterout=newBufferedWriter(newFileWriter("file-copy.txt"))){Stringline;}}

7. Read lines from input stream. Create a while loop in which the lines are read into the previously createdStringvariable until it readsnull. In other words, it reads data from the file until it reaches the end of the file.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"));BufferedWriterout=newBufferedWriter(newFileWriter("file-copy.txt"))){Stringline;while((line=in.readLine())!=null){}}}

8. Write lines to the output stream with line separator. In the body of the while loop, write the read lines to the output stream and append the line separator at the end of each line.BufferedReaderreads lines, but omits the line separator.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"));BufferedWriterout=newBufferedWriter(newFileWriter("file-copy.txt"))){Stringline;while((line=in.readLine())!=null){out.write(line+System.lineSeparator());}}}

9. Catch and handle IOException. Add a catch block to the try-with-resources block, catchIOExceptionand, in case of an error, print the stack trace to the console.

   publicstaticvoidmain(String[]args){try(BufferedReaderin=newBufferedReader(newFileReader("file.txt"));BufferedWriterout=newBufferedWriter(newFileWriter("file-copy.txt"))){Stringline;while((line=in.readLine())!=null){out.write(line+System.lineSeparator());}}catch(IOExceptione){e.printStackTrace();}}

10. Run the program. Contents of thefile.txtare copied to thefile-copy.txtwhich, if it doesn't exist yet, is created.

Method 3

Using Data Streams

Data streams handle binary input and output of primitive data types and Strings. Data streams make it very easy to read and writeboolean,char,byte,short,int,long,float,doubleandStringvalues. Most common usage of data streams is in network applications (e.g. instant messengers).

1. Create a main method. You can addthrows IOExceptiondeclaration to the method signature to avoid adding the catch block to the try-with-resources block.

   publicstaticvoidmain(String[]args){}

2. Create a try-with-resources block. Try-with-resources block will automatically close streams for us.

   publicstaticvoidmain(String[]args){try(){}}

3. CreateDataOutputStreamin the block statement. In the constructor of the created input stream, createFileOutputStreamand specify an arbitrary file name inside its constructor. In this article, we will name itfile.txt.

   publicstaticvoidmain(String[]args){try(DataOutputStreamout=newDataOutputStream(newFileOutputStream("file.txt"))){}}

4. CreateDataInputStreamin the block statement. In the constructor of the created input stream, create aFileInputStreamand specify file name from the previous step inside its constructor. In this article, we named itfile.txt.

   publicstaticvoidmain(String[]args){try(DataOutputStreamout=newDataOutputStream(newFileOutputStream("file.txt"));DataInputStreamin=newDataInputStream(newFileInputStream("file.txt"))){}}

5. Write some data to the output stream. In the body of the try-with-resources block, write String,boolean,intand other primitive data to the output stream.

   publicstaticvoidmain(String[]args){try(DataOutputStreamout=newDataOutputStream(newFileOutputStream("file.txt"));DataInputStreamin=newDataInputStream(newFileInputStream("file.txt"))){out.writeUTF("wikiHow is a great place to learn");out.writeBoolean(true);out.writeInt(10430);out.writeDouble(20.9d);out.writeFloat(23.10f);}}

6. Read data from the input stream and print it. Read the data in the same order as it has been written and print it to the console.

   publicstaticvoidmain(String[]args){try(DataOutputStreamout=newDataOutputStream(newFileOutputStream("file.txt"));DataInputStreamin=newDataInputStream(newFileInputStream("file.txt"))){out.writeUTF("wikiHow is a great place to learn");out.writeBoolean(true);out.writeInt(10430);out.writeDouble(20.9d);out.writeFloat(23.10f);System.out.println(in.readUTF());System.out.println(in.readBoolean());System.out.println(in.readInt());System.out.println(in.readDouble());System.out.println(in.readFloat());}}

7. Catch and handle IOException. Add a catch block to the try-with-resources block, catchIOExceptionand, in case of an error, print the stack trace to the console.

   publicstaticvoidmain(String[]args){try(DataOutputStreamout=newDataOutputStream(newFileOutputStream("file.txt"));DataInputStreamin=newDataInputStream(newFileInputStream("file.txt"))){out.writeUTF("wikiHow is a great place to learn");out.writeBoolean(true);out.writeInt(10430);out.writeDouble(20.9d);out.writeFloat(23.10f);System.out.println(in.readUTF());System.out.println(in.readBoolean());System.out.println(in.readInt());System.out.println(in.readDouble());System.out.println(in.readFloat());}catch(IOExceptione){e.printStackTrace();}}

8. Run the program. Data is written to thefile.txtand then read and printed to the console.

   wikiHowisagreatplacetolearntrue1043020.923.1

Method 4

Using Object Streams

Object streams handle binary input and output of Java objects. Like data streams, object streams make it very easy to read and write primitive data types and Strings, but also any object that implementsSerializableinterface. Most common usage of object streams is in network applications where both server and client are written in Java.

1. Create a serializable class. Create a class that implementsSerializableinterface. Name the class however you'd like, but in this article we will name itWikiHowian.

   publicclassWikiHowianimplementsSerializable{}

2. Create a few private (serializable) variables. SinceWikiHowianclass represents a person, we will create two String variables for first and last name and anintvariable for age.

   publicclassWikiHowianimplementsSerializable{privateStringfirstName,lastName;privateintage;}

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