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Java : PushbackReader with Examples

Last Modified : August 29, 2024 Published : April 3, 2023

PushbackReader (Java SE 22 & JDK 22) with Examples.
You will find code examples on most PushbackReader methods.


Summary

A character-stream reader that allows characters to be pushed back into the stream.

PushbackReader (Java SE 22 & JDK 22)

Class diagram

try (final var reader = new PushbackReader(new StringReader("abcd"), 3)) {
    System.out.printf("%c%n", reader.read()); // a
    System.out.printf("%c%n", reader.read()); // b

    reader.unread('X');
    reader.unread('Y');
    reader.unread('Z');

    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // c
    System.out.printf("%c%n", reader.read()); // d
    System.out.println(reader.read()); // -1
}

Fields declared in FilterReader

in

Please see the link below.

Fields declared in Reader

lock

Please see the link below.

Constructors

PushbackReader (Reader in)

Creates a new pushback reader with a one-character pushback buffer.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    System.out.printf("%c%n", reader.read()); // a

    reader.unread('X');

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // b

    reader.unread('Y');

    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // c

    reader.unread('Z');

    System.out.printf("%c%n", reader.read()); // Z
    System.out.println(reader.read()); // -1
}

try (final var reader = new PushbackReader(new StringReader("abc"))) {
    reader.unread('X');
    reader.unread('Y');
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : Pushback buffer overflow

PushbackReader (Reader in, int size)

Creates a new pushback reader with a pushback buffer of the given size.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abcd"), 3)) {
    System.out.printf("%c%n", reader.read()); // a
    System.out.printf("%c%n", reader.read()); // b

    final char[] cbuf = {'X', 'Y', 'Z'};
    reader.unread(cbuf);

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // c
    System.out.printf("%c%n", reader.read()); // d
    System.out.println(reader.read()); // -1
}

try (final var reader = new PushbackReader(new StringReader("abc"), 3)) {
    final char[] cbuf = {'W', 'X', 'Y', 'Z'};
    reader.unread(cbuf);
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : Pushback buffer overflow

Methods

void close ()

Closes the stream and releases any system resources associated with it.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    System.out.printf("%c%n", reader.read()); // a
    System.out.printf("%c%n", reader.read()); // b
    System.out.printf("%c%n", reader.read()); // c
    System.out.println(reader.read()); // -1
}

// An example without a try-with-resources statement.
final var reader = new PushbackReader(new StringReader("abc"));
try {
    System.out.printf("%c%n", reader.read()); // a
    System.out.printf("%c%n", reader.read()); // b
    System.out.printf("%c%n", reader.read()); // c
    System.out.println(reader.read()); // -1
} finally {
    reader.close();
}

void mark (int readAheadLimit)

Marks the present position in the stream.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    reader.mark(0);
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : mark/reset not supported

boolean markSupported ()

Tells whether this stream supports the mark() operation, which it does not.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    // Always returns false.
    System.out.println(reader.markSupported()); // false
}

int read ()

Reads a single character.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    System.out.printf("%c%n", reader.read()); // a

    reader.unread('X');

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // b

    reader.unread('Y');

    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // c

    reader.unread('Z');

    System.out.printf("%c%n", reader.read()); // Z
    System.out.println(reader.read()); // -1
}

try (final var reader = new PushbackReader(new StringReader("abc"))) {
    reader.unread('X');
    reader.unread('Y');
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : Pushback buffer overflow

boolean ready ()

Tells whether this stream is ready to be read.

PushbackReader (Java SE 22 & JDK 22) 
final var path = Path.of("R:", "java-work", "aaa.txt");
System.out.println(path); // R:\java-work\aaa.txt

Files.writeString(path, "abc");

try (final var reader = new PushbackReader(Files.newBufferedReader(path))) {
    System.out.println(reader.ready()); // true
    System.out.printf("%c%n", reader.read()); // a

    System.out.println(reader.ready()); // true
    System.out.printf("%c%n", reader.read()); // b

    System.out.println(reader.ready()); // true
    System.out.printf("%c%n", reader.read()); // c

    System.out.println(reader.ready()); // false
    System.out.println(reader.read()); // -1
}

void reset ()

Resets the stream.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    reader.reset();
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : mark/reset not supported

void unread (char[] cbuf)

Pushes back an array of characters by copying it to the front of the pushback buffer.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abcd"), 3)) {
    System.out.printf("%c%n", reader.read()); // a
    System.out.printf("%c%n", reader.read()); // b

    final char[] cbuf = {'X', 'Y', 'Z'};
    reader.unread(cbuf);

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // c
    System.out.printf("%c%n", reader.read()); // d
    System.out.println(reader.read()); // -1
}

try (final var reader = new PushbackReader(new StringReader("abc"), 3)) {
    final char[] cbuf = {'W', 'X', 'Y', 'Z'};
    reader.unread(cbuf);
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : Pushback buffer overflow

void unread (char[] cbuf, int off, int len)

Pushes back a portion of an array of characters by copying it to the front of the pushback buffer.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abcd"), 3)) {
    final char[] cbuf = {'X', 'Y', 'Z'};

    System.out.printf("%c%n", reader.read()); // a

    reader.unread(cbuf, 0, 3);

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // b

    reader.unread(cbuf, 1, 2);

    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // c

    reader.unread(cbuf, 2, 1);

    System.out.printf("%c%n", reader.read()); // Z
    System.out.printf("%c%n", reader.read()); // d
    System.out.println(reader.read()); // -1
}

void unread (int c)

Pushes back a single character by copying it to the front of the pushback buffer.

PushbackReader (Java SE 22 & JDK 22) 
try (final var reader = new PushbackReader(new StringReader("abc"))) {
    System.out.printf("%c%n", reader.read()); // a

    reader.unread('X');

    System.out.printf("%c%n", reader.read()); // X
    System.out.printf("%c%n", reader.read()); // b

    reader.unread('Y');

    System.out.printf("%c%n", reader.read()); // Y
    System.out.printf("%c%n", reader.read()); // c

    reader.unread('Z');

    System.out.printf("%c%n", reader.read()); // Z
    System.out.println(reader.read()); // -1
}

try (final var reader = new PushbackReader(new StringReader("abc"))) {
    reader.unread('X');
    reader.unread('Y');
} catch (IOException e) {
    System.out.println("IOException! : " + e.getMessage());
}

// Result
// ↓
//IOException! : Pushback buffer overflow

Methods declared in FilterReader

read, skip

Please see the link below.

Methods declared in Reader

nullReader, read, read, transferTo

Please see the link below.


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